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267

CHAPTER–XII

UNDERFREQUENCY RELAYING AND

LOAD SHEDDINGEr.K. Mounagurusamy

CE / P&C

- Load shedding is essential in emergencies to keep the system in tact.

- Effect of under frequency operation of system

- Boiler outputs reduce due to reduction of draft fan speed.

- 10% reduction in speed of feed pumps reduces output by 30% and hencereduction of turbine generator output.

- Cooling effects of generators get reduced and hence affects the thermal

limits.

- Stator voltage is proportional to speed of generator and hence MVAR

output decreases, desinged output is not feasible.

- 10% reduction in frequency reduces turbine capacity by 0.9%. Lowfrequency operation may result in vibration and probable resonance of low

pressure blades leading to blade fatique.

- Pull out torque of induction motors is inversely propertional to squre offrequency.

- 10% reduction in frequency will increase the operating time of protection

relays by 10%

- Instrument errors increase

- Accuracy of energy meters adversely affected

- Transformer core losses increase

- 10% reduction of frequency 10% reduction of KVAR output of capacitors.

- reactive power consumption increases in ballest lamps- 10% of frequency reduction increases 16% of consumption of reactive

power in air conditioners and 63% in T.V.Sets.

POWER SYSTEM PROTECTION DURING DECLINING FREQUENCY:

When there is a sudden loss of generation due to any tripping of large generator,

the system frequency immediately drops. If the tripped unit is compartively small, the

system is not affected.

If the tripped generator or loss of generation power is large, effect is serious. Ifthere is sufficient reserve spinning governors take up the problem.

If there is not sufficient spinning power, the frequency will go down depending onhow much generation was lost and how much was system demand.



268

CHAPTER–XII

If the frequency declines much, some other generators provided with under

frequency protection to protect their machine also trip and the effect is cummulative andthe system may go black out.

If some load shedding is done when the frequency declines sufficient to keep the

frequency in limits, the system will survive. This kind of load shedding is automatically

done by the use of under frequency relays.

Soft ware package are available now-a-days to exactly arrive at the settings ofthese relays in stages and if properly set and put into effect without manipulations, thesystem stability will be well within the safety.

If the automatic load shedding is not effected properly, the stability of the system

will certainly be under question.

Normally the under frequency tripping scheme control wrests with the local

operating people. If trip links are provided in this system, there are possibilities of

keeping the trip link open due to the known reasons but the implications of such an actionwill now be understood clearly, it is hoped.

Ref: “Philosophy of under frequency relaying”

Article by Er. R. Venkataraman,

Assistant Engineer,

Office of the S.E/T/E.

Published in TNEB Engineers Association bulletin

U/F SYSTEM PROTECTION IN TNEB AS ON APRIL 2001

To get separated from Southern grid during disturbance the following inter-statefeeders are tripped with RPF and Under Frequency relay combination.

1) 400KV Sriperumbudur – cuddapah will trip at 100MW (Export to cuddapah)

when frequency is at 48 Hz with time delay of 0.5 sec.

2) 400KV Salem – Bangalore will trip at 300MW (Export to Bangalore) When

frequency is at 48Hz with time delay of 1.0 sec.

When these 400KV feeders get tripped the TNEB with Kerala system

gets separated from Andera Pradesh and Karnataka.

II If frequency is not improving due to Generation – Load mismatch, Loadrelease through Under frequency relays set at 47.8 Hz/Inst is obtained. Selected 110 KVfeeders would trip on Under Frequency relay to effect a load relief of about 650 MW.

III On further decline of frequency persisting sub – islanding schemes to getfollowing block – islanding will be effected.



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CHAPTER–XII

a) ETPS (combined with BBGTPS) Block:

Under this block two conditions viz.. with or without Generation inBBGTPS are envisaged. The feeder in this block would be tripped at47.6 Hz/ 0.75

sec. When there is no Generation at BBGTPS additional relief of Padi SS &

Sembium SS is added. Operator on duty at ETPS act depending on availability of

Generation at ETPS to match the load in the block.

b) GMR Vasavi Diesel Generation Plant Block:

This block would get separated at 47.6 Hz/0.75 sec. In this block –islanding also, two conditions ie.. for 180 MW and 100 MW generation level at

M/S GMR plant are envisaged. When Generation drops to 100 MW, additionally

at chindaripet would be tripped.

c) NCPTS (Combined with TCPL Generation) Block:

At 7.6 Hz/2 sec, the NCTPS (Plus TCPL) will go with base loadsaccording to Generation in two stages viz.. i) When generation at NCTPS is less

than 450 MW with TCPL Generation. This block will have Korattur,

Koyambedu, Kadaperi., Tharamani, Mosur loads according to the two conditions

of Generation level.

House load operation of two units at 47.5 Hz/3 sec. Is restored. Also one

unit will go on H/L at 52 Hz/1 sec.

d) Neyveli Thermal Power Station Block:

(Generation 1700 MW load 664 MW). This islanding scheme operates at

47.6 Hz/2 sec with Generation @ TS1 & TS2 and selective 110 KV & 230 KVfeeders of Cuddalore, Perambalur, Deviakurichi, Villupuram 230 KV, Villupuram

110 KV and Eachengadu Substations for base load. All the 400 KV feeders at

TS2 will be connected to under Frequency trip at 47.6 Hz/2 sec. The excessiveGeneration in this block will be reduced by running selected units on H/L. The

scheme will be supervised by Neyveli Authorities.

e) Mettur Thermal Power Station Block:

(Generation 800 MW Load 612 MW) this block too gets islanded at

47.6Hz/ 2 Sec. This block will have Salem, Mettur, Singarapet, Hosur,Thiruvannamalai and Erode loads as base loads.

House load operation is not possible for these units due to design

problems.



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CHAPTER–XII

f) TTPS – Hydro Block :

This block gets islanded at 47.6 Hz/2 Sec. Under three conditions viz., i) 5machines availability @ TTPS plus Hydro area Generation ii) 4 machines

availability @ TTPS plus Hydro area Generation iii) 3 machines availability @TTPS plus Hydro area Generation. Depending on load Generation study thefeeders that are tripped at 47.6 Hz/2 Sec. Separately for the above three conditions

are communicated.

Apart from this certain other feeders at 47.6 Hz/3 sec. Are tripped to offsetadditional load within the islanded zone.

Under Frequency relay on Aliyar Power House to automatically changethe machines from condenser mode to Generator mode at 47.6 Hz/0.5 sec. Is

installed.

House load operation of machines 4 & 5 in TTPS is set at 47 Hz/5 secs.

iv) Since MAPS will go on H/L at 47.78 Hz at 4 sec. Itself separate

islanding is not provided for these machines.

Kalpakkam units are connected for H/L. In stage I unit auxiliary loads of

24 MVA will be transferred to Generator at 47.78 Hz/1 Sec. At 47.78 Hz/4 sec

the unit will go on H/L.



271

CHAPTER-XIII

POWER-LINE CARRIER

COMMUNICATIONEr. M. Arunachalam

EE / GRT

INTRODUCTION:

The Power Line Carrier Communication terminals are created and commissioned

at various substations. The values for the required characteristic input and outputquantities for the system are to be followed as per 1) IEC Recommendation 495-1974 and

as per Indian Standard IS 9482-1980. The tests on the terminals are to be done as per the

method indicated in Indian Standard IS 10706-1983 of latest versions.

Units and levels & Measurement methods:

The units are in Decifal, and terms used in the system are Attunation, compositloss and Return loss. The PLCC systems is functioning in the range of 30 KHE-500 with

maximum power lost in line. The receiving equipments has little effect on transmitting

end the losses are expressed db-attenuationPower Line: Xdb = 10 log P1/P2

Absolute power level Xdbm = 10 log P/1mw

Relative power level Xdbr = 10 log P/P ref.Voltage level Xdb = 20 log V1/V2

Current level Xdb = 20 long I1/I2

(When the scalar ratios of currents or voltages are the square roots of the corresponding

power ratios.).

1 Mw in 600 ohm

= 0.775V

= 1.291ma.

COMPOSIT LOSS:

The input of stem having impedence Z is fed by a source with internal impedence

Z1, the composit loss in Decibel is given by 10 times log 10 Ratio of power PO – meet

the source would give upto an impedence Z1, to the power P it sends through the system

to its terminating impedence Z2.

Composit loss = 10 log10 P0/P dB.

Insertion loss:

10 log10 P1/P2 dB.

Where P1 is the power available to the system without the insertion of a network.

P2 is the power at the output with insertion of network.



272

CHAPTER–XIII MISMATCH LOSS:

10 log P0/P dB

Number of decimals by which the power in the load in the matched conditions wouldexceed the power actually flowing in the load.

RETURN LOSS:

10 log10 Po/Pr dB.

Number of decibels by which the power in the load in the matched condition wouldexceed the reflected (Return) power with connection to be actual load.

INTER MODULATION:

In a non-linear Network to which two or more sinusoidal signals are appliedsimultaneously, a series of additional sinusoidal signal will arise, there are all Harmonics

and inter modulation produces of the applied signals.

Among the inter modulation produces of two signals = m1 t1 + m2t2, the old

order products, the two 3 order products frequencies (2 f1+f2) and (2f2-f1) are harmful,being closes to f1+f2.

Measurement of Impedence.

V1/V2 = R+1x1/R ; V1/V2 = 10 P1-P2/20

1x1 = R. 10 P1-P2/20 – R

Ex: h = 0dB. P2 = -43.5 dBR = 1r

1x1 = 10 43.5/20 = 150r.

Insulation level of Line Trap:

Residential voltage by nominal discharge

0.5mH 31.5 5.4

Front-of-wave Impulse

Sparkover voltage of the arrester

P1 dB

V1

O

O

V

1 X 1P1 dB



273

CHAPTER–XIII

Peal : 26 KVInsulation levels of Tuning device and Line Trap.

Typical factory impulse voltage: 90 KV

Impulse Test voltage of L.T. 75 KV

Front of wave impulse spark over voltage of arrester .. 62 KV

The performance of Line Trap can be assessed in terms of its EFFECTIVE

RESISTANCE.

Tappling loss of a line trap is a measure of the loss of power sustained by carrier

frequency signal due to the finite blocking ability of the line trap. It is defined in terms of

the ratio of the signal voltages across an impedence equal to the characteristic impedence

of the line with and without shunt connection of the line trap. It is expressed in decibels(db). Rating of the Tapping Loss::

The value of the tapping loss as determined by the shunt connection of the resistancecomponent only of the line trap impedence. (Tapping loss based on blocking resistance).

Tapping Loss::

Due to R+jx = 10 log (1+0.25+N)/N2+P2 db

Where N = R/Z & P = 1 x 1 /2.

Due to R only 20 log ( 1 + 1 /2N) db Due to 1x1 only 10 log (1+0.25/P2) db

Band width of line trap

That frequency band V f1

Within which the blocking impedance does not fall short of a specified value.

OR

That frequency band V f2 within which the tapping loss does not exceed a specified

value.

Rated band width:

Bandwidth expressed in terms of

Rated blocking impedance or rated tapping loss

V f1N or V f2N.



274

CHAPTER–XIII

V f1N Band width expressed in terms of rated blocking impedence.

V f2N Carrier frequency band within which the rated tappingloss does not exceed a specified value.

BLOCKING REQUIREMENTS:

Permissible variation of the blocking impedence and tapping loss quantities

should be within the band width of the line trap.

A maximum loss of 2.6 db for both tapping loss and rated tapping loss this

corresponds to Line trap blocking resistance 1-41 times the characterisitc impedence of

the transmission line.

TYPICAL CASE:

Line trap blocking resistance: 570 ohms.Transmission line characterestic impedence of a single conductor phase to earth

impedence – 400

TEST ON LINE TRAPS:

Type Tests

1) Measurement of inductance of the main civil.

2) Measurement of Temperature Rise

3) Insulation tests.

4) Short time current tests

Routine tests

Measurement of blocking impedence

Measurement of tapping loss.

Measurement blocking impedence

2b

By means of a bridge method from which Resistance and Resistive and Reactive

components may be read off.

Measurement circuit.

Measurement of Tapping Loss(A7)



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CHAPTER–XIII

2L I

V0

At = 20 log (V1/V2) dB

Z are resistors equal to characteristic impedance of the line.

V1 = VO/2 V2 = V

Coupling capacitor of coupling Device coupling capacitor and compiling device from a

carrier frequency filter for efficient and connection of CF currents to high voltage line.

High frequency characteristic of coupling capacitor.

Equivalent series resistance 40 r

Stray capacitance of Low for CC

voltage terminal 200 pf and for CVT 300+0.05 Capacitance

Stray conductance of low 20 pvs for CCvoltage terminal 50 pvs for CVT

High frequency current – to with stand atleast 1A ( )

value of current equival

to a power of 400 w for a terminal resistance 4400 ohm.

ROUTINE TESTS:

1. Capacitance at power frequency

a) in the standard tem. range for testing.b) at rated power frequency

c) at sufficient low voltage to ensure No internal breakdown.

2. Voltage tests

a) Duration 1 min.

b) Test voltage between high voltage and earth terminals.

c) Low voltage terminal shall be earthed.

3. A.C. test voltage

Value corresponding to insulation level.

4. D.C. test voltageValue twice the RMS value of the AC test voltage.

3

4

L

2LG

V



276

CHAPTER–XIII

5. Test between the low voltage and earth terminals.

AC voltage of 10 KV RMs.

Duration 1 Minute

6. Capacitance and tangent of the loss angle after the voltage tests.

a) at Rated voltage

b) at Rated frequency.

Measured capacitance shall not differ from the rated value by

more than – 5% + 10%

Tangent of the loss angle.

Limits of permissible variation subject to agreement.

The purpose of measurement is to check uniformity of production.

Typical value less than 0.5 x 10-3

Coupling Device :

Coupling Device is connected together with coupling capacitor The turning of the

coupling capacitor is to component of the coupling capacitor.Impedence; in order to promote the efficient transmission of carrier frequent

signals.

Turing device:

It matches the impedence between the power line carrier frequency connection.

TRANSFORMER

Galvanic Isolation between primary and secondary terminals of the coupling device to

drain to earth of the power frequency current devived by the coupling capacitor.

DRAIN COIL:

If limits the volt ge surges coming from the power line at the terminals of the

coupling device.

LIGHTING ARRESTORS:

Direct and efficient earthing of the system when necessary of the primary terminals of the

coupling device.

Carrier freq. requirements.



277

CHAPTER–XIII

composite lost : not more than 2 dB

Return loss : preferably not less than 12dB

Nominal line 200-400 ohm

side impedence phase to earth coupling

400-700 ohm phase-phase coupling

Nominal equipment 75 ohm (unbalance)

sideimpedence 150 ohm (Balanced)

Destoration and Inter modulation Atleast 80 dB Below peak envelop power

INSULATION REQUIREMENT

Power freq. Level 5 Kvrms 1 min. isolation Transformers

Impulse level To with stand 1.2/50 impulse voltage 10 KV (peak)

(Peak value equal to twice the value of the impulse sparkovervalue of the main Arrestor.

TESTS ON COUPLING DEVICE (ROUTINE/ACCEPTANCE)

1. Composit loss

2. Return loss

3. Power Freq. voltage test

MEASUREMENT OF COMPOSIT LOSS.

Z2

21N V0 V

CF

Generator.

Loss = 20 Log 10 V0/2v √2 ½ dB.



278

CHAPTER–XIII

MEASUREMENT RETURN LOSS.

CF Generator

Return loss: 20 log (V1/ V11) dB

V1 is the voltage measured by the Web meter (V) with switch closed.

V11 is the voltage measured by the voltmeter (V) with switch

The line boide and equipment side return loss shall preperably he not less than 12dB.

In certain cases values less than 12 dB may require to be accepted.

DISTORTION AND INTER MODULATION TEST

Apply to the secondary terminals of the coupling device, two generator, set on twodifferent frequencies conveniently located within the available bandwidth of the coupling

device, Measure across an impedence equal to the line side impedence connected to the

primary side by means of test capacitor, two signals are obtained, whose power is equal

to one generator of the nominal peak envelop power. Power frequency test of Isolatingtransformers.

Power fre. voltage of 5 KVrms for one min.

J

Z1

2

2GV0

V

TC f Generator 21G GF1

F1

Selective c f

receiver



279

CHAPTER–XIII TEST ON DRAIN COIL:

Measurement of Impedence at power frequency.

Impedence at power frequency between the primary terminal and the earth

terminal as low as possible and in no case in excess of 20 ohm.

The frequency bandwidth, within which the composite loss does not exceed and

the return loss does not fall short of the specified values.

For coupling devices ICE REC 495 (1974) mentions for line side and equipmentside Impedence. A return loss greater than 12 dB Referred to the normal values, but

impractice this figure may be difficult to achieve.

For PLC terminals IEC REC 495 (1974) specified a Return loss greater than 10

dB referred to the nominal value of carrier frequency impedance.

C.F. CONNECTING CABLE:

150 ohm balanced

Electrical characteristic

Resistance Max 23.4 ohms

Insulation Resistance Min. 10,000 M. ohm/km

Test voltage 50 H 2 min.

wire-wire – 500 VRMS

wire-shiled – 4000 V RMS

Mutual capacitor – 31 n /km

Earthing at equipment end

Eliminates power freq. current circulation.

May cause high voltage across the wdgs of the coupling transformer which will need tobe designed for this duty. Maintenance personnel will need to take precaution against thepossibility of potential differences during faults, between cable screen and thelocal earth.

B) Coupling device ad carrier terminal not part of same earthmesh.

Earthing at earth potential differences may be high in the case of a fault and the

circulating currents in the screen may be dangerous.

Earthing at equipment end only the common practices to earth only the one side to thescreen at the carrier equipment end. By use of Balanced cables some of the above

problems can be avoided.



280

CHAPTER–XIII

APPLICATION OF PLCC SYSTEM

Analogue signals of frequency variation type.

Speech

Signals

TeleprotectionTelecontrol

Teleprinting

and Telefax.

As per IEC 495, IS 492, CC, TT Dissortion per 1 H droft in FSK Channel N 0.5 at 200

Bd.

Possible utilisation 4KH

Speech 300-2400 H

Pilot 2400-2700 V

Signals - 2700 H – 3660 H

V.F. Band 0.3 - 3.7 KH

Speech 0.3 - 2.4 KH

Dial tF6 2.58 KH

Signals 2.76 – 37

IF Freq. 16.45 KH

IF Band 12.7 – 16.15 KH

As approved by a national Authority the carrier frequency range

40 KH – 500 KH

Basic carrier frequency Band

for a single one way channel 2.5 KH, 4 KH

Nominal CF Band.

Band for a particulars one way PLCC channel.

e .g. 2.5, 2, 7.5, 10 kh4, 8, 12, 16 KH

Nominal Impedence 75 r unbalancedAt CF output 150 r balanced



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CHAPTER–XIII

RETURN LOSS

10 dB R/2 = 1.925

Nominal C.F. Power is the permissible Emission power for which the equipment is

designed comparable with the requirements for superiors emissions available at CF

output acc resistance load equal to nominal load impedance. Mean CF Power averagedover a time sufficiently long compared with the cycle time of the lowest modulating freq.

During which average power assures its highest value.

Ratio between PEP and manpower depends all factors in multiple signal. Speech

level, with or without compressor. No type and level of signals, may be assumed to be

between 8.5 & 10 ds. under normal service condition speech levels (Relatine)

Four wire

Transmit Receinee

Range of 0.60-17 dBr. 3.5 to + 8 dBr.Suggestion

-3.5 dBr – 3.5 dBr.

-14 dbr + 4 dbr

Two RecommendationTransmit 0 dbr

Receive -7 dbr.

Balanced Normal Impedence 600 R

Return loss

Not less 14 db

Group delay distortion:

Suggested limits

300 – 3400 HZ CCITT M - 10 20300 – 2400 HZ

Group delay distortion of a pair of transmitting and Receiving PLC Terminus for dataTransmission where speech channel is used for data transmission.

For 300 –3400 HZ

Starts 500 HZ - 3ms

600 HZ - 1.5ms

1000 HZ to 2600 HZ - 0.5ms

2800 HZ - 3ms

For 300 – 2400 HZ500 HZ - 3ms

600 HZ - 1.5ms

1000 to 1900 HZ –0.5ms2100 HZ - 3.0ms



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CHAPTER–XIII

AUTOMATIC GAIN CONTROL :

For a variation of CF input signal level of 30 db, the U.F receive level

speech/signalvaries of db.

LINEARITY :

As a function of UF input level over all loss of the speech circuts not differ by

more than + 0.3 dbr from overall loss at 0 dbmu.

For any input level between –10 dbm & 0 dbmu

Example 800 HZ

- 3.5 dbm - -3.5 dbm )(- 5.5 dbm - -5.5 dbm )( ± 0.3db

- 8.5 dBm - -8.5 dbm )(- 11.5 dBm - -11.5 dbm )( +0.3Db

- 13.5 dBm - -13.5 dbm )(

Limiter action :-

Increase in VF signal level of +15 dBm. Increase in CF output level must be +3

dBm.

Noise generated within the terminal weighted Telephone noise not be exceed 60

dBm op.Noise generated within the terminals has limited significance, as underoperational conditions , the corona noise is dominant, in the order of –40 dBm opunder operational conditions a more realistic value is –55 dBm op.

CROSS TALK:

Due to signal channels, either individually or collectively the system shall notgive rise to a weighted disturbance power in the speech circuit of more than –60 dBm op.

Signalling input and output , the pulse distortion should exceed 5ms.

VOLTAGE REQUIREMENTS:

Power supply : DC; 500V DC 1 mohm(both terminal connected together and earth )

1000V 1.2 /5 pulse for terminal not isolated from earth.

AC 2000V ms power frequency 1 min both terminals connectedtogether and earth .

CF input and output terminals ; terminals isolated from earth, 2000V ms power frequency

1 min. Both terminals connected together and earth Terminals not isolated from earth

3000N 1.2 /50 pulse



283

CHAPTER–XIII

V.F Signalling and AlarmFree from earth. 500V DC 1 min.

VFT channels its frequency and Tolerences :-

Channel Number

CCITT Recommendation R35 R37 R38 ANominal Modulation Rate 50 100 200 Bd.

Capacity of Homogenous 24 12 6

VFT channels in a standardCarrier system with 4 KHZ spacing ;

Lowest mean frequency 420 480 600 HZ

Higher mean frequency 3180 3120 3000 HZ

Permissible deviation from the

Frequency at sending end ±2 ±3 ±4 HZ

Difference between two characteristic

Frequency in the same channel 60 120 240 HZ

Maximum in PLC system :- ±3 ±4 ±6

Noise in PLC system :-

Mainly caused by the power system operation..

Two main type of Noise : Substained white – moisse – like voltages (Random noise).Irregular discharges across insulators and conductors. (Carona and brush discharge)

Impulse type noise:-Shortsparks and bursts of high amplitude caused by,

1. Operation of Isolators.

2. Operation of breakers.3. Short circuits.

4. Flash over

5. Atmospheric discharges.Interference caused in PLC system due to HVDC system.

Other PLC system :

Sources external to power systems

Maritime Aeronautical system

Broad casting service.System operating in MF and IF bands.

Reuse of me PLC frequencies:



284

CHAPTER–XIII

Reuse at a geographically spaced distance which ensures a level difference of preferably

60db between the useful signal and disturbing signal.

dB, dBm, dBu

10 log p1/p2 dB20 log v1/v2 dB

Abritute Levels1mw = 0 dBm

U = 0.7751 = 0 dBu

40 dBm 10W 600 ohm 77.5V + 40 dBu+40 dBm 10W 150 ohm 38.7V + 34.0

+40 dBm 10W 75 ohm 27.4V + 31.0 dBu

Standard Limits for transmission quality of Data transmission. One of the most important

factor affecting the data transmission quality is the distortion in time of the significantinstances (known as telegraph distortion).

The degree of signal distortion must be kept within certain limits, the ultimate

objective being that the degree of distortion on received signals should be complaiable

with the merging of the receiving equipment.

The distortion limits,

600 Bands – leased circuits - 20 - 30%1200 Bands – leased circuits - 25 - 35%

Degree of tolerable distortion (%)

Modulation rate 50 Bd 100 Bd 200 Bd

Channel spacing 120 HZ 240 HZ 480 HZ

Inherent inochronumdistortion with normal

reception level 5 % 5 % 5%

Incase of slow level variationof +8.7 dB to 17.4 dB with

respect to normal receptionlevel 7% 7% 7%

Inpresence of interfrence

by a single wave freq.

equal to either of twocharacteristics frequencies

with a end of 20 dB below

the signal level of thetest channel. 12 12 10

With introduction of afrequency of the

signals. 5 5 5



285

CHAPTER–XIII

Distortion in a data channel causes Loss / Frequency distortion group delay distortion

Variation with time in over all loss

Random circuit NoisePhase filter

Single tone interference

Frequency errorHarmonic Distortion

Text distortion due to white Noise

VFT FM 240 100 Bd

Channel level –17.5 dBmFor a Noise level of –24 dB.

For 50 Bd distortion is 12.5%100 Bd distortion is 20%

Text distortion due to frequency distortion.

48 HZ – 13.5%

-8 HZ – 13.5%

Distortion in series connected VFT channel for 120 – 50 Bd.

For 4 Nos. of Series connected VFT channel,

For the normal level distortion will be 7%If the level is above normal, the distortion varies minimum for 4 Nos 8%

Where as for a reception level below normal about 17.4 dB.

4 Nos. of Series connectedVFT channels, distortion becomes 12%

Distortion in FSK channel due to frequency change of 1 HZ

For 120 50 Bd 2.08 %

240 100 Bd 1.04 %

480 200 Bd 0.52 %

600 600 Bd 0.31 %Limits for maintenance of Telephone type circuits for Data transmission Telegraph

distortion limits.

Leased Switched300 Bd 20 – 25 20 – 25 %

600 Bd 20 – 30 25 – 30 %1200 Bd 25 – 35 30 – 35 %

Bit error rate (max) Leased Switched

300 Bd 5 – 10-5 10-4

600 Bd 5 - 10-5 10-3

1200 Bd 5 –10-5 10-3



286

CHAPTER–XIII

PERIOD OF MEASUREMENT IS 15 MIN

Block Error Rate:

Example: Period of measurement = 15 min

No. of Bits transmitted = 1080000Length of sequence = 511 Bits.

No. sequences transmitted = 2113.

Maximum Permissible line loss:Total loss planning

Value as per IEC 5 dB.

Dielectric loss in capacitance, loss in coupling devices, loss in CF cable, loss incarrier sets operating in parallel. (0.5 – 1.0 dB (IEC)).

PEP = 1010 =40 dBm

Coupling loss at Max. Permissible Min.Permissible S/N Noise

One line end line loss line loss Ratio level

Min. of 2 HZ132 KV 5 dB 43 dB - 8 dBm 25 - 33dBm

220 KV 5 dB 33 dB - 2 dB 25 - 23dBm

400 KV 5 dB 23 dB + 12 dB 25 - 13dB

Power alocation in a multi purpose PLCC system is determined by the followingproperties of the sub channels.

Noise band width.

Required signal to Noise Ratio.Method of modulation.

Assumption.

Sum of voltages of individual sub channels at carrier frequency is equal to the voltagescorresponding to the PEP. of the transmitter. The speech limits rise is 0 dB. For is used

for all signal channels. operating range for all sub channels should be the same.

S/N ratio for speech 25 dB

for signalling channel 15 dB.

Noise power in a sub channel is proportional to its Noise Band Width.

Allocation of power in various sub channels of PLC terminals for speech plus signals

without teleprotection.

Criteria: Power proportional to Noise band width in AM channels, (Speech and

Pilot) power in FM signalling channel 6 dB lower than in equal Band width AM



287

CHAPTER–XIII

channels.

Sub channel Noise Band Power Voltage Level relative

Width HZ Ratio Ratio to speech

Speech 2100 2.5 10 0 dBn

Pilot dial 80 1 2 - 14 dBnFor 120 (50Bd) 80 ¼ 1 - 20 dBn

For 240 (100Bd) 160 2/4 1.5 - 17 dBn

For 480 (200Bd) 320 4/4 2 - 14 dBn

Calculation of Required Level In Speech Channel.

Level in speech+

Sum of all sub channel, equ. Channel mn

= dBm (max) – 20 Log - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - -Equ. channel No. for speech

dBu (max) = Voltage level corresponding to PEP of transmitted.

PLc terminal level: 10 WATT PEP

40 dBm PEP

34 dBm / 150 n

Example of calculation

Sub channel Eq. channel No.Speech 10

Pilot 2

-------12

-------

Speech level = 34 – 20 log 12/10

= 32 – 4 dB/150 nLevel Pilot = -14 dBr to speech

= 18.4 dB/150 n

NOTE:

Channel No. Channel specimen Type of medulatorAmplifier Frequency

001 – 024 120

151 – 165 170301 – 308 360



288

CHAPTER–XIII

Power Allocation:

Pr = PPEP – 20 log (nsi √Bsi/Br + √BZS/Br + √Brc/Br + √A.Bs/Br)

Pr = Signal level of Reference Channel dBm.PPEP = Peak envelope power – dBm.

B = Noise Band width CHz.

Fs = Tel. Sig. Channel.

Rl = Reduced carrier.A = 10 without compander.

1 with compander.

R = Reference channel.

Example:

PEP = + 40 dBmOperation mod : Speech only

Suppressed carrier 300 – 2400 Hz.

Pr = 40 –20 log (√80/80 + √10 x 2100/80= 15 dBm

with reduced carrier

Pr = 40 – 20 log (√80/80 + √200/80 + √10 x 2100/80)= 14.52 dBm.

Example:

PEP = 40 dBm

Operation mode : Speech + Data300 to 2400 Hz

1-Sub channel 200 Bd2-Sub channel 100 Bd earth

Suppressed Carrier.

Pr = 40 – 20 log (√80/80 + √320/80 + 2 √160/80 + √10 x 2100/80) = 13.14 dBm

Reduced carrier

Pr = 40 –20 log (√80/80 + √320/80 + 2 √160/80 + √200/80 + √10 x 2100/80) = 12.54 dBm

Line Alternation:

Several modes of carrier signal propagation take place simultaneously on a multi

conductor line.

Main Characteristics of Natural Modes:

Each mode has its own specified propagation loss, Velocity and characteristic

impedance.

The modes are independent of each other. The phase voltage at any location is the

sector sum of the phase mode voltages at that location, similarly the phase currentis the vector sum of the mode currents.



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CHAPTER–XIII

NUMBER OF MODES:

3 modes in the case of single circuit line with 2 earth wires grounded at each

tower.7 modes in the case of double circuit line with one insulated earth wire.

Coupling arrangements should be chosen that the above transmitting power of

lower loss mode.For practical coupling arrangements, such as phase to earth, phase to phase or

inter circuit coupling, the transmitting power is generally injected in the form of a mode

mixture, part of it much high loss (ground) ground mode, this resulting in a certain model

conversion loss.

Line Alternation line

+ = L11 + 2 ac + aadd

aadd : Additional loss caused by discontinuities e.g., Coupling

circuit, transposition etc

L1 : alternation constent of lower loss mode

√f

= 0.07 ---------- + 10.7 dB / pam

√dC n

f = Frequency in KHZ

de = Diameter of phase conductor (nm)

n = Number of phase conductor in bundle.Approx. + 10 % Upto 300 KHZ : + 20 % Upto 500 KHZ

Line Voltages above 150 KV

Earth resiotivity around 100 – 300 rm.Additional alternation due fault distance.



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CHAPTER-XIV

HV AC TESTBy R&D

SOME DETAILS ON DIELECTRIC TESTS:-

The dielectrics break down due to several factors like increased voltage

application, temperature, the age of dielectric materials, presence of moisture and othercontaminants.

When an arc is struck through an insulation, say of generator, it punches a pinhole

through the material. The result of the pinhole may not be felt immediately and an arcmay continue without causing damage for some time. Internal damages which take place

in voids in the dielectric erode electrical insulating materials causing serious damage. At

sometime minor faults can cause a short circuit causing considerable damage and may beleading to major shut downs.

The following are some tests used for assessing insulation properties:

a) IR value measurement with meggers, P-I value tests (10 min to 1 min

value)

b) Hipot tests (D.C and A.C):

RECOMMENDED TEXT VALUES ARE:

I. GENERATORS: (1 MIN. TESTS)

a) A.C tests for new winding or coil - 2E + 1

Subsequent test - 80% of first test.

Old machines - 0.6 (2E + 1)

Where 0.6 is the derating factor.

b) D.C. testsA.C to D.C Conversion factor of 1.4 may be used.

i.e 11 KV A.C = 11 x 1.4 KV D.C.

c) Example I: 11 KV Old m/c: A.C Hipot value = (2x11)+1x0.6 = 13.8KV

d) Example: 11 KV old Gen. D.C hipot testValue = (2x11)+1x0.6x1.4

=19 KV

e) Cables. (1 min)



291

CHAPTER-XIV

A.C TESTS.

New Cables – 2.5 x Uo Where Uo is the phase to neutral KV rating of cable.

If the Cable is 11/6.35 KV

test value = 2.5 x 6.35

If the cable is 11/11 KV (normally used in generators)

test value = 2.5 x 11 = 27.5 KV

D.C TEST:-

A.C to D.C conversion factor of 1.4 may be used. An abstract of CIGRE report28.8.1988 given below will be interesting to go through.

The necessity for such a A.C voltage test level is since the m/c phase to Neutralvoltage may reach (1.2 x 11 KV) When a m/c is separated from grid due to some valid

reason the m/c voltage may reach 1.2 times the ratio voltage. If an earth fault occur in one

phase of cable the voltage in other phase of the Gen. may go to 11 KV to neutral in highimpedance earthed generators. The gen should withstand this value.



292

*CHAPTER–XIV



293

*CHAPTER–XIV



294

*CHAPTER–XIV



295

CHAPTER–XIV

C) Tan delta and Capacitance tests on generators

For a good insulation the Capacitance is almost constant at all voltages, but forinsulation containing voids, the capacitance value increases with increase in voltage due

to discharge in void. Tan delta test is a sensitive test for delection of moisture content,voids, crack and deterioration etc. Any steep value in the tan delta indicates someabnormal condition. Absolute values are not useful generally. Comparison with previous

test results help.

There is a correction between increase in loss tangent (tan delta) and capacitancewith voltage and the energy dissipated in discharging voids.

D) The other tests available are partial discharge test and 0.1 HZ test.

SCOPE:

This covers the high voltage AC test conducted on equipments at site to measure

the leakage current.

APPLICATION:

This test is done on the stacks of 110 KV & 230 lightning Arresters, at rated

voltage.

PERIODICITY:

The test is done at the time of commissioning, thereafter yearly.

TEST PROCEDURE:

TEST CIRCUIT:

Varia Voltmeter

HV Testing

Transformer

Specimen

Under test



296

CHAPTER–XIV

Test equipments

HV Testing Transformer 220V/60 KV, 600 VA

Ammeter 0-10 mA with resolution of 0.1 mAVoltmeter 0-250 V AC

Variac 230 V/0-260, 5A.

The Lightning Arrester to be tested is completely isolated both from supply end

and from ground.

The connections are given as shown in the circuit diagram. The voltage is appliedgradually on the LA under test using the variac, keeping an eye on the ammeter &

voltmeter readings. The leakage current readings are noted at say 30%, 60% & 100% of

the MCOV rating of the Arrestor. Care should be taken not to exceed the MCOV. The

Voltage should be reduced as soon as MCOV is reached. Normally the test is done oneach stack separately.

Precautions:

The IR value of the LA is to be tested before conducting the HV AC test.

While testing individual stacks of a LA, it should be ensured that the stack is not

kept on the ground while testing.

The test voltage should not exceed the MCOV values for any stack. The HV leads

from the HV testing Transformer should not be very close to conducting surfaces and

adequate clearance shall be maintained.

Significance of the Test:

A surge arrestor normally acts as an insulator to normal system conditions, hencethis insulation property is, as in any insulation system, subject to certain deterioration.

Hence a power frequency leakage current test at the rated voltage of the Arrestoris a practical field test to determine the condition of arrestors in service.

Results and Analysis:

The leakage current values have to be interpreted on a comparative basis,

emphasis is on variation from earlier recorded values than on absolute values. However a

limit value of 3 mA is taken as a criteria. Also, the leakage current value at rated voltageshould not exceed the minimum level recommended by the supplier. The readings are to

be used more as trend analysis for detecting deterioration/degradation in the Arrester

components.



297

CHAPTER–XIV

Reference:

TNEB Code of Technical Institution/1990.

HV DC Test:Scope:

This covers the high voltage DC test conducted on equipment at site to check the

voltage withstand capability and the leakage current.

Application:

The test is done on equipments, in which HV AC test cannot be effectively done

due to high capacitance and consequent power requirement of the testing apparatus.

Typical applications include test on Generator Stator Coils, H.T. motors, Cables,

Busbars etc.

Periodicity:

Normally the test is done after overhaul, recommissioning as per field

requirements.

Test Procedure:

Test Circuit:

HV Testing

Transformer

HV

ToSpecimen

Variac

A

L

R

C

Ammeter

Diode



298

CHAPTER–XIV Test equipments:

HV Testing Transformer - 220 V/60 KV, 600 VA

Diodes - HV Rectifier Diodes

Ammeter - 1 mA – 10 mA Range

Variac - 1 phase, 5A

If the test specimen is a HT motor, the 3 phases of the stator winding terminals

may be shorted together and the High Voltage lead should be connected to it. If test can

be done on separate phases, the same may also be done. The HVDC is to be applied

gradually, preventing any overshoot of the ammeter. The leakage current may be

measured at the rated voltage after about one minute.

In the case of cables, while conducting the test on one phase, the other two phases

in a 3 core cable should be earthed.

Precaution:

The HV DC test must be done only after conducting the IR value test (with a 5

KV megger) and only if the IR value is found satisfactory.

As the capacitance of the specimen, would be normally high especially in the case

of cables, proper care should be taken to sufficiently discharge the specimen after the test.


The normal leakage current values would be in the range of 0.05mA - 0.5mA.

Dissolved Gas Analysis test:

Scope:

This covers DGA test of Transformer oil samples using Gas chromatography

technique to detect and quantify dissolved gases in the oil.

Application:

The test is applied in case of HV Transformers mainly to detect incipient faults that maydevelop inside the Transformers and generally to diagnose the condition of the

Transformers in service and to suggest future action.



299

CHAPTER–XIV

Description:

The Transformer in service is subject to electrical & thermal stresses resulting in

liberation of gases from the hydrocarbon mineral oil used in the Transformers. Cellulose(paper insulation) also is involved in the formation of gases, which are dissolved in the

oil. Gases may be formed, due to natural aging and also as a result of faults. Basically,

the mechanism of gas formation in oil includes oxidation, vapourisation, insulationdecomposition, oil breakdown etc. An assessment of these gases, that are dissolved in the

oil, would help in diagnosing the internal condition of the Transformer. Operation with a

fault may seriously damage the equipment and it is useful to detect the fault at a very

early stage of development.

In the case of fault, its type & severity may be inferred from the composition ofthe gases and the rate of gas formation. In the case of incipient faults, the gases formed

are partly dissolved in the oil, hence periodic analysis of oil samples for the amount and

composition of dissolved gases forms a means of detecting faults.

DGA involves the following steps:

(a) Sampling of oil

(b) Extraction of gases from the oil

(c) Analysis of the extracted gases using gas chromatograph.

(d) Calculation of concentration of gases in PPM.

(e) Interpretation of results.

Periodicity:

The DGA is done on all power/auto transformer of 110KV class & above on

yearly basis and on special occasions warranted by service conditions. In the case of new

Transformers the test is recommended one month after commissioning and thereafter

yearly. A DGA test one month before expiry of the guarantee period of the Transformer

is also recommended.

TEST PROCEDURE:

Equipment used:

(a) The Gas extraction plant consisting of magnetic stirrer, vacuum pump, mercury

reservior, degassing system.

(b) Gas Chromotograph.

(c) Output unit namely Integrator and PC

The Gas – Chromatographic system consists of a carrier gas stream supplied by a

gas cylinder, a sample inlet /injection port, a chromatographic column, detectors, and an

output recorder.



300

CHAPTER–XIV

The carrier Gas Nitrogen obtained from cylinder is passed through flow regulatorto the column. The carrier gas passes through the sample inlet system where it picks up

the sample to be analysed. The carrier gas sweeps the sample being injected into its

stream and enters into the column where the separation takes place.

Absorption columns are used for the separation of gaseous mixtures. Molecular

sieves Poropak Q type absorbents are used to separate CO, CO2, H2 gases. Silica gel typeabsorbents are used to separate hydrocarbon gases.

Detectors (Flame Ionisation and Thermal conductivity detectors) are used in

detecting the Gases and works on the principle of thermal conductivity (TCD) of thegases and the electrical conductivity of gases which have been partially ionised The FID

is used for hydrocarbons and the TCD for atmospheric gases like CO, CO2, & Hydrogen.

The Gas extraction plant is first evacuated with the help of the rotary vacuum

pump. When sufficient vacuum is achieved, oil is let into the degassing vessel and stirredtill complete degassing is achieved. Using the mercury column, the evolved gases are

compressed to the known volume.

The Gases are drawn by means of airtight syringes and injected into the Gas

Chromatograph, after the Gas Chromatograph is properly set up with carrier Gas etc. Thedetection and quantification of gases take place in the Chromatograph. The

Chromatograph is calibrated by means of a standard gas mixture containing a suitable

known amount of each of the gas components to be analysed to establish the calibration

curve and retention time. An Integrator connected to the output of the Chromatographgives the proportional area in units for different gases. The method of calibration involves

measuring the area of each peak and retention time, identifying the gases correspondingto each peak by comparison with the chromatogram obtained by calibration & obtainingthe gas values in PPM. The PPM values of the gases are calculated by comparing with

standard gas values and the quantity of dissolved gases in PPM is than calculated for each

gas.

Precaution:

The samples must be collected, labeled, stored, Transported and tested with

proper sampling, storing and testing procedures to obtain accurate results.

Analysis & Interpretation:

There are several methods for interpreting the results of the DGA test. Firstly acheck is made by comparing the concentration levels with levels that are permissible in a

healthy Transformer depending upon the service age of the Transformer. These

permissible concentration levels for gases are tabulated, for reference.

Then, in case of higher gas levels, than the permissible levels, or in cases wheregas levels show abnormal increasing trend from previous recorded values, the Roger’s

method of diagnosis or the 3 ratio method prescribed in IS 10593 may be used for

interpretation.

Reference:

IS 1866, IS 9434, IS 10593, CPRI Publications.



301

CHAPTER–XIV

LIMITING VALUES

IS 1866 – 1983------------------------------------------------------------------------------------------------------------

TEST EQU. VOLTAGE METHOD LIMIT------------------------------------------------------------------------------------------------------------

ELECTRIC STRENGTH, Min ≥ 145 KV IS : 6792 50

KV < 145 ≥ 72.5 KV 40

< 72.5 KV 30

WATER CONTENT ≥ 145 KV IS : 335 25PPM, Max < 145 KV 35

SPECIFIC RESISTANCE ALL V IS : 6103 0.1@ 90, 10 E 12 Ohm-Cm Min

TAN DELTA ≥ 145 KV IS : 6262 0.2@ 90, Max < 145 KV 1.0

ACIDITY ALL V IS : 1448 0.5Mg KOH/g, Max

I F T, N/m, Min ALL V IS : 6104 0.015

FLASH POINT Min ALL V IS : 1448 125 orDeg C, Min Max. decrease of 15

SEDIMENT AND/OR ALL V IS 1866 NIL

PRECIPITABLE SLUDGE

PERMISSIBLE GAS CONCENTRATIONS

GAS <4 YEARS 4-10 YEARS >10 YEARS

1 HYDROGEN 100/150 200/300 200/300

2 METHANE 50/70 100/150 200/300

3 ACETYLENE 20/30 30/50 200/150

4 ETHYLENE 100/150 150/200 200/400

5 ETHANE 30/50 100/150 800/1000

6 CARBON MONOXIDE 200/300 400/500 600/700

7 CARBON DI OXIDE 3000/3500 4000/5000 9000/12000



302

CHAPTER–XIV

Furan Analysis Test:

Introduction:

Paper is the major solid insulant in Transformers. While there are a number of

tests to monitor the condition of the oil in the Transformer, till recently there was nopractical technique available for condition assessment of the solid insulation in theTransformers.

A new testing method has emerged in which condition of solid insulation is

assessed by analysing the degradation of products of cellulose paper called furanic

compounds using High Performance Liquid Chromatography (HPLC) or any othersuitable equipment.

Application:

The test is specially applicable to Transformers that have put in more than 10

years of service life and also in cases where the involvement of cellulose is suspected infaults that have been detected irrespective of service age of Transformers.

Furan compounds:

Furanic compounds commonly referred to as furans, are products of degradation of

cellulosic materials and are dissolved in the oil. The furanic compounds that are detected.

quantified and analysed are

2 – Furfural dehyde5 – Hydroxy methyl – 2 furfural

2 – Acetyl furan

5 – Methyl – 2 – furfural

2 – Furfural alcoholof these 2 – furfural dehyde is found to be the most commonly monitored furan

compound.

Periodicity:

The periodicity for this has not been established but it is suggested that areference test value for all Transformer in the 10 th year of service and yearly testing from

the 15th

year onwards may be adopted presently.

Test Procedure:

Equipments:

Equipments such as High Performance Liquid Chromatograph, visible rangespectrometer are used in Furan Analysis. However the HPLC is the standard equipment

used.

Method:

(a) Furanic compounds in the oil samples are extracted from a known volume

of test specimen.

(b) A portion of the extract is introduced into an HPLC system equipped witha suitable analytical column & UV detector.

(c) Furanic Compounds in the test specimen are identified and quantified by

comparison to standards of known concentration.



303

CHAPTER–XIV

Result and Analysis:

The furan compounds are analysed on a trend basis. The concentration levels are

compared with previous values and the assessment of solid insulation as healthy, initial

stage of degradation, failure levels etc are made and appropriate action taken.

Reference

IEC 1198/1993

ASTM D 5837-95

CPRI Publications.

Transformer oil tests:

(a) Electric Strength (BDV)

Scope:

This covers test on oil samples of Transformers, which are inservice and use

uninhibited insulation oils and complying with the requirements of IS 335 when fillednew.

Definition:

The voltage at which the oil breaks down when subjected to an ac electric field

with a continuously increasing voltage contained in a specified apparatus. The voltage is

expressed in KV.

Application:

The test is applicable to Transformers of any rating and switch gears.

Periodicity:

The test is done on an annual basis along with all other oil characteristic tests and

more frequently if condition of the oil/equipment warrants. However the BDV of oilsamples from Transformer of all voltage class & from OLTC shall be tested on a

quarterly periodicity, separately with locally available test kits.

Test procedure:

The test is done with a test cell, made by glass or plastic, which shall be

transparent and non-absorbent, with an effective volume of 300 to 500 ml and preferablya closed one. The electrodes are mounted on a horizontal axis and shall be 2.5 mm apart.

The test procedure is begun by adjusting the sphere gap of the electrodesaccurately by the use of 2.5 mm gauge (supplied with the kit).



304

CHAPTER–XIV

Initially some of the oil from the sampling container is poured out to clean the tip

of the sample container. The test cell shall be cleaned by rinsing with the test oil twice

before filling the test oil for the test. The oil, then, should be poured gradually, avoidingformation of air bubbles.

The oil is filled to a height of 40 mm from the axis of electrodes. The test cell

with oil is then placed in the testing unit. A period of 5 minutes is allowed for the oil to

settle. Then voltage is applied at the rate of rise of 2 KV/second. The voltage is thusincreased to a value where the oil breaks down and the corresponding voltage is noted.

The test is carried out six times on the same oil sample filling with intervals of 2 minutes.

The Arithmetic mean value of the six readings is taken as the BDV of the oil sample.

Precaution:

The sample must not be exposed to atmosphere and should be as near to the actual

oil in the Transformer as possible, in all aspects..

The sample container may be shaken upside down to get a homogenous samplefor test.

The container electrodes etc may be rinsed thoroughly with test sample, prior tothe commencement of the test.


The test values are interpreted as per IS 335 for new oil and as per IS 1866 for oil

in service.

For oil in service the limit values are as follows:

Equipment voltage Limit (Minimum)

145 KV and above 50 KV

Between 72.5 KV and 145 KV 40 KV

Less than 72.5 KVA 30 KV

Reference:

IS 335, IS 1866, IS 6792

(b) Flash Point:

Scope:

This covers test on oil samples of Transformers, which are inservice and useuninhibited insulating oils and complying with the requirements of IS 335 when filled

new.



305

CHAPTER–XIV Definition:

It is the temperature at which the oil gives off so much vapour that this vapour,

when mixed with air, forms an ignitable mixture and gives a momentary flash on

application of a small pilot flame under the prescribed conditions.

Application:

The test is applicable to Transformer of all ratings.

Periodicity:


more frequently if condition of the oil/equipment warrants.

Test procedure:

The test equipments used are pensky-martin closed cup apparatus, thermometersand variac.

The cup is cleaned well by rinsing twice with the test oil. Oil is filled upto the

marking provided and is placed in the test apparatus. The oil is heated and from about100’C onwards, a small pilot flame is used to ignite the mixture and the temperature at

which this mixture gets ignited is noted and recorded as the Flash-Point.


Minimum limit is 125’C or maximum decrease of 15’C for all voltage class.

Reference:

IS 335. IS 1866, IS 1448

(c) Neutralisation Value (Acidity)

Scope:

This covers test on oil samples of Transformers, which are inservice and use

uninhibited insulating oils and complying with the requirements of IS 335 when fillednew.

Definition:

It is the measure of free organic and inorganic acids present in the oil. It is

expressed in terms of the number of milligrams of potassium hydroxide required toneutralize the total free acids in one gram of the oil

Application:

The test is applicable to Transformers of all rating.



306

CHAPTER–XIV

Periodicity:



Test Procedure:

The materials used for the test are indicator bottle containing universal indicator

with PH value of 4 & 11, clean, dry glass test tubes and a color chart calibrated with

neutralisation number values.

The test procedure is, 1.1 ml. of sampling oil to be tested is accurately pipetted

into a clean dry test tube. To this 1 ml of Isoprophyl, alcohol. 1.0 ml of 0.0085 N Sodium

Carbonate solution are added. Then, to this five drops of the universal indicator are addedand gently shaked.

0.0085 N of Sodium Carbonate solution is prepared by dissolving 0.085 N ofSodium Carbonate in 10ml of distilled water to get 0.0085 N of sodium carbonate

solution.

The resulting mixture develops a color depending on the PH value of the mixture.This color is compared with the standard chart, which gives the approximate

neutralisation value ranging from 0 to 1.0.


Maximum limit for all voltage clause is 0.5.

Reference:

IS 335, IS 1866,

(d) Specific Resistance (Resistivity)

Scope:

This covers test on oil samples of Transformers, which are in service and use


Definition:

It is the ratio of the dc potential gradient in volts per centimeter paralleling the

current flow within the specimens to the current density in amperes per squarecentimeters at a given instant of time and under prescribed conditions. This is

numerically equal to the resistance between opposite faces of a centimeter cube of the

liquid. It is expressed in Ohm-centimeter.



307

CHAPTER–XIV

Application:

The test is applicable to Transformers of all ratings.

Periodicity:

The test is done on an annual basis along with all other oil characteristic tests andmore frequently if condition of the oil/equipment warrants.

Test procedure

The equipments needed for the test are million megohm meter, oil cell, oil cell

heater.

The oil is heated upto 90’C and 500 V d.c. applied, and after one minute the

megohm indicated is noted and the Resistivity value is calculated with appropriate

multiplication factors and cell constant.

Results & Analysis

Minimum limit is 0.1x10^12 Ohm-cm at 90’C for all voltages.

Reference:

IS 335, IS 1866, IS 6103.

(e) Dielectric Dissipation Factor (Tan delta)

Scope:

This covers test on oil samples of Transformers, which are in service and useuninhibited insulating oils and complying with the requirements of IS 335 when filled

new.

Definition:

It is the Tangent of the angle (delta) by which the phase difference between

applied voltage and resulting current deviates from 1/2 radian when the dielectric of thecapacitor consists exclusively of the insulating oil.

Application:


Periodicity:





308

CHAPTER–XIV

Test procedure:

The equipments required are Dielectric constant test kit, oil cell, oil cell heater.

The oil cell is thoroughly rinsed with the sample oil to be tested and about 35 ml

of oil is taken in the cell and heated to 90'C. Then 500V AC is applied to the terminals of

the oil cell. The Tan delta bridge is balanced by adjusting the potentiometers to get nulldeflection. The Tan delta value obtained is recorded.

Results & Analysis:

The maximum limit for Tan delta at 90'C is 0.2 for voltages of 145 Kv & above

and 1.0 for voltages below 145 KV.

Reference:

IS 335, IS 1866, IS 6262.

(f) Interfacial Tension:

Scope:



Definition:

It is the force necessary to detach a planar ring of platinum wire from the surface

of the liquid of higher surface tension that is upward from the water-oil surface. It isexpressed in dynes/cm. or N/m.

Application:


Periodicity:



Test procedure:

The apparatus required are tensiometer, fine platinum ring, glass beakers.

Before starting the test, all glass beakers are cleaned with isoprophyl alcohol and

acetone. The platinum ring is also cleaned with isoprophyl alcohol & acetone. Thetensiometer is placed in a horizontal plane.



309

CHAPTER–XIV

About 20-25 ml. of distilled water is taken in the sample container and is placedon the adjustable platform of the tensiometer. The platinum ring is suspended from the

tensiometer. The adjusting platform is raised till the platinum ring is immersed in the

water to a depth not exceeding 6 mm and at the centre of the glass beaker.

Now gradually, the platform is lowered, increasing the torque of the ring system

by maintaining the tension arm in the zero position. As the film of water adhering to the

ring approaches the breaking point, slow adjustment is made to ensure that the movingsystem is in the zero position when rupture occurs. The surface tension of the water is

noted. The value is normally 71 to 72 dynes/cm.

Now the tensiometer scale is brought to zero and the adjustable platform is raised

until the ring is immersed to a depth of about 5 mm in the distilled water. The sample oilto be tested is poured slowly along the walls of the beaker over the distilled water. The

platform is slowly lowered, increasing the tension of the ring system. The IFT is the scale

reading at which the ring breaks free from the interface.

Results & Analysis:

The minimum limit for all voltage is 15 dynes/cm.

Reference:

IS 335, IS 1866, IS 6104

(q) Water Content:

Scope:



Description:

This test is for the determination of water content usually in the range of 0-75

ppm in the oil.

The Karl-fisher method is used. The method is based on the reaction of water withIodine and Sulphur-di-oxide in Pyridine/methonol solution.

Application:


Periodicity:





310

CHAPTER–XIV

Test Procedure:

The materials required for the test are methanol with less than 0.02% water

content, Karl-fisher Reagent, Titration vessel.

The titration vessel is made moisture free.

The Karl fisher Reagent and the methanol are taken in the two sides of the burette

to levels. A certain quantity of methanol is allowed in the test vessel. The pointer willshow end point as water. The electro magnetic stirrer should rotate at a speed of 150-300

rpm. Karl Fisher Reagent is allowed into the vessel to neutralise the water. When all the

water is separted, the pointer will show Karl Fisher Reagent-O'. A known quantity of

water say 20µl is introduced with a syringe. The pointer will once again show waterindication. Steadily and gradually the Karl Fischer Reagent is added continuously so as to

bring the pointer to Karl Fisher 'O' position. The initial and final readings are noted. The

difference is the volume of Karl fisher required to neutralise 20µl of water. The sameprocedure is repeated with sample oil and the water content present in the oil is calculatedusing the formula (20 X K.F. required to neutralise the Oil X 103 ) / ( 25 X 0.88 X K.F.

required to neutralise water).

Results & Analysis:

The minimum limit for Transformers of voltage class 145 KV & above is 25 PPMand for voltages below 145 KV is 35 ppm.

Reference:

IS 335, IS 1866, IS 2362

(h) Sludge Test:

Scope:


uninhibited insulating oils and complying with the requirements of IS 335 when filled

new.

Description:

This test is conducted to determine the presence of sediments and perceptiblesludge in the oil.

Application:


Periodicity:

The sludge test is carried out when the IFT value of oil is very low say below 13Dynes / cm.



311

CHAPTER–XIV Test procedure:

11 ml of the test sample oil is pipetted in a clean conical flask. 100 ml. of Hexane

or N-heptane is added to this oil. The mixture is shaken well and is kept in a dark place

for 24 hours. At the end of 24 hour, it is checked for any precipitation in the oil. If anyprecipitation is observed, the sample oil contains sludge.

Results & Analysis:

For all voltage class sludge should be NIL.

Reference:

IS 335, IS 1866.

Note:

All Indian standards referred versions are the latest versions revised/amended

from time to time.

Test procedure for measurement of Tan delta and Capacitance of equipments.

1. Scope: This covers the method of measuring the dielectric loss properties of the

insulation system of equipments by measuring the Tan delta and Capacitance values.

2. Definition: Tan delta is the tangent of the dielectric loss angle of an insulation

system. It is also referred to as dissipation factor or dielectric loss factor.

3. Significance of Tan delta value in insulation systems:

In an insulation system, the dielectric loss is given by V2 WC tan delta watts. If

the dielectric power loss is more, the dielectric strength of the insulation would bereduced. The Tan delta is affected by moisture, voids and ionization in the Insulation.

Hence it is indicative of the quality of insulation.

4. Principle of Tan delta and Capacitance measurement for HV equipments.

4.1 The High Voltage electrical equipments have conductors HV and LV separated by

an insulating medium. It can also be a conductor or winding with an HT terminal and theLV terminal connected to ground. These systems can be represented as two and three

terminal capacitors. An example of a two terminal capacitor is the bushing of an

equipment. The central conductor is one terminal and the mounting flange (ground) is the

other terminal. An example for a three terminal capacitor is a bushing with a Tan deltatest tap. In this case the central conductor is one terminal, the test tap is the second

terminal and the mounting flange is the third terminal. Likewise most of the HV

equipments can be visualised as capacitors with simple and complex insulation systemsand these can be measured with a test set that can measure both grounded and

ungrounded specimens.



312

CHAPTER–XIV

In the ideal case, the capacitance current leads the voltage by 90'. But in practice,

in all insulation systems, there exists a loss current Ir which is small in magnitude but in

phase with the voltage, as shown above. The total current I, therefore leads the voltage byan angle which is less than 90°. The angle by which it is less than 90° is known as theloss angle delta and in all practical cases, the magnitude of Ic and I are same as Ir is very

small and the power factor and dissipation factor tend to be the same.

In the above diagram Dissipation factor = tan delta; As the important

characteristic of a capacitor is its dissipation factor, it is measured and monitored as adiagnostic test of insulation systems.

5. Application:

The test is conducted on the following:

(1) Power and Auto Transformer Bushings

(2) Power and Auto Transformer Windings

(3) Generator stator coils

(4) Current and Potential Transformers.(5) CVTs

(6) Any other HV equipment where insulating condition is to be tested.

6. Periodicity:

The test is done at the time of commissioning and thereafter yearly and on actualrequirement depending on the conditions of the equipment.

4.2 The Vector Relationship:

Q

S

IC

VIr

I



313

CHAPTER–XIV

Test procedure:

There are two basic versions of testing (i.e.) Grounded specimen test andungrounded specimen test. The circuit diagram are shown below:



314

CHAPTER–XIV



315

CHAPTER–XIV

The circuit connections are given as shown above depending on whether the

specimen is grounded or floating. The Input voltage is raised gradually through a variactill the desired HV Voltage is reached for the specimen. The bridge circuit consists of a

differential transformer, R-C network, known standard capacitor (Cn) and the unknown

specimen (Cx) under test. The same HV voltage is applied to both the known andspecimen capacitors. The currents through the two capacitors pass through the differentialTransformer, which is balanced by means of adjustment of the bridge capacitors, which

are provided with multiplication selectors. Once the bridge is balanced for the

capacitance value the capacitance selected is read directly from the multipliers. The tandelta is then adjusted to get the balanced horizontal position in the Oscilloscope. The

value of Tan delta is also directly read from the bridge Tan delta selector with appropriate

decimals.

Precautions:

(1) It is always preferable to conduct the Tan delta test after the IR value test has beendone and found satisfactory.

(2) The test voltage should not exceed the rated voltage of the equipment, under test.

(3) Adequate safety precautions are to be taken when the test is on, Inadvertent entry

to testing area must be prevented by proper measures.

(4) Bushings etc. should be well cleaned and the test must be carried out in dry

weather condition.

(5) Make sure the input voltage variac is in the 'O' position before the start of the test.

(6) Interference from neighbouring live lines should be minimum. Modern kits withinterference suppression circuits are preferred while testing in yards etc.

(7) For Generator windings and higher capacitance specimen's the variac and thetesting Transformer should be of higher rating to carry the increased charging current.

Test value Interpretation:

In the case of Bushings the ISS prescribes a maximum value of 0.007 for oil

impregnanted condensor bushings and 0.020 for noncondenser bushings. These arevalues for new bushings and for bushings, windings and other equipments that areinservice trend monitoring is the best suggested course for proper analysis of the test

results.

Reference:

1. MWBTan delta and Capacitance kit operating manual.

2. IS2099-1973.



316

CHAPTER-XV

MAINTENANCE OF PROTECTION

RELAYSEr.K. Mounagurusamy

CE / P&C

9.1 SITE VISITS:

During the site visits, the following inspection works may be done in the protectionand control rooms and arrangements may be made to coordinate with other

departments for necessary works:

1. The room should be tidy and clean2. Sufficient lighting should be there

3. There should not be any leakage of water

4. Sun rays should not fall directly on panels.

5. The panels should be vermin proof.6. The inside of the panels should be free from cobwebs, dust, hanging loose wires etc.

7. The room temperature should be with in limits.

8. The outside of the panels should be clean.9. All the relay covers should be tightened and clean.

10. Fault recorders should be in working condition.

11. All the relay catalogues and drawings should be well maintained and be available ineasily traceable location. A list of these items may be readily available.

12. General condition of the batteries should be checked and reported to the concerned if

any improvement is required.

9.2. MAINTENANCE TESTING OF RELAYS:

All the protective relays have to be tested ONCE in a year and calibrated.

The procedures for testing should be well studied and understood. Latest digitalrelays have self test facilities and these relays need testing once in 5 years only as per

the manufacturers. Otherwise periodic testing is extremely important, as almost allthe protective equipments are passive for most of the time. They are called upon to

act only when abnormal conditions occur.

9.3. GENERAL PRECAUTIONS ON TESTING AND HANDLING OF RELAYS:

- Examine relay coils like current coil, voltage coil, flag coil, D.C. auxiliary coil,

timer coil etc. for continuity.

- Check for burns on contacts, sticking up of moving parts, meeting surface and fixed

contacts.



317

CHAPTER–XV

Experience shows that moving parts normally stick to the mechanical back stop.In armature attracted relays, there is remanence magnetic sticking up also. The starters in

L3WYS distance relays have this problem. Contacts sticking up with backstop have been

experienced frequently in EE relays. These should be cleaned each time without fail withtrichloroethylene (good to clean oil and grease), CTC (good to remove carbon), or white

petrol (good to clean disc jewel bearings).

9.4 HANDLING OF ELECTRONIC EQUIPMENT:

a) A person’s normal movements can easily generate electrostatic potentials ofseveral thousand volts. Discharge of these voltages into semiconductor devicesparticularly chips when handling electronic circuits can cause serious damage, which

often may not be immediately apparent but the reliability of the circuit will have been

reduced.

b) Do not remove the modules unnecessarily. However, if it becomes necessary to

withdraw a module, the following precautions should be taken to preserve the highreliability and long life for which the equipment has been designed and manufactured

- ensure that you are at the same electrostatic potential as the equipment by

touching the case.

- Handle the module by its front plate, frame or edges of the PCB. Avoidtouching the electronic components, PCB track or connectors.

- Do not pass the module to any person without first ensuring that you areboth at the same electrostatic potential. Shaking hands achieves

equipotential.

- Place the module on an antistatic surface or on a conducting surface whichis at the same potential as yourself.

- Store or transport the module in a conductive bag.

- If you are making measurements on the internal electronic circuitry of anequipment in service, it is preferable that you are earthed to the case with a

conductive wrist strap.

- Wrist straps should have a resistance to ground between 500 K – 10 m

Ohms.

- If a wrist strap is not available, you should maintain regular contact withthe case to prevent the build up of static.

- Instruments used should be earthed to the case whenever possible.

- Re-soldering may affect the capacitance of the circuitry.

9.5 Take precautions to avail line clear on the equipment to be tested. Place greenflags in the panel under test.

9.6 Ensure that P.T voltages are not available to the relay under test. P.Ts in

generators should be kept isolated : otherwise back feeding of high-voltage to theGen. is possible.



318

CHAPTER–XV\

9.7. Mark down the existing physical position of potentiometer, time dial pointers etc.

with a pencil. This will help restore in case of inadvertent changes during

handling.

9.8. Actuation of certain relays like Generator differential or split phase relay mayrelease CO2 in generators. Hence, proper isolations of CO2 circuits should be

ensured.

9.9. Actuation of certain relays could operate LBB schemes. Precautions should be

taken, while testing LBB and BB relays, extra care should be taken to isolate theTRIPPING Circuits. In some cases, BB relays and other relays may be in same

core of C.T. Unless care is taken, the ENTIRE SUB STATION may go BLACK

OUT.

9.10. There may be necessity to change some settings during testing. Original settingsshould be restored by making entries in site register.

9.11. Some wiring may need removal for testing. They should be entered in register andbefore closing the job the wiring should be restored promptly. Any removal of

TB. links should be treated similarly. If ferrules are not available in the removed

leads, temporary ferruling should necessarily be done before removing.

9.12. The fuses removed should be entered in the site register to enable putting back

without fail.

9.13. Cartridge type fuses should not be checked with higher range in multimeters or

for continuity buzzer. It should give zero ohms in an accurate low range

multimeter since failed fuses also give continuity in high ranges.

9.14. Current can be injected to the relay without removing them C.T leads. Removal isnot a must but this should be judiciously done. Refer to 9.9 above.

9.15. Earth fault selection relays in some distance relays need shorting during testing to

avoid overloading.

9.16. Temporary wedges placed should be removed back.

9.17. The relay coils and the auxiliary switching relays are not continuous rated. Hence

they should not be engaged continuously.

9.18. Some operations like test closing of breakers could lead to L.T. supply

changeovers unwantedly and even they may back charge the machine.Precautions have to be taken.

9.19. Once L.C. is availed, any operation is the responsibility of the engineer who hasavailed the L.C. but it shall be done with information to operator concerned.

9.20. The maintenance engineers should also witness the relay tests to the extentpossible since they are the owners of the relays.

9.21. While test tripping the breakers through the relays, the manually picked up relays

should not be released until the breaker has tripped since the relay contacts are not

designed to break the trip coil current. When the breaker trips, the trip coil current

will be broken by the breaker auxiliary contact.



319

CHAPTER–XV

9.22. After normalising, the availability of D.C. voltage, P.T. voltage at the relay inputs

should be confirmed. The load current passing through the C.T. should be

confirmed by measuring the voltage burden between the current coil terminals,

noting down the load current also in the register.9.23. Do not try to do any modification to the wiring or change in settings without

analysing fully and without having consultation with superiors unless other wise

situation warrants, in which case ratification should be later obtained.

9.24. Do not assume that the scheme drawings are always correct. Some modifications

could have been done and not marked. Always have a suspecting eye.

9.25. Any modification done should be communicated to all concerned who shouldincorporate them in the drawings in their offices without fail.

9.26. History of settings and trouble shooting should be entered in permanent registers.

9.27. Faulty operations or LED indications should be checked.

9.28. Wherever master relays are available, all the connected relays should be testoperated to ensure the picking up of master relay. Test tripping of breaker can be

checked through master relay.

9.29. All alarm/ annunciator points should be checked without fail.

9.30. P.T. voltage availability, D.C. aux. supply availability across all the relaysterminals should be confirmed.

9.31. Voltage burden at the relay current terminals after normalising the equipment

should be measured and recorded in the test report also noting down the loadcurrent at the time of burden measurement.

9.32. It is preferable to note down in the glass cover of the relays the date of last testdone.

9.33. LOAD ON C.Ts

The peak load on the lines, feeders and substation transformers may be reviewedfor any possible overloading of C.Ts beyond the limits once in 3 months and

entered in a separate permanent register called “Peak Load on C.Ts”. The C.Tscan be overloaded by 20% continuously.

9.34. RECORDS:

An official test circuit diary for each type of relay shall be maintained in hand,

containing the test procedure, precautions to be taken, isolation to be done, model

test result, settings adopted etc. Relay catalogues should invariably be on hand.

All the testing works and results should be first recorded at site in a permanent register/note books with printed page numbers to avoid tampering of details later. The test results

shall be authenticated by the engineer present. Names of the testers should be entered.

The test results may then be entered in the specified form and sent to higher officers.

Standardised specimen test report form is enclosed in Annexure.2 B/A means beforeadjustment and A/A means after adjustment. Changes may be done in the form if

necessary to suit local conditions. Any abnormality noticed during the testing may berecorded under the column “Remarks” in the test report.



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CHAPTER–XV

The test schedule with tests done date and tests due date shall be displayed conspicuously

in the office room in a Fixograph or in a board so as to review them frequently.

The details of the tests done may also be recorded in a permanent register with pagevar

allocation for each relay. A few pages together may be allocated for each relay or set ofrelays in the case of 3O/L. One register may be put up for each substation or for moresubstations combined.

A specimen of one page of the register for a relay is given below:

------------------------------------------------------------------------------------------------------------

Feeder/Line/Transformer : S/S :Relay details : (Make, Type, Model, Sl.No., rating,

D.C.aux. voltage etc.)

Settings Range available :Settings adopted :

C.T.Ratio available :C.T.Ratio adopted :V.T.Ratio adopted :

Date of Commissioning :

------------------------------------------------------------------------------------------------------------Sl. Date of Date of Remarks Signature Signature

No. Last Next of of

test test Protection Reviewingdone. Due. Engineer. Officer.

------------------------------------------------------------------------------------------------------------

1) Deails ofsettings changed

with reference

letters no.

2) Details of any

defects.

3) Details of

modification

4) Details of

“Obsoletion”

Communicatedby the

suppliers.

-------------------------------------------------------------------------------------------------



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CHAPTER–XV

T E S T R E P O R T

1. Name of S/S. :

2. Name of Feeder/Line :

3. (a) Relay : (Ex. Distance Relay Main 1/Main 2)(b) Make : (ABB)

(c) Type : (Ex. RAZOA)

(d) Sl.No. :

4. Nature of Test : Special/Routine (State reason if it

is special)

5. Date of last testing :

6. Date of this testing :

7. Page No.of Test record (site) Book : (Including Volume No.)

8. Testing instruments used : (Ex: TURH KIT, WICO megger,

5A ammeter, 150 V Volmeter)

9. Test Results:

(a) Relay (Ex: For O/C relay)----------------------------------------------------------------------------------------------------------

Test Time Time Obtained Remarks

Current Exp.

R O Y O B O

B A A A B A A A B A A A

----------------------------------------------------------------------------------------------------------

P.U. 2 Amp. 2.1 2.1 2.1 -- 2.0 --

4 A 1 Sec. 1.4 1.0 1.1 -- 1.0 -- Time dia

adjusted8 A 0.7 Sec. 0.9 0.71 0.75 -- 0.65 -- in R

phase20 A 0.3 Sec. 0.5 0.31 0.31 -- 0.28 -- relay.

----------------------------------------------------------------------------------------------------------

(b) Checking of Flag or LED (indications of relays) and the annunciator points.

(c) Meggering

C.T.Sec. to Earth

(d) C.T.Burden (VOLTAGE MEASURING IN THE C.T.SECONDARY CIRCUTT AT

REIAY TERMINALS)

R-N = V : Y-N = V : B-N =V

Load Current:

(e) P.T.Voltage

R-Y = R-N =

Y-B = Y-N =

B-R = B-N =

(f) Trip Circuit testing (test tripping the breakers through relay)

(g) Remarks:

1) Checking of all fuses

(Sd) TESTER ASST.ENGINEER ASST.EXE.ENGINEER.



322

CHAPTER-XVI

GAS INSULATED SUB-STATIONSEr.K. Mounagurusamy

CE / P&C

The informations given be low are abstracted from several ASEA GIS equipmentbooklets.

Gas insulated Sub-station of certain types takes up only about 10% of the area of

conventional Sub-stations.

Figure-1 shows below the comparison, for a volt level upto 170 KV.

HISTORY: Use of SF-6 gas for breakers was started in mid sixties.

GIS programmes were launched in seventies. In early 1977, first GIS was

commissioned by ASEA in Sweden upto 420 KV. Now GIS of several thousand KV areavailable.

At lower levels of voltage three phase systems are used.At UHV levels single phase systems are used.

ADVANTAGES:

1) The area required is very much less

2) Quicker and simpler erection

3) Easier maintenance4) Insensitive to influences of surroundings

GAS PRESSURE:

The higher the gas pressure (density), the higher will be the insulation strength of

the gas and smaller the dimensions of the enclosure. Normal pressure is 7 bars.

Fi ure-1



323

CHAPTER–XVI

In some designs, the equipment can withstand the rated voltage also when the gas

pressure decreases to atmopheric pressure provided no switching is done.

COMPONENTS:

- Conductors

- Insulators

- Enclosures- Gas

- Spacers

SPACERS: - forms a solid insulation, in parallel with gas, between the conductor in the

centre and the surrounding earthed enclosure. The earthed enclosure is in the form of

metallic tube. In the centre of this there is the conductor which is supported and held in

place by insulating cones called spacers. The space between conductor and enclosure isfilled with SF 6 gas at overpressure. See Figure 2 to 5.



324

CHAPTER–XVI FIGURE - 2

FIGURE - 3

FIGURE - 4



325


FIGURE - 6



326

CHAPTER–XVI



327


FIGURE - 8



328

CHAPTER–XVI

The spacers have to withstand mechanical forces from gravity, apparatus function,

pressure differences between gas sections, earthquakes and short circuit currents. Disctype spares are also used.

CONDUCTORS:

Consists of aluminum tubes with joining contacts at the ends. Current is transmitted via

the spring loaded contact member to the copper parts and against which the contact

member rests. These are later welded to the aluminium parts.

JOINTS:

There are angled joints and T-Joints

EXPANSION JOINTS:

Expansion joints are provided partly to compensate for the tolerance during manufacture

and partly to allow for thermal expansion.

OTHER COMPONENTS: Like disconnectors, CTs, VTs etc. are shown in figure

below:

FIGURE-9 Disconnector straight

1. Fixed contact

2. Moving contact

3. Operating devise



329

CHAPTER–XVI 1. FIGURE-10

Angled and T disconnectors

Disconnectors can be operated by Motor devices.

`

T-disconector

Fig - 10



330

CHAPTER–XVI

- Shows continuous position indication- Possibility of using the earthing switches as a test probe for measuring contact

resistance and polarity of instrument transformers

- Can be located in the same housing as disconnectors but also elsewhere.- Can be operated either manually or Motor operated

- There are two types such as fast operating and slow operating

FIGURE – 12

VOLTAGE TRANSFORMERS

- Voltage transformers can be set up where it is required i.e. on bus bars and outgoing

circuits.



331

CHAPTER–XVI Figure – 13

CURRENT TRANSFORMER



332

CHAPTER–XVI



333

CHAPTER–XVI

The bushing can be adopted to any existing oil filled or PEX cable

- Also used for High potential testing of GIS bus bars etc.

EXTENSION:

The GIS can be extended usually by lengthening the bus bars and adding more

breaker groups provided necessary space is provided in the building. Erection sequence

must be checked in detail. Another question to investigate is the procedure of testing afterinstallation of the new parts.

SAFETY: The probability of anybody being injured in a GIS will 0.000025 per year or once

per 1300 years. GIS is said to be 40 times safer than conventional sub-stations TESTING OF GIS:

1) Testing of Gas:

Non return valves are provided to fix the gas density switches. After removing theswitch assembly, external gas hoses can be connected and gas filling, draining, testing

can be done.

2) Breaker testings:

Since the poles are inside the gas tank, approach to do the timing tests, primary

injection through CTs were difficult. For one end, the earth switch end which is insulatedbefore the earth connection can be used. For the other end the earth switch cannot be used

since all the three phases are looped inside the SF-6 chamber and only the neutral is

brought out. Hence, the cable ends which was at a distance of 100 meters from S.S. were

used for the above tests. The layout of cable system is shown in Figure-17. This was alsoused for hipot testing the cables. D.C. hipot testing of GIS has to be avoided.

FIGURE – 16: SF-6 AIR BUSHING



334

CHAPTER–XVI



335

CHAPTER–XVI



336

CHAPTER-XVII

REVIEW AND ANALYSIS OF TRIPPINGSEr.K. Mounagurusamy

CE / P&C

ALL THE TRIPPINGS SHOULD BE REVIEWED.

Analysis of the operation of protective relays or the scheme is very important for aprotection engineer.

The following types of operations need analysis:-

- Maloperation i.e False tripping in the absence of primary fault.

- Incorrect operation or unwanted operation during a primary fault.

- Failure to operate.

The protection engineer may carry out the analysis in the above lines and do the needfulfor improvements. The action taken may be reported to the head office for scrutiny. There

are always possibilities for human error in the protection works and hence a scrutiny by

another agency is an Absolute Necessity.

All the trippings of transmission and sub-transmission level lines and transformers at

Substations should be reported by T.M. to the concerned head office in the form given

below:

Transmission line fults:

A line fault is a condition where electric current follows an abnormal path due tofailure or the removal of insulation which normally confines it to the conductor.

Insulation is usually either air or high resisting material which may also be used

as a mechanical support. Air insulation can be accidentally short circuited by birds,rodents, snakes, monkeys tree limbs, unintentional grounding by maintenance crew etc.,

or broken down by over voltage due to lighting or weakened by ionisation due to fire.Organic insulation can deteriorate due to heat or ageing or can b broken down by overvoltage due to lighting, switching surges or faults at other locations.

Porcelain insulators can be bridged by moisture with dirt salt or industrial

pollution or can develop a crack due to mechanical forces. In such cases the initiallowering of resistance causes a small current to bee diverted which hastens the

deterioration or ionisation causing this current further to increase in a progressive manner

until a flash over occurs.



337

CHAPTER-XVII

Overhead transmission lines are most vulnerable for lighting strokes. More than

50% of electrical faults of overhead lines are known to be caused by lightning. As per

Van C. Warrington, all faults occur within 40 degrees before voltage maximum at linesover 100KV. The shield wires intercept most direct strokes and allow them to be

conducted harmlessly to ground. Some time, they could reach the conductors below the

shield wire. In such cases, the lightning surge will bee distributed in all directions of thelines connected, depending upon the point of incidence. For example, a lighting strike

penetrating the shielding system and terminating on a phase conductor would generate

traveling waves of the same magnitude and polarity propagating in opposite directions.

Some times, these waves may attenuate and die without any problems. Most of

the times, they keep on propagating on the line. Of all the line insulators are in healthy

condition, the surges reach the terminal substations and be bypassed to the groundthrough lighting arresters. In this case, protection needs to operate and line will remain

healthy since surge current is by passed within micro seconds.

If any of the lines insulators re weak, it can undergo flash over due to the surge.

“The possibility of even the direct stroke causing a flash over near voltage zero is

minimised by the fact that the lighting stroke lasts only one or two microseconds and, ifthe line voltage were near zero at the moment, there would be nothing to sustain the flow

of power after the stroke. Although the stroke current may be upto 100,000A there is less

then a coulomb in a stroke, so there would be no cloud of ionised air maintaining a lowresistance path until the voltage built up” (Van C. Warrington : vol 2). Once a flash over

occurs, there will be system frequency follow current depending upon the fault level and

the arc will not extinguish till the system voltage is interrupted by the protection. This

means that both end relays of a tied line should operate and isolate the line. A single end

tripping will not suffice. Many a times, the flash over does not damage the insulator andthe line can be recharged. This is called a “passing faults”. Short circulating the line

insulator by snakes, birds etc., as discussed before, will also come under this category.

But, if the insulator gets damaged by the flashover, it will not withstand the power

system voltage if reenergized and the protection will again operate. This is a kind“permanent fault”. There are different types of permanent faults which are not discussed

here.

The flashover may occur in more than one towers due the lighting surge wave. Ifone such flashover leads to a permanent damage in the second zone of a distance relays

and another flashover causes a temporary flashover in its first zone coverage, both endrelays will trip on first zone and may cause confusion when analysing by the protectionengineer.

Single end trappings should be treated in a special manner. From the discussions

so far made, at will be clear that there can not be single end trippings at all! But, they dooccur. A tall tree ay swing and touch a conductor in the second zone but may withdraw

before the second zone time of the relay. In this case, only the other end will trip on first

zone. A jumper may get open and fall on the tower arm in one side and the tripping willbe single en only. A conductor may snap and tall to ground in only one side of the lines

and the result will be single end tripping. Hence, the protection engineer shall not take

granted any single end tripping which is very rare. If the cause is not established clearly,the protection system should be checked thoroughly in the case of single end trippings.



338

CHAPTER-XVII

Lighting need not even come in direct contact with power lines to cause problems,

since induced charges can be introduced into the system from nearby lighting strokes to

ground. Although the cloud and earth charges are neutralised through the established

cloud – to – ground path, a charge will be trapped on the line. The magnitude of thistrapped charge depends on the initial cloud to earth gradient and proximity of the stroke

to the line. Voltage induced on the line from the remote stroke will propagate along the

line causing similar problems as that of direct stroke.

When a lightning directly strike a tower or the earth conductor the tower has to

carry huge transient currents. If the tower footing resistance is considerable, then thepotential of the tower would rise steeply with respect to the line and consequently the

insulator string would flash over. This is known a “BACKFLASHOVER”. It is clear

that too many trippings on temporary faults may also indicate more tower footingresistance, needing improvements.



339

CHAPTER–XVII

TM

From To

The Superintending Engineer

Asst.Exe.Engineer/Shift Copy to the Exe. Engineer/O

110 KV/line tripping Copy to the EE/GRT (MRT)

message Copy to the AEE/GRT (MRT)

1. Name of Sub-Station :

2. Name of line :

3. Time & Date of tripping :

4. Relay indications )

at both End. )

5. Is the line radial or )

Tied at both ends )

6. Load on the line prior )

to tripping MW, MVAR, )

AMPS. )

7. Bus voltages recorded )

before tripping – at )

the time of tripping - )

after tripping. )

8. Special observations )

like grunt in generators,flickering of lamps )

oscillations in panel )

meters. )

9. Any other simultaneous )

trippings of 132 KV )

lines or distribution )

lines. )

10. Climate :

11. Time and date of )

normalisation )12. Remarks :

Asst. Exe. Engineer / Shift.



340

CHAPTER–XVII

1) Every grid and upstream radial feeders tripping shall be reviewed monthly.

2) Even correct trippings of grid feeders and upstream radial feeders should also be

reviewed and classified as “IN ORDER” and reported to higher office. A correct

tripping in the view of one engineer (may be inexperienced) may be a wrong one.

There are instances that single end tripping of grid feeders have been classified as IN

ORDER in some cases without analysis. Correct single end trippings of grid lines are

also possible but extremely remote – a line getting open and conductor making

ground fault in only one side.

3) Review of transformers and generators shall be reviewed then and there. Our old

practice is that the review should be made within 24 hours. It is felt that this is even

now very essential. Maloperation of any equipment i.e. radial lines, tie lines,

Transformers, Generators shall be analysed within 24 Hours.

4) Correct operations shall be classified as due to

- Weather

- Lightning

- External incidents

- Failure of line or equipments

- Overload- System disturbance

- Cause not known.

5. Incorrect relay operations shall be classified as due to

- Design limitations

- Inadequate or Incorrect settings

- Construction defect

- Maintenance defect

- Failure of relay component

- Caused by pilot channel

- Personnel errors

- Incorrect application of relays

- Unexplained.



341

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6. Relay tripping registers shall be maintained by protection wing as well as substations

O&M wing.

7. Protection engineers should be knowing how to calculate the fault level at any point

in the system. Fault level of local substations should be calculated by them and

exhibited in the premises conspicuously.

8. Some of the interesting review and analysis are discussed below:

I. WRONG CONNECTION OF GENERATOR – ROTOR EARTH FAULT

RELAYS AT ALIYAR POWER HOUSE AND SHOLAYAR POWER HOUSE.I.

The Generator rotor earth fault relays were with wrong connections at Aliyar

Power House and Sholayar Power House.2 since their commissioning. The relays were

not operating during normal conditions though there was an earth fault existing in the

rotor and were operating “Correctly” for a short moment during shutdown sequences.

The circumstances which warranted the tracing of the fault and action taken to rectify the

defect are narrated in the following lines:-

On 23.12.79, 27.7.80 and 1.9.80 the rotor earth fault relay of the 60 M.W. Hydro

generator at Aliyar Power House acted for a short-while during normal shutdown

sequences soon after the shutdown impulse was given.

Every time the relay was tested and found to be normal. The details of the I.R.

value of the rotor circuits meggered on 23.12.79 are not available and the I.R. value of

the rotor circuits meggered subsequent to the operation of the relay on 27.7.80 and 1.9.80

were low and was of the order of 0.2 to 0.3 M. Ohms.

No serious thought was given for the relay operation on 23.12.79 considering it as

freakish. Only after a recurrence on 27.7.80 the matter was studied in detail.

The relay was acting just for a moment during the shutdown sequence and it was

not acting during normal running of the machine or during shutdown time and this

required a deep study of the subject.

While going through the original schematic drawing of the Generator on 30.9.80

it was observed that the rotor earth fault relay was given wrong connection.



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The scheme as per the given drawing is shown below :

In this connection scheme, the rectifiers in the bridges of the relay will permit

flow of current when the circuit is closed by earthing the point ‘A’. This current would bedue to the D.C. source voltage available at the terminals 9 & 10 of the relay. It could be

seen that the D.C. voltage on t he rotor is in “subtractive series” connections with the

D.C. source voltage of the relay. Hence, if the earthing point is slowly moved from pointA towards point B, the resultant voltage across the relay coil would be V9-10 – V A F.

As long as V9-10 is greater than VA-F, there would be a flow of current through the relay

element R. When VAF becomes greater than V 9-10, the resultant voltage would not be

able to drive any current through the circuit since there are the rectifiers in the bridge ofthe relay which will not permit any flow of current when they are supplied with a voltage

of reverse polarity. This means that only a very small zone of the motor from the point A

towards B was under protection of the relay so far. (It was confirmed later that the relaycurrent was zero even when the point B was earthed solidly.)

In the first look, it seemed that the problem has further confused since it wasoperating during a particulars period of shutdown sequence, though it was connected in a

“non-operating way”. On further analysis, the “wrong connection” was found to be the

cause for the momentary operation of the relay during the shutdown sequence alone as

explained below.

The Generator at Aliyar has “de-excitation scheme” during shutdown sequence

i.e. as soon as the shutdown impulse is given, the main exciter voltage is reversed rapidlyto cause “de-excitation of the rotor” before the tripping of the field breaker. When the

main exciter output voltage is reversed, it comes in “additive series” with the D.C. supply

voltage of the relay i.e. the relay gets “correct connection” accidently for a moment and if

a rotor earth fault is persisting it measures and indicates and this a what had occurred onall the three occassions. This was got proved on 5.9.80. Necessary modification in the



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scheme was down on 5.9.80 by interchanging the internal wiring leads in the terminal 9

& 10 of the relay, after getting oral approval of the Divisional Engineer/GRT/Thudiyalur.

The machine was running and when the relay was put back in service after modification,the relay acted immediately. A persisting rotor earth fault was suspected. On earthing t he

rotor through a 5 K resistor, there was measured a leakage current of 0.75 m.amps. So farit was not detected by the relay and after modification it has detected.

Even with the original wrong connection, the relay should have detected the earth

fault when the machine has come to rest i.e when the rotor voltage has come to zero. This

was not there and it could be explained as below with an example.

Let the D.C. source voltage of the relay be 55 V The setting current required for

the relay element to pickup is 1.1 m.amps.

Therefore I.R. value detected by the relay

55= = 50000 Ohms.

1.1 ma

This much of low I.R. value will be detected by the relay if the fault is in anyportion of the rotor when the machine is shutdown.

Let the rotor voltage be 55 Volts.

Assuming that the connections are O.K, and a fault of 100000 Ohms occurs atpoint B when the machine is running.

Total voltage available for the )relay element ) = 55 + 55 = 110 V

Therefore the leakage current or )

the operating current through ) 110the relay element ) = - - - - - - - = 1.1 ma

) 100000

Hence the relay could operate i.e a fault of 100000 ohms at point B could be

detected by the relay only when the machine is in service and the same fault would go

undetected when the machine is shutdown since the relay current in the case would beonly 55/100000 = 0.55 ma i.e. the “aid” voltage of the rotor is not available now.

The relay available at Aliyar Power House is of English Electric make type VME.

The same type relay was available at Sholayar Power House 2 also. When the studies

were going on at Aliyar Power House, the scheme at Sholayar Power House.2 waschecked for comparison. It was found that the very same defect was there also. That relay

was also operating for a moment several times when the machine was tripping on faults

since 1971. The relay has not picked up during normal shutdowns as was operating in thecase of Aliyar Power House due to the fact that the de-excitation scheme comes into

operation only during fault trippings of the machine at Sholayar Power House.2 and is

not coming during normal shutdowns. The modification was also carried out at Sholayar

Power House 2 afterwards.



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Such defects with similar type of rotor earth fault relays could be detected only by

test-earthing both ends of the rotor while the machine is running. Any testing by earthing

the rotor when the machine is not in service or by earthing any one end of the rotor whenthe machine is in service is not the complete one and will not reveal such defects.

II. ROTOR LIFT PROTECTION AT KADAMPARAI POWER HOUSE:

Top beam

Setting

Electrical ContactGap = 1.2 m.m

Rotor bracket beam

During over-speeds or any unbalance problems, the rotors of the generators may

get lifted up in the case of vertical machines. At Kadamparai, rotor lift protection is givento trip the machine. When the rotor bracket beam lifts up by 1.2 mm (Original setting),

the protection will operate.

The machine was tripping frequently from 11.4.91. It was tripping.

a) before synchronism

b) after synchronism

c) When the load was changed

d) Even when the machine was running smoothly.

After struggling continuously for 8 days, the reason was found to be rather funny.

Whenever the side doors of the generator was opened for some reason or other, the entiretop platform with the beam bent and moved down by 1.2 mm due to downward suction of

air caused by all blower fans.

Prior to 11.4.91, all the blower fans could not be switched on due to problems in

some fans.

III. TRIPPINGS OF GENERATOR 2 at KUNDAH POWER HOUSE. 3 ON

BUCHHOLZ INDICATION:

Generator No. 2 at Kundah Power House 3 tripped on transformer buchholzindication and Generator O/V relay indication. There was no air or gas in the buchholz

relay and the machine was put back in service. Though the machine was running OK, the

machine was shutdown to probe further for the tripping.

The cable coming from the transformer was found damaged. Thinking that this

could be the reason, the machine was re-serviced. The machine tripped again after one

day with the same relay indication.



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Staff who checked the Buchholz relay by dismantling it said that the diaphragm

was weak and that could be the cause.

But, it was approached from another angle. Why the Generator O/V relay hasacted? Had the Buchholz relay first acted, the machine field breaker would have trippedimmediately along with the main breaker and there could not be any reason for the

machine voltage to rise.

It was decided to test the O/V relay. It was operating even for normal voltage of11.2 KV on the machine. And, when the O/V relay acted, Buchholz trip alarm came.

There were two culprits:

1) Wrong calibration of O/V relay

2) Wrong connection of annunciator.

IV. PERSONNEL ERROR:

The generator at one Power House was reported one day to have tripped withoutany relay indication except master relay operation.

After thorough checking of Generator, transformer, Cables, Protection systemnothing could be found out. Everyone was hesitant to restart the machine but the

concerned operator said the machine can be restarted. Here was the clue:

On further interrogation with the operator, Switch Board Attendant and other

staff, the truth came out. When the operator had observed some oscillations of somemeters, he thought that something was wrong with the machine and operated the

emergency push-button.

V. ANOTHER PERSONNEL ERROR:

Machine. 1 at one Power House was reported to have tripped on Generator

differential relay. When the operator on duty was contacted over phone, he said that whenhe wanted to shut down unit.1, he just put his hand on the L.T. breaker switch of

machine. 1 and at that instant the machine had tripped.

Not a deliberate, but an upset boss shouted at the operator over phone :

“Are you playing? How can it trip when you just touch it? Do you think that I am a

fool? Do you think that I do not know what happened?”

Immediately, the operator surrendered and accepted that he had done a wrong

operation by paralleling the L.T. system of machine 1 and machine.3 after tripping hemain breaker of machine.1.

Shouting helps some times for analysis:



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VI. MALOPERATION OF TRANFORMER DIFFERENTIAL RELAY:

At one Sub-Station, both the transformers were tripping one differential relay

frequently for through faults i.e for known fault on downstream radial feeders. This washappening for many years and several testes on transformer and relays were in vain.

The ultimate reason was very simple. The differential relay p.u. setting was 15% but

the transformer had tap range upto 17.5% with OLTC. When the tap moves to extreme,position, the mismatch current was sufficient to operate the relay.

VII. MALOPERATION OF DISTANCE RELAYS:

When the author joined at Saudi Arabia, the first assignment was the analysis of

the frequent tripping of a double circuit feeder outgoing from a Power House for reverse

faults, Er. Arunachalam who has contributed some chapters in this Manual was theprotection incharge. Though the problem could not be identified by them so far, it was

not at all a problem for both of us.

Attacked the first point and found that the C.T. connection were opposite.

The problem was set right without availing a shutdown and also without

succumbing to the threat from the local boss that both of us would be sent to fail if

anything wrong happened.

This particular analysis is so simple that it does not deserve inclusion in this

manual but this is included to show the capability and standard of the protectionengineers in our board on comparison. The problem had caused several black outs to thesystem there but was not given due though for several years.

The author wishes to mention on more things – purely personal:

In one committee meeting held to finalise the procedures to commission a new

substation, 10 out 12 people were from India and nine out of the ten were from TamilNadu.

VIII. DOWN TO THE EARTH PROBLEM

During a pre-commissioning test in a Sub-Station, a transformer differential relay

type MBCH, a static relay, was not behaving properly.

When the relay was tested by another engineer next day, the relay behaved

correctly.

The reason was:

The first engineer tested the relay keeping it outside the case. That was his usual

method.

The second engineer did by his method by keeping it inside the case.



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Later, on enquiry from an engineer from the relay manufacturing company, the

reason given was that the static relay will misbehave if its chassis is not earthed properly.

Proper earthing is provided inside the case and when the relay is racked out, the earth islost.

IX. BLIND APPROACH:

For the known fault on a distribution feeder from Sub-station in Saudi Arabia,

several feeders were tripping simultaneously even in other substations very far away.

How to approach?

One of the author’s Colleagues, a distribution engineer, hailing from our board,

came up with suggestion one day. He was telling that one particular feeder which had a

very high pick up setting had never tripped on similar occasion. On that basis, he

suggested to revise the settings of all other feeders.

The suggestion looked very childish. Comparing with the peak load the settings

were more than sufficient in all feeders.

However, the subject was digged further.

A phenomenon called “COLD RUSH” was explained in an article appeared in the

lectures at PSTI, Bangalore. There was not much explanation but it gave a starting point.

On further searching, it was found that the “Cold-Rush” is a very big problem

where loads are predominant with Air-conditioners, even in United States. Several blackout have occurred.

What is a “Cold Rush”?

When a fault occurs at a particular location in a system, the system voltage drops.When the voltage drops to 70% and below, the A.C. units stall. Even if the voltage is

restored immediately, they take a very high current of 5 times the full load current till the

bleeding of pressure system completes.

This takes more than a minute and hence the load on the healthy feeders suddenly

shoot up to several times the full load, causing the tripping.

The problem was solved temporarily by increasing the P.U. settings as suggested

by our colleague, though by layman approach.

The correct solution for this problem is to provide U/V tripping in all the air

conditioners.



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X. DISTRIBUTION OF EARTH CURRENTS IN HIGH VOLTAGE SYSTEM:

The theory of this subject is dealt in several books including GEC measurements

book and Russian books. Chances are very remote for the protection engineer to go into itdeeply but one of our former engineers, Er. Srinivasaraghavan, Disvisional Engineer(Generation) has produced a very good article on this subject in MSEB. Journal dated

June 1952. A reproduction of the full article (since not even one word is extrawritten)

will certainly help to guide our engineers.

ELECTRICITY DEPARTMENT JOURNAL

IT IS THE GENERAL practice to earthed the neutral in high voltage transmission

systems, at one voltage transmission systems, at one point only, that is at the sending end.

In case of earth fault in one of the phases, the earth current flows from the fault to the

earthed neutral through earth and actuates the earth fault relay and trips the breakers, thusisolating the fault. There have been instances where star/star transformers with tertiary

delta have been connected at the end of transmission system, the neutral point on the

H.V. side of these transformers being brought out and connected to earth. Thus theneutral is earthed also at a point, other than at the sending end. In such cases, earth fault.

Current flows not only from the fault to the sending end neutral but also from the neutral

point of the star/star transformer, though this is beyond the fault. The distribution of thefault current is as shown in diagram I.



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In this diagram G is the generator feeding into a transmission line through adelta/star step up transformer T1, T2 is a step down transformer connected to the end of

the transmission line, the connections of the windings being star/star with tertiary delta

and the neutral point on the H.V. side being connected to earth.

The effect of the current flowing into the fault not only through the faulty phase,

but also through the healthy phases from beyond the fault on the operation of protectiveequipments is interesting. A few examples that have actually occurred some years black

in the Department’s E.H.T. net work are mentioned here:

(i) Coimbatore –Madurai-Koilpatti 66 KV line – (Diagram 2).--The neutral pointsof the star connected auto-transformers at Coimbatore were solidly connected to earth: in

addition, the neutral pint of the star/star transformers with tertiary delta at Koilpatti end

was earthed. For earth faults in the lines between Coimbatore and Madurai, the 66 KVOCB on the outgoing line to Koilpatti a Madurai end used to trip out: the 66 KV fuses on

the transformers at Koilpatti end also used to blow out on certain occasions. The remedy

was either to isolated the neutral at Koilpatti or raise the setting of the earth fault relay onthe Koilpatti line at Madurai end sufficiently high to prevent its operating under such

fault condition. The former course was adopted.

(ii) Coimbatore-Prianaickenpalayam – Nellitharai 11 KV line. Mettupalayamwas original fed from Nellitharai S.S. by stopping down the 66 KV voltage to 11 KV

through delta/star transforms, When Nellitharai sub-station was abolished after changing

over to feed from Coimbatore. The 66 KV/11 KV transformers were left at Nellitharai forsome time and this used to be kept energised at 11 KV from Coimbatore end and isolated

on the 66 KV side. Under such conditions, there have been cases when the 11 KV OCB

at Nellitharai tripped for a fault on the line between Coimbatore and Nellitharai (vidediagram 3).

In both these instances the operation of the OCBs beyond the point of the fault areevidently due to flow of earth fault current from the neutral of the transformer at the

remote end in these case at Koilpatti and Nellitharai.

XI. PROTECTION ENGINEERS’ PROBLEMS:

Another good article written by Er.G.A. VISVANANTHAN, in MSEB. Journal

(date not known) is also reproduced since this is also very illustrative:

IT IS NOT very uncommon to have certain unexplainable operation of relays in spite of

very careful selection of relay settings. In many such cases definite faults were found to

exist outside the sphere normally scrutinized by the protection engineer. It is, therefore,

necessary that the engineer should proceed with tan open mind to investigate suchapparent maloperations. The following occurrence is an example:-



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At Tiruvarur in the Mettur Electricity System, a 11 K.V. feeder is taken for abouta mile from the Government sub-station to the South Madras Electric Supply licensee’s

power house from where a number of feeders are taken out to the licensees; various

stations. At the Government sub-station the feeder is protected with 2 overload and oneearth leakage relay and at the licensees’ Power House, the incoming and the outgoing

feeders have also 2 overload and one earth leakage relay; some four years back

complaints were being received from that station that for earth faults on any of thelicensees’ feeders, the relay at the Government sub-station end only would trip, thus

causing supply failure to the licensee’s entire area.

The testing of relays and O.C.Bs. in the Government sub-station and gradation ofsettings of relays at both the ends of the feeder and those on the out-feeders at the

licensee Power House did not stop this occurrence. Finally it was decide to check up the

connections of and test the relays and O.C. Bs. at the licensees’ Power House.

On examinations, it was found that on each feeder, the connexions were as shown in

the sketch below with an earth connection at “a”

This explains the non-operations of the earth leakage relays at he licensees’ end for

an outside fault, while tripping the relay at the Sub-station end. This earth connexion was

removed and the relays and O.C.Bs. were tested. From then on wards the relays operatedsatisfactorily.

G. A. VISVANATHAN.



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EXPERIENCES IN PROTECTION FIELDEr.K. Mounagurusamy

CE / P&C

FEEDER TRIPPINGS DUE TO SINGLE PHASE FUSE BLOW-OUT:(Arti cle by: Sri. S. Raghunatha Rao B.E; D.E(E) & Sri. P. Narayanan B.E; D.E(E)

The blow-out of the H.G. fuse on the H.V. side on one phase of the Delta/StarPower Transformer at a Sub-station may cause feeder trippings on L.V. side.

There was recently an occurrence of this nature at Poonamallee Sub-station, whenthe 33 K.V. H.G. fuse on the yellow phase of single 3-MVA: 33/11 K.V. Transformer in

service at the time below off. Of the four numbers 11 K.V. feeders taking off the station,

three tripped on over-load blue phase while the fourth feeder was standing.

A review has indicated that the fuse blow out should have preceded the feedertrippings, the blow out being caused by mere over-load over a period of time during the

peak period. (Two strands of 21 SWG timed copper wire were used for the fuses in the

absence of OCB control on the H.V. side.)

The trippings of the 11 KV. Feeders are analysed with the help of vector diagramsgiven below : --

With the blowing out of the H.V. fuse on the yellow phase the voltage vectors ofphases R and Y on the primary and r and y on the secondary side collapses, Y becoming

Y’ and r and y moving to r’ and y’. This results in only half the normal voltage being

impressed across the windings R and Y of the primary and a single phase secondarysupply with normal voltage between the blue phase and neutral and half the normal

voltage between the red and yellow phases and neutral. Consequent upon the full

secondary voltage being available only between the blue phase and neutral there shouldhave been a disproportionately heavier drawal of power on the blue phase. The three 11

K.V. feeders, which were already fairly loaded at the time of occurrence all tripped on

overload blue phase, the heavier drawal on this phase, resulting in load currentsexceeding the overload settings. The fourth feeder, which was also in service at that

time, is understood to have had practically negligible load and the fact of this feeder not

tripping is perhaps explained by the failure of the load on the blue phase of this feeder to

reach the plug setting value, notwithstanding the heavier drawal of power on this phase.

EARTH FAULT RELAY:

There is a big article on this subject by ER.K.S. DORAISWAMY, DivisionalEngineer on this heading published in December 49 of MSEB. Journal. The conclusion

is given below:

The current flow in a residually connected earth fault relay in series with 2 O/L

relays, is only a fraction of the unbalanced current. The true replica of earth fault currentwill not be flowing through the E/L relay particularly when it setting is very low.



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Number of tests were done in the MRT. Lab at Coimbatore and results and

tabulated. The readings show that with the E/L relay plug setting at 20% the E/L relay

sets only 40% of current in the faulty phase and the balance current flows through theother phase relays. At 70% P.U. setting, the current sharing is 79%.

BREAKER MECHANISM FAULT:

For a fault on Aliyar – Sholayar feeder.2 in 1980, all the 110 KV feeders

emanating from Aliyar tripped at remote ends. The relay had operated in Aliyar-Sholayar

feeder.2 also and the breaker had also tripped.

The system was normalised without too much digging out.

On deeper investigation next day, the relay contacts of Sholayar feeder.2 at ALR

end had burnt and damaged.

Why the contacts should burn?

Suspected the breaker and the timings were measured. Much increased. This was

due to heavy friction in the mechanism.

In cricket, the match is not over till the last ball is bowled, Kapil says. In

protection, the investigation is not over till the cause is traced out.

CONCLUSION:

It should be clearly understood that only maticulous, strict adherence to rigid

testing standards and indepth knowledge of tripping analysis go a long way in ensuring

the correct operation of protective gear and elimination of unwanted operation or minormishaps which often prove very costly. In spite of everything, only 80% of the faults are

still cleared correctly by the protection systems as per experts. This is why – protection

is an ART where perfection is impossible.

SOME EXPERIENCES IN THE FIELD WORKS:

To start with, item 1 is reproduced from our old MSEB (Now TNEB) Journal

June 1960 – an article “Operation and Maintenance Problems” written byEr. S. Mohammed Ali, then Divisional Electrical Engineer.

1) “Know what you are doing”

In many of our potential transformers, the secondary yellow phase is connected to

earth and the neutral left insulated. A section officer look out the P.T. for routineoverhaul. While reconnecting the wires, he did his job all right but finding the neutral

was without a connection, he attached an earth wire to it as is done on any distribution

transformer. In a few minutes after energising it, the P.T. was found burnt out. Thismistake can be attributed to (i) ignorance and (ii) not marking each terminal while

disconnecting. It is a good practice that while disconnecting wires in any terminal board,each terminal is clearly tagged.



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It is equally good practice to record the order of parts dismantled when handling

any mechanical equipment. What is dismantled in a few seconds may take hours to refit

if you have lost sight of the order of things.

2) Earthing of P.T. secondary at Moyar

Originally, the P.T. secondary Yellow phase was connected to earth and the

neutral left insulated when Moyar was commissioned in 1952.

After all the distance relays and synchronising scheme were completely replacedand modified in the years 1992/93, the Yellow phase earth remained in Yellow phase. As

per the manufacturers of the new distance relays provided, the P.T. neutral needs earthed.

This was corrected in 1997. The implications can be set aside but the overlooked isoverlooked.

3) P.T. failure at Maravakandy:

When the commissioning tests were done on 14-6-92 at Maravakandy Mini

Hydro Power House (1 x 750 KW) in Nilgiris, at the time of building the machinevoltage to its rated value of 3.3 KV one of the two PTs of V-connected machine PT got

burnt out.

A spare P.T. was erected on 26.2.92 and it also got burnt out when energised.

When the contractor brought replacement P.T. on 20-7-92, he informed that theyhave supplied 3.3 KV / √3 110 / √3 P.T. so far instead of 3.3 KV / 110 V PTs with the

name plate of 3.3 KV / 110 V ratings.

Any site done with station L.T. supply will not reveal the defect.

4) Mixing of P.T. wiring with C.T. wiring:

When the P.T. secondary circuits were meggered on 25-3-1978 in the station L.T.

supply circuit of one of the generators at Sholayar PH I (Commissioned in 1971), the

circuit was giving zero IR value. The reason was found to be the wiring mixing betweenP.T. and C.T. circuits. One C.T. was actually feeding the potential coil of an energy

meter. The Polarity connections of the CTs were also opposite.

5) Mixing of A.C. supply with D.C. circuit at Sandynallah S.S.

When the routine meggering of D.C. circuits was done in 1971 at 110 KV

Sandinallah S.S. in Nilgiris it was found that there was wiring mixing between station LTsupply and station DC supply. If annual meggering was done effectively, this could have

been identified early.

6) Loss of P.T. supply at Moyar PH

The layout of the 110KV buses at Moyar Power House existed in service in 1997

is given in Figure 12.1.



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Normal operational procedures are:

- Keep 189 A – PT and 189 B – PT isolators closed energising the 110 KV feederBus PT and taking PT loads on this P.T.

- Keep transformer bus dead

- Keep the 189 A and 189 B isolators of the generators which are running and keep

them open in idle generators.

- When LC is needed in 110 KV feeder bus P.T. close any one of the isolator of

transformer bus of running generators and avail the relay loads on Transformer

bus P.T. De-energise the feeder bus P.T. by opening both its isolators. Such L.Cswere being availed monthly for maintenance works.

Let us assume that all the three machines are running. But P.T. is on L.C.Transformer bus P.T. is fed from machine 1.

Suppose, machine 1 trips on fault. Its OCB trips and machine 1 voltage goes to

zero thereby the bus P.T. looses its supply. All the distance relays will trip on loss of PTsupply, causing black out at Moyar.

The same black out will happen if the operator shuts down machine 1 and opensthe 189 A and 189 B without knowing the implications.

It was told that there were many cases of all feeders tripping simultaneously

before at Moyar end only.

Another problem is the non-existence of a true bus coupler isolator. Any

inadvertent penning of one of the isolators in all the generators and PT, there is thepossibility of separation of the two buses. If Bus PT is Bus A in such an eventuality,

faults on Singera feeder 2 and Gobi feeder 2 will not be sensed by the P.T. in service

leading to possible non operation of protection.

By connecting both bus P.Ts to both buses and introducing a bus coupler 189

AB – BC as shown in figure 12.2 solved all the above problems.

Even now 189 A isolators of the generators are useless since the 110 KV lightningarrestors of transformers are connected through 189 B isolators only.

Action is being taken to remove the copper tubular bus bars of transformer bus

completely.

In the authors opinion, the design of the bus arrangement is non-standard. With

50% of the bus structure materials, a simpler bus with the same facility could have been

designed and erected. Even now, a comprehensive operation with one bus is not possiblesince the feeders do not have bus selection facility. Selection arrangement can be made

but very laborious. This can be done if MUSHEP comes.



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7. Emergency operation with one P.T. of less V.A capacity:

When one of the 110 KV P.T. failed at Udumalpet in 1991/1992, a smaller V.A.capacity was temporarily used till the correct capacity P.T. was arranged. All the

metering circuits were kept switched off to keep the P.T. load under capacity and the S.S.was operated with calculated risk.

8. Emergency operation with one transformer of less capacity at Aliyar Power House:

When one single phase transformer of 24 MVA capacity failed, a transformer of12 MVA was connected in the bank with other phase transformers of 24 MVA and the

Aliyar machine was operated for some days till the correct size transformer was arranged.

The load and the generator was limited to the capacity of the small size transformer. Themachine had negative sequence relay and it was kept in service without any problem.

9. Need to test C.T. at rated current:

Due to the non-availability of suitable loading transformer one 800 A.C.T. wastested O.K. with 400 A and put in service but the ratio did not keep up when the load

went beyond 400 A. This shows that, the CTs should be invariably tested for its rated

current.

At Kadambarai, the generator ring CTs are rated 8000 A. Loading transformer

was available to inject only 1000 A. Hence, 8 turns of current injecting lead were

toroidally would through the C.T. and the tests were done for 8000 A. Such torodial

winding may not be possible in sub-station C.Ts but their maximum rating is 1200 A onlyand hence no problem exists.

10. Tripping of generator differential relays at sholayar PH-1

There were frequently maloperation of generator differential relays of bothmachines at Sholayar PH 1 for through faults on 110 KV feeders since commissioning in

1971. Suspecting the metrosils connected across the CT secondaries they were removedon 12-11-79 and thereafter there was not even a single such wrong tripping.

11. Negative sequence relay operation at Kadamparai

Unbalance current (1000 A, 1000 A, 1800 A) was noticed in machine 4 on

29.8.90. The observation was ignored assuming that the transducers would have been

faulty. When the load picked upto 70 MW on 30.8.90, the machine tripped on negativesequence relay.

Many tests like measurement of D.C. resistance of generator circuit, measurementof generator impedance primary injection by injecting current just after 230 KV CTs in

the yard – in vain.

Primary injection was done after the 230 KV breaker in the yard. CT secondarycurrents were less than expected in R and Y phases.

Reached the location. Y Phase limb of the 230 KV ABCB was showing 500Ω inclosed condition.

Lesson: Don’t make assumptions.



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CHAPTER–XVIII

12. Tandem rod problem in 110 KV OMCB:

During routine maintenance works on a 110 KV OMCB (BHEL) at 230 KV/110

KV S.S. at Udumalpet, the timing was incorrect in one of the limbs. The reason was theloosening of bolts in the tandem rod.

13. Problem with core balance CTs in Cables:

In one of the sub-stations where core balance CT was used for earth fault

protection in the outgoing cable of a distribution line, the earth fault relay did not operate

for a known earth fault in the cable.

It was found that the earthing of cable sheath was not made properly.

The earth fault current has gone through the C.T. and also returned through the

C.T. getting cancelled each other in the C.T. Hence no out put from C.T.

Correct sheathing is shown in figure. Current first goes through the cable core,returns through sheath and again returns through the sheath. The sheath currents through

the CT gets cancelled and the cable core current remains.

14. Protection tripping through ‘Local’ control of breaker:

In a section of a system with 8 No. grid feeder breakers commissioned under one

contract, a fault occurred in one of the lines.

The protection operated O.K. and isolated the fault. The Operator went to the yardfor inspection and tried to test charge the line through local control from the breakermechanism box. All the incoming breakers to that S.S. tripped at the remote ends. The

distance relay in the above faulty line operated but the breaker did not trip.

On investigation, the protection scheme was so designed that the protection

tripping was not effective when the breaker control was on ‘LOCAL’.

We have already said that the protection tripping should be effective irrespectiveof the position of the local-remote control switch of the breaker.



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CHAPTER–XVIII

15. A Terrific Experience:

While test charging after a fault tripping the operator at 230 KV S.S. Udumalpet

one day observed that there was no current in one phase of a 230 KV feeder going to the

nearby S.S. at Myvadi. The protection did not operate will not operate if it was on open jumper.

LC was availed and a through inspection of the line was done by lines wing and

everything was O.K.

Test charged the feeder. The current was still missing.

Shut down the feeder. 400 Volts 3 phase voltage was injected from 230 KV

Udumalpet S.S. and bulbs connected at Myvadi a end of the line were burning O.K. in all

the 3 phase.

The protection wing was again moving in the yard for further probing. Suddenly,

one person shouted and alerted others to sit down. A vertical live 230 KV jumper from

the bus going to the isolator got unclamped at the top end and was hanging down just atthe safe clearance over the head of the inspecting persons.

16. Operation during L.C.

This happened when Kundah PH I was a dead end. A double circuit 110 KV line

was there between Kundah PH-2 and Kundah PH-1. All the machines were shut down at

Kundah PH-1.

A shutdown was needed in line No. 1 shut down was issued and LC was availed

in line 1 at both ends. Line work was taken up and LC returned and everythingnormalised.

But the SBA at Kundah PH I found that the 110 KV bus RVM was not recordingfor so many hours. On deeper investigation, it was found that the operator had tripped

line 2 at Kundah PH-1 while issuing the L.C.

How much negligence?

Clear instructions over the step by step operations before issuing a line clear onequipment should be available in every SS/PH at the operators table.

Sub-log book should be essentially maintained. The author has heard a story of

attending repair works in the cooler of a healthy generator when the cooler was defectivein other unit.



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CHAPTER–XVIII

17. Primary injection and Bus bard stability problem:

The above figure shows the portion of a 33 KV GIS system. When the B/B CTwas tested for ratio by inserting the current cable through the available external hole

marked as “C”, the CT behaved accurately.

When it was tried to do the primary injection by injection current in between A

and B through conductor, the CT behaved erratically.

THE PROBLEM could not be solved for many months inspite of the visits of

expert from the country where it was manufactured.

The foreign company finally brought a flux Camera which can take photo ofmagnetic flux. It was seen that there was flux linking between points D and E through F.

This was found to be due to missing of an insulating washer provided in one of the fixing

bolts of flange F. Actually, there should not have been electric conductivity between Gand H but the defect was the existence of continuity due to the missing washer as was

concluded by the company engineers.

18. Real life is like that:

I. After the tripping of a generator, the hydraulic operator in a power house wasshouting to the electrical operator:



364

CHAPTER–XVIII

“Hey – the machine has tripped – but the shaft is still rotating”

II. No rains – No problem:-

Load dispatch engineer asked to put one Generator on bars immediately.

Operator replied that the rotor is outside.

Load dispatcher advised that since there were no rains, the rotor could well be

outside and asked to put the machine first.

III. Deserving appreciation:

The rewinding works of an induction motor was going on. One big engineercommented to the electrician, “what you are doing is wrong. There should be only 4 leadscoming out. How come there are six leads?”

Finally, after successful completion of the work, the big engineer recommended

himself for appreciation and got it too.

IV. Yet to design:

This is in sixties. A proposal was sent through the hierarchical ladder to the

Canadian Company who erected Kundah system to provide a transformer in a circuit tosolve a problem. The company replied:

“We are yet to design a D.C. transformer”.

V. More careful:

One engineer was more careful that he wanted to get approval from the foreign

company who had supplied the 250 V.D.C. generator equipment as to whether varnishingcould be done to improve its I.R. value.

VI. How is it?

Boss: Which fool gave you the degree?

Subordinate engineer: The same fool who gave it to you.

VII. Betting by the author:

Primary injection was going on in the Bus bar protection C.T. in a sub-station.One young engineer was doing the test. The author was witnessing. The testing guy

found it very difficult to drive current in the loading transformer when he tested oneparticular phase. He said to his assistant to check whether any secondary of the C.T. wasopen.

The author intercepted and asked? “How is it? You are sending current only

through the C.T. primary and a big bus bar jumper of about 5 meters. How can the C.T.

open circuit can impede the current in the loading transformer?”

The testing guy was very sure. The author was very adamant. The author offeredto make a bet and the testing guy immediately accepted. Supplying Pepsi to all present

was the bet.

The author had to supply Pepsi to all finally.Experience always speaks.



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CHAPTER–XVIII

VIII. Protection Mind:

One engineer saw a chicken in his dream inside the Kadamparai PH. (How can a

chicken get into the under ground Power House?) He was telling about this dream to hisnearest people.

Within two or three days, there was the fire accident at Kadamparai. It was sug-

gested by somebody in presence of the then Chairman to send down through a basket –

pulley – rope system a chicken through the vertical tunnel upto the Power House location

to ensure that fire had extinguished completely. These are also protection thinking anddreaming.

IX. Getting shouted gives un-forgetting pleasure:

The top brass from Madras who is famous for shouting is in site to witness the

scheduled commissioning of a big equipment. One small protection engineer could not beavailable for the commissioning. He gets the nod from his local boss to be away on theday of commissioning on an unavoidable family function. The news reached the shouting

boss. He shouts, “what? What do you mean? nothing doing. No commissioning

tomorrow. We will wait for you. Go and finish your job and join us.” This shouting isunforgettable. Only the protection engineer can get so much of importance – every one of

you know.

X. Masters:

The author has innumerable number of masters in this world. Leaking rain waterwas dripping over a villager silting in a bus. He was not at all caring. He neither bothered

nor enjoyed. He was as calm as a baby in its mothers’ arms. He is also a master of theauthor. The author thinks of him whenever he faces such a situation now and then and

asks himself how the villager in bus would have acted under this circumstance?

Like wise many masters. The author is blessed always with very good bosses

anywhere in the world.

Most of the bosses like Er. K. Narayanasamy, Er. B. Ranganathan and Er. K.R.Syed Abdul Subhan are his masters in many ways. When the author makes an analysis of

tripping or when he drafts a letter or when he faces a labour union, he asks himself, “How

Engineer …… will analyse this tripping?” The author gets some more depth. His mastershas “assessed” the author as “GIVES IMMEDIATE SOLUTIONS IN THE FIELD” and

“CAPABLE OF TACKLING ANY PROTECTION PROBLEM”

- Million dollars boosts indeed.

The author has started thinking confidently after getting these assessments that he

could give himself a solution to any problem in life also.

The 26 year old protection engineer of electrical equipment has understood nowthe way to protect himself from any hazards in life and his own I.R. value is > --------.



366

CHAPTER-XIX

UNDER GROUND CABLESEr. M. Arunachalam

EE / GRT

A.3.1. 11KV & 33KV POWERCABLES.

A.3.2. 110KV OIL FILLEDPOWER CABLES.

A.3.3. 110KV XLPE POWER

CABLES.

A.3.4. 230 KV XLPE POWERCABLES.

A.3.5. PILOT CABLES.

A.3.6. LV CABLES.



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CHAPTER–XIX A.3.1. 11KV & 33KV POWER CABLES.

Test No. Test Description Standard Ref. Remarks

1. Sheath Insulation &continuity Test.

2. Insulation test for

cable core.

3. Phasing test

4. H.V. test

IEC-52

ISI-

IR value for

sheath>100 meg

ohms, And for cableconductor>500 megohms

A.3.2. 110 KV OIL FILLED POWER CABLES.


1. Oil flow test

2. Impregnation Test

3. ConductorResistance test.

4. Capacitance test

5. Sheath insulation by

5KV megger.6. Cross bonding test.

7. Tightness of links in

Junction boxes

8. Test for SVL by

2.5KV Megger.

9. High Voltage Test.

IEC-141-1

ISI-

IR value moreThan 100 Meg

ohms.

Test at 245KVDC for 15

Minutes.



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CHAPTER–XIX A.3.3. 110 KV XLPE POWER CABLES


1. Insulation Test IEC-8402. Conductor

Resistance,

capacitance &

inductance test.

ISI-

3. Sheath insulation

Test.

4. Cross bonding Test.

5. Link box tightnesscheck

6. SVL test by 2.5 KVmegger

Test at 185 KVDC for 15minutes.

7. H. V. Test

A.3.4 230 KV XLPE POWER CABLES.


1. Insulation Test

2. Conductor

Resistance,

capacitance &inductance test.

IEC – 840

ISI-

3. Sheath insulationTest.

4. Cross bonding Test.

5. Link box tightness

check

Test at 385 KV DC

for 15 minutes.6. SVL test by 2.5 KV

megger

7. H.V.Test



369

CHAPTER–XIX A.3.5. PILOT CABLES.


1. Insulation Test by5KV Megger

IR value>100 Megohm

2 Loop Resistance

Test.

3. Cross talk test &coupling Test.

4. Noise level

measurement

A.3.6. LV CABLES.


1. Phasing &continuity check.

IEC- 227-2

2. 2KV insulation test.

3. Visual inspection,

size & ratings

confirmation.

IR value More than

100 Meg Ohms.

Note: Annual DL H.V. test on cables in generalings station should be dispensed with and

the DL H.V test should be conducted after ratification of fault conditions.



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CHAPTER-XX

BATTERIES

Checking of Value Regulated Lead-Acid Batteries in conjunction with the

commissioning of plant.

1. GENERAL

This is a general guideline for bringing Valve regulated lead acid (VRLA)

batteries into serviceTest Record 1 WAT 910037-006 is to be used during testing.

For commissioning of freely vented lead acid batteries, please see provision 1

WAT 910034-007.

The installation, commissioning and maintenance instructions given by eachmanufacturer shall always be complied and should be read at the same timesas this guideline.

2. RFERENCE DOCUMENTS.

- Installation drawings and instructions from the manufacture providedtogether with the battery, regarding storage, erection, initial charging etc.

- Commissioning instruction for rectifies 1 WAT 910034-005

- Manufacturers manual for rectifier.

3. TEST EQUIPMENTMultimeter class 1.5

Test leads

Voltmeter for DC class 0.2 (Digital multimeter)Thermometer

Rubber gloves, goggles, eyecup, cold water and saline solution in squeeze

bottle for eye wash.

4. SAFETY PRECAUTIONS

4.1 Hydrogen gas.

When lead acid batteries are being charged, oxihydrogen gas is liberated. Tominimise the risk of explosion, the following precautions must be taken:

- Ensure that the space around the battery is adequately ventilated. Ensure

ventilation according to local standards. Use Swedish standard SS 408 01

10 if no local standard is available.

- Smoking is to be prohibited - Prior to touching the call caps, remove any static electricity by placing

the hand on the edge or the side of the respective battery cases.



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CHAPTER–XX

4.2 Chemical stuff.

The valves of the battery must not be blocked or opened. Filling of cells is

not possible, since the electrolyte is immobilized and the battery already

filled when delivered.

Severe damages on the battery container might cause acid to leak. Thereforethe same safety precautions as the vented batteries are necessary:

- Use protective goggles.

- Washing facilities are to be available close to the battery.

- Electrolyte on the skin, must be washed with plenty of soap and water.

- If electrolyte gets into the eyes wash with plenty of clean water and get

immediate medical attention.

Lead compounds are poisonous. Always wash your hands after working withthe battery.

4.3 Electrical current

Valve regulated batteries are always electrically alive and the risk of shortcircuit (and electrical sparks, see 4.1) must be prevented.

- Use insulated tools only to make connections to the battery, taking care

not to over tighten beyond manufacturer’s recommended torque value.

- Check the circuit and make sure it is safe before making a connection to

the battery.

- Before working on the battery, always remove personal metal effects,such as rings, watches, bracelets, necklaces etc.

4.4 Temperature.

For lead acid batteries in general and especially for valve regulated batteries

it is of utmost important to keep the temperature at a steady level of 20 C° (See fig. 1).

5. PREREQUISITES.

Chargers which are connected to the battery shall already been commissionedpreviously.

6. VALVE REGULATED BATTERIES.GENERAL INFORMATION

6.1 Description of basic technology

Over the past years VRLA batteries have been introduced as an alternative tothe conventional lead acid and nickel cadmium batteries. This new type is

advertised as “sealed” or “maintenance free”. The correct designation is

“valve regulated” according to IEC 896-2 (draft).

In a VRLA cell the net water consumption is strongly reduced by means of a

recombination of the oxygen at the negative electrode and by preventing thehydrogen from being generated.



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However, the oxygen recombination does not work at 100% efficiency.

Some oxygen escapes from the electrochemical system. Furthermore,

evolution of hydrogen cannot be fully suppressed. This means that water losswill take place already during normal conditions and particularly during

charging with high currents and high temperatures. These water losses arenot possible to replace.

The valve, which must open at high pressure, is a very important detail. If

the valve sticks open (or is removed for any reason), this will lead to oxygen

ingress with resultant discharge of the cells and ultimately total dry out. If itsticks, internal pressure build-up will create a severe bulge of the cell

container leading to eventual fracture. In the extreme case it may rise to an

explosion.

NOTE:

Valve regulated batteries are not sealed.

6.2 Two VRLA designs

The VRLA batteries are divided into two main groups depending on the way

the gas recombination is achieved:

- Absorbed (starved) electrolyte

- Gelled electrolyte

Absorbed electrolyte cells are built up of pasted plates with microporousglass fibre as separators.

The electrolyte is absorbed by the pores of the electrode active materials and

the separator. The separator is not saturated with acid and the acid free pores

are used for transferring the oxygen from the positive to the negativeelectrode.

The gel electrolyte is immobilised by the addition of silicon dioxide to thesulfuric acid. The oxygen is transported through micro cracks in the gel.

The plates can be designed as for FVLA with pasted or tubular plates. The

separators are normally made of microporous plastic.

6.3 Float charge

Due to the limited acid volume and consequently the need for high aciddensity in the absorbed cells (1.29-1.30 kg/I), the float charge voltage will be

somewhat higher than for other lead acid batteries. For this reason the

absorbed VRLA cells must be charged with a higher float charge level of2.27 V/cell with given tolerances.

The gelled type has the same density as the FVLA type i.e. 1.24-1.26 kg/I

and accordingly has the same float charge level of 2.23-2.25 V/cell.



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CHAPTER–XX

This discrepancy must of course be considered when calculating the number

of cells so that the upper limit is not exceeded.

Absorbed Gelled

Density 1.29-1.30 kg/I 1.24 kg/IFloat charge 2.27 V/c 2.23 V/c

The manufacturer must give detailed information of recommended float

charge voltage.

6.4 High rate charge (boost charge, equalizing charge)

In order to avoid gas development and risk of dry-out, all manufacturers

advise limitations for voltage, current and time when recharging VRLA cells.

It is therefore of importance to follow the instructions from the manufacturer.

6.5 Ambient temperature

The higher ambient temperature, the higher float current at a given voltage

level.

High float current causes high internal temperature and escape of gas, whichwill dramatically decrease the lifetime of the battery.

This is valid for all lead acid batteries. See fig 1.

100 %

Fig 1.

50 %

0 10 20 30 40

Due to the exothermal oxygen recombination reactions, VRLA batteries

develop much more heat inside the cell than the conventional cells.

Furthermore, as there is no free acid, the heat dissipation is not supported byconvection.

Under extreme conditions, the battery can be subject to successive increase

of float current and temperature until it is destroyed. This phenomenon is

called “thermal runaway”.

Life Time

Temperature



374

CHAPTER–XX

A temperature compensated charger can be installed in the DC system.This charger can decrease the float charge level at high temperatures and

therefore marginally improve the situation but not restore the lifetime due tohigh temperatures. See typical values fig 2.

Fig 2.

0 5 10 15 20 25 30 35 40

In general, the gelled batteries have a larger electrolyte volume than theabsorbed type and are more resistant to a drying out. This is a generalguideline and differences between different makes may change this picture.

6.6 Ripple

There is no difference between valve regulated batteries and conventional

lead acid batteries regarding the acceptance of ripple current. The ripple

current must be limited to a value recommended by the manufacturer(Normally 5A/100Ah). Otherwise the corrosion on the positive grid and the

internal temperature will increase.

6.7 Discharge performance

The absorbed type has a very good high rated discharge current performance.Therefore this technology is highly suitable for UPS systems, diesel engine

starting and DC systems where large current peaks are required after a long

discharge period.

For the gel type, the peak loads might increase the nominal battery capacity

and consequently the cost. Gel-technology is worth its price for applications

with low discharge current without extreme peak loads at the end of the

discharge period.

240

235

230

225

220

Plant voltage Per Coil (V)

Temperature C’



375

CHAPTER–XX

6.8 Classification and Lifetime

EUROBAT has classified the VRLA batteries into 4 groups with particular

reference to – Performance- Safety- Design Life

*10 + year – High integrity

Telecommunications, nuclear and conventional power plants, oil andpetrochemical industry and other applications where the highest security is

required.

*10 year – High performance

In general terms, this group of batteries have comparable design lifeperformance as in the 10 + year – High integrity group. However,

requirements for performance and safety are not as severe. The requirement

for capacity is 95% at first cycle and 100 % at 10:th cycle.

*5-8 year – General purpose.

Safety requirements and design life related tests are not as stringent.

*3-5 year – Standard commercial.

This group of batteries are at the consumers end and are popular in smallemergency equipment.

There are some gelled batteries on the market today which cannot reach100% after first cycle and shall be classed in the 10 year – Higher

performance group.

The difference in lifetime expectancy between the two VRLA types at 20

degrees C is;

* Absorbed technology – appr. 10 years for the 10+ and 10 year groups.* Gel technology – appr. 12-15 years

7. RECEIVING, UNPACKING AND STORAGE.

Inspect the battery upon arrival and check that the goods delivered arecomplete and that all cells/blocks are undamaged.

Under no circumstances shall the cell/blocks be lifted by their terminal

pillars.

There is no need to remove the terminal covers before the erection of the

battery set.

If the battery cannot be installed immediately, store all parts in a clean anddry room.



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CHAPTER–XX

It is advisable to check the voltage for each cell/block after the unpacking.This can be done without removing the terminal cover (for most of the

makes). The recommended lowest voltage is given by the manufacturer.

(appr. 2-10 V/cell)

In order to ensure that the batteries can be charged easily after a long period

of storage, it is recommended that the batteries should not be stored morethan the following periods without recharging (typical values);

6 months at 20 °C4 months at 30 °C2 months at 40 °C

For this reason it is very important that the design office is informed about

any delays at an early stage so that the delivery of the batteries can becoordinated with the start of commissioning.

8. INSTALLATION

The installation section of the battery manual and the installation drawing for

the special project must be complied completely.

Care for space between cells/blocks and for good ventilation in the roomwhere the battery is accommodated.

9 . COMMISSIONING CHARGE

NOTE! It is very important to follow the instruction from each manufacturer

regarding;

- applied float voltage and high rate voltage (if recommended).- current limit

- time period for charging

- temperature when charging

Some manufactures make a distinction between initial charge forimmediately load connection and initial charge for site acceptance test.

Generally, the absorbed typed are more sensitive to high voltages and largecurrent. For this reason batteries require most time to be initially charged,

especially when the battery is subject for site acceptance test (up to 6days

charging for some manufactures).

The voltage applied to the battery set is calculated according to: nextrecommended charging voltage (n = number of cells)

If a temperature compensated charger is installed, the float charge voltage

shall be adjusted according to recommendation from the manufacture or

according to fig 2. In this case the alarm level for under/over float charge

voltage is set to 2-3% of the set float value.



377

CHAPTER–XX

The charging current (1 charging) is expressed as % of Ah capacity 0.1 xC10 means that the current shall be limited to a value of 10 % of the nominal

10 h capacity.

For a 100 Ah nominal capacity it is 10 A. Before initial charge it might be

necessary to derate the current limit of the charger (1 set) so that the current

is limited to the recommended value; I set = I charging

When the station load is connected it is advisable to set the current limit in

accordance with actual configuration in the power station (substation) and the

recommended value given from the design office; I set = I charging + I load

10 . SITE ACCEPTANCE TEST (CAPACITY TEST)

The site acceptance test must be carried out in the period between completion

of the commissioning charge and the introduction of an operating load on thesystem.

The capacity test is normally performed during 5 hours, 10 hours or the

battery duty period. The following instruction will apply to the 5 and 10hours discharge.

- Read the ambient temperature.

- The charger shall be connected to the battery until the start of the

discharge. It is recommended to check the voltage for each cell/bloc after

completion of the initial charge but before disconnection the charger andstart of the testing.

- If nothing else is specified the discharge current is given in the

manufactures catalogue at an end voltage of 1.80 volt/cell for 5 hour or 10hours discharge.

- The battery load until shall be connected to suitable terminals where thestation loads and rectifier are disconnected and where the battery load unit

connections are protected by fuses/circuit breakers. See fig 3



378

- If possible, makes a rough current setting on the battery load unit before isconnection to the battery.

- Connect the battery load unit and make a final adjustment of the current. Itis very important to that the discharge current is supervised and kept at an

accurate level of +/-1 % throughout the test

- Make a note in the test sheet at what time the test is started and at what

time the test is finished.

- The battery voltage is to be measured 6 times during the discharge period.

- Battery terminal voltage is to be measured the first 3 times and individual

cell voltage is to be measured the last 3 times (at 80-90-100% discharge).

If the battery terminal voltage is measured at the load bank, the voltage drop

in the cables between battery and load bank has to be considered

- Voltage drop between the battery terminals and the cell connections shall

be checked during an early stage of the discharge. All cells are checkedand the voltage must not exceed 5 mV. Connections where the voltage

drop is larger must be investigated and carefully adjusted.

Successively as the test is performed, enter the test results in Test Record 1WAT 910037 – 006.



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CHAPTER–XX

The acceptance test must be supervised to prevent deep discharges and the

recharging must be commenced immediately after the discharge with a

voltage, current and time equal to the method for putting the batteries intoservice without test.

1. ACCEPTANCE CRITERIA FOR COMMISSIONING TEST

Two criteria must be regarded for acceptance of the site test.

-cell/bloc voltage deviation

-capacity

The cell/bloc voltage deviation has a very wide range for new VR batteries. It

is recommended to check the voltage for each cell/bloc after completion of theinitial charge but before disconnecting the charger and start of the testing. In

this stage the deviation can reach a level of +0,2/-0,1 V/cell for gelled cells

but less for absorbed type.

At the end of a site test the cell/bloc voltage deviation shall not vary more

than +/-0.06 V from mean value and the battery voltage shall not be bellow

the predestinated end voltage (normally the number of cells multiplies with1.80 V/cell).

The capacity test shall be interrupted when the battery voltage has reached theend voltage.

If the happens for instance at the 4,5 hour reading for a 5 hour test, it indicates

that the capacity is only 90 % (4,5/5).

For adsorbed type in*10+ year – High integrity group not less than 100 %

capacity is accepted.

For gelled type which is in the 10 year – High performance group 95 %

capacity at first cycle is accepted.

Temperature correction for other temperatures than 20° C must be done asfollows:

K(t°) = capacity at temperature t° = time for discharge x discharge current.

K(20°) = capacity at 20° = K(t°) / f

f = correction factor given by the manufacture. If nothing is specified thisfactor can be calculated as; 1+0,006 x (t-20)



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ABB Substations Inspection and test record Record form No:

1 WAT 910037-006

Description:

Batteries valv requlated lead acid

Order No.: Sheet

1

With capacity test Drawing No.: Cont2

Customer.: Customer ref.:

Erection site.: Ref.:

A. neral Data and Information

Battery type…………………………….Manufacturer:………………………….

Nominal Voltage:………………………Nubmer of cells:……………………….

Temperature compensated charger ___

B. Initial charging:

Charging coltage:……………………… VCharging current. (current limit of the charger):……………………. A

Charging time:………………………….h

C. Cell voltage after initial charging:

Float voltage:………….V Time between initial charging and capacity test….h

Ambient temperature:…………………. °C

Cell. Volt Cell Volt Cell. Volt Cell. Volt Cell. Volt Cell. Volt

No. age No. age No. age No. age No. age No. age

1 21 41 61 81 101

2 22 42 62 82 102

3 23 43 63 83 103

4 24 44 64 84 104

5 25 45 65 85 105

6 26 46 66 86 106

7 27 47 67 87 107

8 28 48 68 88 108

9 29 49 69 89 109

10 30 50 70 90 110

11 31 51 71 91 11112 32 52 72 92 112

13 33 53 73 93 113

14 34 54 74 94 114

15 35 55 75 95 115

16 36 56 76 96 116

17 37 57 77 97 117

18 38 58 78 98 118

19 39 59 79 99 119

20 40 60 80 100 120Test carried out

Date. Sign.:

Customers approval

Date. Sign.:



381


1 WAT 910037-006DescriptionBatteries vlav regulated lead acid

Order No.: Sheet.:2

With capacity test Drawing No.: Cont.:

3Customer.: Customer’s ref.:

Erection sale.: Ref.:

D. Capacity test.5 hours test:___ 10 hours test:___ ..........hours test:____End voltage/cell 1.80 V/cell_____ Other end voltage/cell.......V/cell

Time between initial charging and capacity test.......h

Ambient temperature............................... °C

Discharge current according to datasheet.........................A

Calculated capacity (discharge current x time)........................Ah

The discharge was started at...............stopped at..................

Individual cell voltage

5 hours test 10 hours test …….hours

Time V Time Time hours

(h) (h) V (h) V

0 0

1.00 1.00

2.30 5.00

4.00 (80%) Sh.4 8.00 (80%) Sh.4 (80%) Sh.4

4.30(90%) Sh.5 9.00 (90%) Sh.5 (90%) Sh.5

Compl. Compl. Compl.test (100%) Sh.6 test (100%) Sh.6 test (100%) Sh.6

NOTE:

Individual cell voltages are noted on sheet 4-6. One sheet for each voltmeter reading at80-90-100% dischargeAt 0 hours the load bank is not yet connected and the voltage indicates the open circuitvoltage of each cell.

The capacity test is to be completed when the battery voltage has reached the endvoltage.

Contact resistance between terminals and cell connectors checked (after 1 hour):______Test carried out

Date. Sign.:

Customers approval

Date. Sign.:

CHAPTER–XX



382

ABB Substations Inspection and test record Record form No:1 WAT 910037-006

DescriptionBatteries vlav regulated lead acid

Order No.: Sheet.:3

With capacity test Drawing No.: Cont.:4

Customer.: Customer’s ref.:


E. Evaluation of results after completed test.

1.Cellvoltage deviation after initial charge with the rectifier still connected at floatcharge mode (section C. page 1)

Mean value...........V/cellMax. value............V/cellMin. value.............V/cell

2. Cellvoltage deviation after completed test (section D, page 2)

Mean value...........V/cellMax. value............V/cellMin. value.............V/cell

3. Extracted capacity.

Extracted capacity after compleated test = discharge current x discharge time:................................................................................................Ah.

Calculated capacity according to section A:...............Ah.Correction with other temperatures than 20°C; Capacity (20°C) = Capacity

(t°C)/correction factor:................................................................................................Ah.

F. Recharging after completed test

Charging voltage...................VCharging current....................ACharging time.....................…h

References to used instruments:

Type:__________________________Identity:_________________________________Type:__________________________Identity:_________________________________Type:__________________________Identity:_________________________________Type:__________________________Identity:_________________________________

Test carried out Customers approval

Date, Sign.: Date, Sign.:

CHAPTER–XX



383


1 WAT 910037-006Description

Batteries vlav regulated lead acidOrder No.: Sheet.:

4

With capacity test

Drawing No.: Cont.:

5Customer.: Customer’s ref.:


Cell voltage after 80% discharge

CellNo.

Voltage

CellNo.

Voltage

CellNo.

Voltage

CellNo.

Voltage

CellNo.

Voltage

CellNo.

Voltage

1 21 41 61 81 1012 22 42 62 82 1023 23 43 63 83 1034 24 44 64 84 1045 25 45 65 85 1056 26 46 66 86 1067 27 47 67 87 1078 28 48 68 88 1089 29 49 69 89 10910 30 50 70 90 110

11 31 51 71 91 11112 32 52 72 92 11213 33 53 73 93 11314 34 54 74 94 11415 35 55 75 95 11516 36 56 76 96 11617 37 57 77 97 11718 38 58 78 98 11819 39 59 79 99 11920 40 60 80 100 120

Test carried out Customers approvalDate, Sign.: Date, Sign.:

CHAPTER–XX



384


1 WAT 910037-006DescriptionBatteries vlav regulated lead acid

Order No.: Sheet.:5

With capacity test Drawing No.: Cont.:6Customer.: Customer’s ref.:

Erection sale.:Ref.:


CellNo.

Voltage

CellNo.

Voltage

CellNo.

Voltage

CellNo.

Voltage

CellNo.

Voltage

CellNo.

Voltage

1 21 41 61 81 1012 22 42 62 82 1023 23 43 63 83 1034 24 44 64 84 1045 25 45 65 85 1056 26 46 66 86 1067 27 47 67 87 107

8 28 48 68 88 1089 29 49 69 89 10910 30 50 70 90 11011 31 51 71 91 11112 32 52 72 92 11213 33 53 73 93 11314 34 54 74 94 11415 35 55 75 95 11516 36 56 76 96 11617 37 57 77 97 11718 38 58 78 98 11819 39 59 79 99 11920 40 60 80 100 120



CHAPTER–XX



385


1 WAT 910037-006Description

Batteries vlav regulated lead acidOrder No.: Sheet.:

5

With capacity test Drawing No.: Cont.:6Customer.: Customer’s ref.:



Cell

No.

Volta

ge

Cell

No.

Volta

ge

Cell

No.

Volta

ge

Cell

No.

Volta

ge

Cell

No.

Volta

ge

Cell

No.

Volta

ge

1 21 41 61 81 101

2 22 42 62 82 102

3 23 43 63 83 103

4 24 44 64 84 104

5 25 45 65 85 105

6 26 46 66 86 106

7 27 47 67 87 107

8 28 48 68 88 1089 29 49 69 89 109

10 30 50 70 90 110

11 31 51 71 91 111

12 32 52 72 92 112

13 33 53 73 93 113

14 34 54 74 94 114

15 35 55 75 95 115

16 36 56 76 96 116

17 37 57 77 97 117

18 38 58 78 98 118

19 39 59 79 99 119

20 40 60 80 100 120



CHAPTER–XX



Contributors of this Manual

1. Er. A.S. Kandasamy M.E.,MI.EEE(USA) CE/Transmission

Contributed a lot of papers on Distribution protection and Metering system.

2. Er. K. Mounagurusamy. B.E.,

Chief Engineer/Protection & communication. The experiences in the generation andtransmission network with contributions to the development of the system.

3. Er. M. Varadharajan, B.E,

Executive Engineer/O&M /Orathanadu

Experiences in generating station protections and distribution protection contributed to

the value information.

4. Er. P. Ponnambalam, B.Sc., B.E.,

Executive Engineer/Sub-Station Erection / Chennai

The experience in the field of erection and testing of equipment contributed to the

manual.

5. Er. M. Arunachalam, M.E.,

Executive Engineer/Grid relay Test/Chennai

The experiences on transmission protection are shared much on this manual.

The contributions are worthy in nature and confined to the transmissions.

Date post:	19-Feb-2018
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