VOCATIONAL TRAINING

ON

UTTAR PRADESH POWER TRANSMISSION CORPORATION

LIMETEDFor

The partial fulfillment of award

Of

B.TECH. Degree

By

SHAILENDRA YADAV

(0705420096)

Electrical Engineering (Final Year)

ACKNOWLEDGEMENT

I am extremely thankful & indebted to the numerous UPPTCL Engineers, who provided vital information about the functioning of their respective departments thus helping me to gain an overall idea about the working of organization. I am highly thankful for the support & guidance of each of them.

I am highly indebted to my project guide, Mr. Ramlal(A.E.), Mr. Mevalal(J.E.), Mr. P.K. Mishra (A.E.-T&C) for giving me his valuable time and helping me to grasp the various concepts of switchyard equipments and their control instruments and their testing.

Last but not the least, I would like to thank my parents & all my fellow trainees who have been a constant source of encouragement & inspiration during my studies & have always provided me support in every walk of life.

SHAILENDRA YADAV B.TECH. FINAL YEAR

ELECTRICAL ENGINEERING B.B.D.N.I.T.M. (LUCKNOW)

Contents

What is an Electrical Substation? Energy growth in UP Grid map of UP Introduction: about substation Overview of substation Single line digram of substation Brief description :

Power transformer Isolators Circuit breaker Lightning arrestor Current transformer Capacitor voltage transformer Wave trap Protective relays Shunt reactor for bus voltage Capacitor bank Clearance at glance Power line communication & SCADA system Other definitions

Appendix References

What is an Electrical Substation

“Electric Power is generated in Power Stations and transmitted to various cities and towns. During

transmissions, there are power (energy) loss and the whole subject of Transmission and

Distribution...

An electrical substation is a subsidiary station of an electricity generation, transmission and

distribution system where voltage is transformed from high to low or the reverse using

transformers. Electric power may flow through several substations between generating plant and

consumer, and may be changed in voltage in several steps.

The word substation comes from the days before the distribution system became a grid. The first

substations were connected to only one power station where the generator was housed, and were

subsidiaries of that power station.

Elements of a substation

Substations generally have switching, protection and control equipment and one or more

transformers. In a large substation, circuit breakers are used to interrupt any short-circuits or

overload currents that may occur on the network. Smaller distribution stations may use reclose

circuit breakers or fuses for protection of distribution circuits. Substations do not usually have

generators, although a power plant may have a substation nearby. Other devices such as power

factor correction capacitors and voltage regulators may also be located at a substation.

Substations may be on the surface in fenced enclosures, underground, or located in special-

purpose buildings. High-rise buildings may have several indoor substations. Indoor substations are

usually found in urban areas to reduce the noise from the transformers, for reasons of

appearance, or to protect switchgear from extreme climate or pollution conditions.

Where a substation has a metallic fence, it must be properly grounded (UK: earthed) to protect

people from high voltages that may occur during a fault in the network. Earth faults at a substation

can cause a ground potential rise. Currents flowing in the Earth's surface during a fault can cause

metal objects to have a significantly different voltage than the ground under a person's feet; this

touch potential presents a hazard of electrocution.

Transmission substation:

A transmission substation connects two or more transmission lines. The simplest case is where

all transmission lines have the same voltage. In such cases, the substation contains high-voltage

switches that allow lines to be connected or isolated for fault clearance or maintenance. A

transmission station may have transformers to convert between two transmission voltages, voltage

control devices such as capacitors, reactors or static VAr compensator and equipment such as

phase shifting transformers to control power flow between two adjacent power systems.

Transmission substations can range from simple to complex. A small "switching station" may be

little more than a bus plus some circuit breakers. The largest transmission substations can cover a

large area (several acres/hectares) with multiple voltage levels, many circuit breakers and a large

amount of protection and control equipment (voltage and current transformers, relays and SCADA

systems).

Distribution substation:

A distribution substation in Scarborough, Ontario, Canada disguised as a house, complete with a

driveway, front walk and a mown lawn and shrubs in the front yard. A warning notice can be

clearly seen on the "front door".

A distribution substation transfers power from the transmission system to the distribution system

of an area. It is uneconomical to directly connect electricity consumers to the high-voltage main

transmission network, unless they use large amounts of power, so the distribution station reduces

voltage to a value suitable for local distribution.

The input for a distribution substation is typically at least two transmission or sub transmission

lines. Input voltage may be, for example, 115 kV, or whatever is common in the area. The output is

a number of feeders. Distribution voltages are typically medium voltage, between 2.4 and 33 kV

depending on the size of the area served and the practices of the local utility.

Energy growth in UP:

Grid map of UP:

Introduction: about substation

400 kv Unnao substation is one important substation of UPGCL & UPPTCL. It is one of the

largest power grids in the state of UP and the north India. It is situated at Dahi Chowki 6.64 km far

from unnao railway station. The construction of this substation completed during 1994-98 by

CGL(Crompten Grives Limted) .The area of this substation is about 300 acre.

The whole substation is divided in four parts:

1. 132kv switchyard

2. 400/220kv switchyard

3. 765kv switchyard

For 400kv &220kv switchyard a common control room is used and for 132kv switchyard

A separate control room used.

Crompton Greaves Limited (CG), an Indian Multinational with manufacturing bases in 8 countries,

have signed the contract on 5th March’2010 with Uttar Pradesh Power Transmission Corporation

Ltd for construction of 765/400 kV Substation at Unnao, in Uttar Pradesh. The value of contract is

Rs 302 Corers .

A 765/400 kV substation is the highest grade system voltage for transmission in India. UPPTCL is

first state utility to enter into 765 kV arena.

The scope of the project includes Design, Engineering, Manufacture, Supply, Erection, Testing

and Commissioning of 8 Bays of 765 kV & 2 Bays of 400kV, along with 7 Nos. of 333 MVA (Single

Phase) 765/400 kV Power Transformers and 7 Nos. of 110 MVAR (Single Phase) 765 kV Shunt

Reactor & 4 Nos. 63 MVAR (Single Phase) 765 kV Reactors. The project is expected to be

commissioned in July 2011.

The project is of strategic importance for entry into market of 765 kV Substations globally and

widens up the horizon for the entire product range of CGL.

Overview of substation

As we said earlier the whole substation is divided in three parts:132kv site ,400/220kv site and 765

kv site 765 kv sit is on under construction. The civil work is completing by L&T Company. Other

part of project Design, Engineering, Manufacture, Supply, Erection, Testing and Commissioning of

Bays will complete by CGL.

In 400/220kv switchyard following outdoor instrument used:

1. One 400kv transfer bus control bus coupler

2. Two 100MVA 220/132kv autotransformer

3. Two 315MVA 400/220kv autotransformer

4. Five 50MVAR shunt reactor

5. Two 63MVAR bus reactor

6. 15 lighting tower

7. SF6 circuit breaker

8. Capacitor voltage transformer(CVT)

9. Current transformer(CT)

In switchyard one room for mulsi fire system and one for generator system is also present.

In 400kv switchyard following lines are present for incoming and outgoing power:

i) Unnao to Lucknow

ii) Unnao to Bareily-1

iii) Unnao to Bareily-2

iv) Unnao to Agara

v) Unnao to Panki kanpur

vi) Unnao to Anpara

vii) Unnao to PGCIL-1

viii) Unnao to PGCIL-2

ix) One bus bar for 400/220kv 315MVA ICT-1 & ICT-2 line.

The buses of 400kv switchyard charged by Unnao - Anpara line. This line is the India’s first line

which is made for 765KV transmission. But till today it is charged by 400kv. In futureit work on 765

kv .

From 220kv switchyard two lines for Lucknow and two lines for Panki Kanpur comes out.

In whole switchyard following main equipment are used:

i) One 400kv transfer bus control bus coupler.

ii) Two 100MVA 220/132 KV auto transformer manufactured from BHEL.

iii) Two 315 MVA 400/220 KV auto transformer manufactured from BHEL.

iv) Five 50 MVAR shunt reactor manufactured from BHEL.

v) Two 63 MVAR bus reactor manufactured from HITACTI.

vi) Circuit breaker from CGL.

vii) Isolators from S&S.

viii) Current transformer from WS and CGL.

ix) CVT

x) Wave trap

xi) Lighting arrester

xii) Surge capacitor

Single line diagram of unnao substation

Brief Description

Of all

Outdoor Equipment

Power transformer:

Various types of transformers have been provided at 220& 400 KV Substation from UPPTCL.

Capacity and voltage ratio wise 100 MVA , 315MVA & 160 MVA and 220/132/11 kV. 400/220 kV,

These transformers are of TELK, BHEL, GEC, NGEF, C & G, Hitachi and Bharat Bijlee make and

have most of the features common except few accessories which may be different. In this

substation all transformers made by BHEL. These

transformers have following main components:

1. MAIN CORE & WINDING.

2. BUSHING :-

(a) 220 kV High voltage bushings:

Condenser type bushings with insulating body and central conducting tube-

backelised with paper wound capacitor have been provided. Innermost of the

capacitor layer is electrically connected to the tube and outermost to the mounting

flange on insulating body. The central tube insulating body and mounting flange are

oil filled assembled. High dielectric Strength oil is filled between central tube and

insulating body. Oil level indicators are provided on the bushing.

(b) 132 kV Medium voltage bushing:

These bushing are also of condenser type and are of similar construction as in the

case of 220 kV bushing in 200 MVA transformers.

In 40 & 20 MVA transformers 132 kV bushings are also of oil filled type in which oil is

filled up when the transformer tank is topped up. Necessary air vent screws are

provided on top of the bushings for release of trapped air at the top of oil fitting.

(c) 66 kV. 33 kV. & 11 kV. Bushings:

These are oil filled bushing and simpler in construction.

3. TAP CHANGER:

The transformers have been provided with on load tap changer, which consists of diverter

switch installed in an oil compartment separated from transformer oil and the tap selector

mounted below it. The tap changer is attached to the transformer cover by means of tap

changer head, which also serves for connecting the driving shaft and the oil conservator.

4. PROTECTIVE RELAYS:

Generally there are two protective buchholz relays, one for main transformer tank and other

for tap changer.

In 40MVA GEC transformers oil surge relay has also been provided in tap changer.

5. PRESSURE RELIEF VALVE:

40 MVA GEC make transformers have been provided with pressure relief valve which

operates in case of sudden pressure formation in side the transformer.

6. COOLING SYSTEM :

100 MVA transformers have been provided with cooling bank installed on separate

structures. These cooling banks have provided with to groups of fans and 2 nos. pumps.

These fans and pumps automatically operate, depending upon the settings of winding

temperature Indicator.

7. TERTIARY BUSING:

100 MVA transformers have been provided with tertiary bushing connected with 11 kv

capacitor and lighting arrestor t absorb switching surges.

ELECTRICAL PROTECTION :

The following electrical protection have been provided on the transformers :-

(i) Differential Protection

(ii) Restricted Earth Fault

(iii) Winding temp high

(iv) Oil temp high

(v) Pressure relief valve

(vi) Oil surge relay

(vii) Over current relay

(viii) Local Breaker Back up protection

(ix) Surge arrestors on HV, MV & LV sides.

The main Tank - The transformer is transported on trailor to substation site and as far as

possible directly unloaded on the plinth. Transformer tanks up to 25 MVA capacity are

generally oil filled, and those of higher capacity are transported with N2 gas filled in them

+ve pressure of N2 is maintained in transformer tank to avoid the ingress of moisture. This

pressure should be maintained during storage; if necessary by filling N2 Bushings -

generally transported in wooden cases in horizontal position and should be stored in that

position. There being more of Fragile material, care should be taken while handling them.

Rediators – These should be stored with ends duly blanked with gaskets and end plates to

avoid in gross of moisture, dust, and any foreign materials inside. The care should be taken

to protect the fins of radiators while unloading and storage to avoid further oil leakages.

The radiators should be stored on raised ground keeping the fins intact. Oil Piping. The Oil

piping should also be blanked at the ends with gasket and blanking plates to avoid in gross

of moisture, dust, and foreign

All other accessories like temperature meters, oil flow indicators, PRVs, buchholtz relay;

oil surge relays; gasket ‘ O ‘ rings etc. should be properly packed and stored indoor in store

shed. Oil is received in sealed oil barrels . The oil barrels should be stored in horizontal

position with the lids on either side in horizontal position to maintain oil pressure on them

from inside and subsequently avoiding moisture and water ingress into oil. The

transformers are received on site with loose accessories hence the materials should be

checked as per bills of materials.

The transformers that are used in Unnao substation have following specification:

Specification of 100 MVA 220/132/11 KV 3-Φ auto transformer:

Types of cooling ONAN ONAF OFAF

Rating of H.V. & I.V.(MVA) 60 80 100

Rating of L.V. (MVA) 18 24 30

Line current H.V.(Amps) 157.4 209.9 262.4

Line current I.V. (Amps) 262.4 349.9 437.4

Line current L.V. (Amps) 944.8 1259.7 1574.6

No load voltage H.V. 220KV

No load voltage I.V. 132KV

No load voltage L.V. 11KV

Temp. Rise winding ˚C 55 55 60

[ Above ambient of 50 ˚C ]

Temp. rise oil ˚C 50 [ Above ambient of 50 ˚C ]

Phase 3

Frequency 50Hz

Connection symbol YNa0d11

Insulation level:

H. V. - LI950 AC395-AC38

L. V. - LI170 AC70

I. V. - LI550-AC230-AC38

Core & winding (Kg.) 54000

Weight of oil (Kg.) 39410

Total weight (Kg.) 127995

Oil quantity (liters) 45300

Transport weight (Kg. ) 69000

Untanking weight (Kg.) 54000

Specification of 315 MVA 400/220 KV 3- Φ auto transformer:

Types of cooling ONAN ONAF OFAF

Rating of H.V. & I.V.(MVA) 189 252 315

Rating of L.V. (MVA) 105 105 105

Line current H.V.(Amps) 272.76 363.68 454.6

Line current I.V. (Amps) 495.96 661.28 826.6

Line current L.V. (Amps) 837.0 1857.0 1837.0

No load voltage H.V. 400KV

No load voltage I.V. 220KV

No load voltage L.V. 33KV

Temp. Rise winding ˚C 55 55 60

[ Above ambient of 50 ˚C ]

Temp. rise oil ˚C 50 [ Above ambient of 50 ˚C ]

Phase 3

Frequency 50Hz

Connection symbol YNa0d11

Insulation level:

H. V. - LI950 AC395-AC38

L. V. - LI170 AC70

I. V. - LI550-AC230-AC38

Oil quantity (liters) : 84550 liter

Impedance volt

315 MVA Base

H.V. position 9/L.V. 71.81%

H.V. position 9/I.V. 11.47%

I.V./L.V. 67.92%

Vector group:

1U

N 2U

3U

3V

2W

1W 2V

1V

3W

Isolators:

In electrical engineering, a disconnecter or isolator switch is used to make sure that an electrical

circuit can be completely de-energized for service or maintenance. Such switches are often found

in electrical distribution and industrial applications where machinery must have its source of driving

power removed for adjustment or repair. High-voltage isolation switches are used in electrical

substations to allow isolation of apparatus such as circuit breakers and transformers, and

transmission lines, for maintenance.

In the substation following type isolators are used for the protection:

Horizontal break center rotating double break isolator:

This type of construction has three insulator stacks per pole. The two one each side is fixed and

one at the center is rotating type. The central insulator stack can swing about its vertical axis

through about 900C. The fixed contacts are provided on the top of each of the insulator stacks on

the side. The contact bar is fixed horizontally on the central insulator stack. In closed position, the

contact shaft connects the two fixed contacts. While opening, the central stack rotates through

900C, and the contact shaft swings horizontally giving a double break.

The isolators are mounted on a galvanized rolled steel frame. The three poles are interlocked by

means of steel shaft. A common operating mechanism is provided for all the three poles. One pole

of a triple pole isolator is closed position.

Pantograph isolator:

illustrates the construction of a typical pantograph isolator. While closing, the linkages of

pantograph are brought nearer by rotating the insulator column. In closed position the upper two

arms of the pantograph close on the overhead station bus bar giving a grip. The current is carried

by the upper bus bar to the lower bus bar through the conducting arms of the pantograph. While

opening, the rotating insulator column is rotated about its axis. Thereby the pantograph blades

collapse in vertical plane and vertical isolation is obtained between the line terminal and

pantograph upper terminal.

Pantograph isolators cover less floor area. Each pole can be located at a suitable point and the

three poles need not be in one line, can be located in a line at desired angle with the bus axis.

Isolator with earth switches (ES):

The instrument current transformer (CT) steps down the current of a circuit to a lower value and is

used in the same types of equipment as a potential transformer. This is done by constructing the

secondary coil consisting of many turns of wire, around the primary coil, which contains only a few

turns of wire. In this manner, measurements of high values of current can be obtained. A current

transformer should always be short-circuited when not connected to an external load. Because the

Pantograph isolator

magnetic circuit of a current transformer is designed for low magnetizing current when under

load, this large increase in magnetizing current will build up a large flux in the magnetic

circuit and cause the transformer to act as a step-up transformer, inducing an excessively

high voltage in the secondary when under no load.

The main use of using the earth switch (E/S) is to ground the extra voltage which may b

dangerous for any of the instrument in the substation.

Circuit breaker:

A circuit breaker is an automatically-operated electrical switch designed to protect an electrical

circuit from damage caused by overload or short circuit. Its basic function is to detect a fault

condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse,

which operates once and then has to be replaced, a circuit breaker can be reset (either manually

or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from

small devices that protect an individual household appliance up to large switchgear designed to

protect high voltage circuits feeding an entire city. Once a fault is detected, contacts within the

circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using

something such as springs or compressed air) contained within the breaker is used to separate

the contacts, although some of the energy required may be obtained from the fault current itself.

Small circuit breakers may be manually operated; larger units have solenoids to trip the

mechanism, and electric motors to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive heating, and must also

withstand the heat of the arc produced when interrupting the circuit. Contacts are made of copper

or copper alloys, silver alloys, and other materials. When a current is interrupted, an arc is

generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the

gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers

use vacuum, air, insulating gas or oil as the medium in which the arc forms. Different techniques

are used to extinguish the arc including:

Lengthening of the arc

Intensive cooling (in jet chambers)

Division into partial arcs

Zero point quenching (Contacts open at the zero current time crossing of the AC waveform,

effectively breaking no load current at the time of opening. The zero crossing occures at

twice the line frequency i.e. 100 times per second for 50Hz ac and 120 times per second for

60Hz ac )

Connecting capacitors in parallel with contacts in DC circuits

Finally, once the fault condition has been cleared, the contacts must again be closed to restore

power to the interrupted circuit.

Types of circuit breaker:

Many different classifications of circuit breakers can be made, based on their features such as

voltage class, construction type, interrupting type, and structural features.

Electrical power transmission networks are protected and controlled by high-voltage breakers. The

definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or

higher, according to a recent definition by the International Electrotechnical Commission(IEC).

High-voltage breakers are nearly always solenoid-operated, with current sensing protective relays

operated through current transformers. In substations the protection relay scheme can be

complex, protecting equipment and busses from various types of overload or ground/earth fault.

High-voltage breakers are broadly classified by the medium used to extinguish the arc.

Bulk oil

Minimum oil

Air blast

Vacuum

SF6

In unnao substation only SF6 circuit breaker is used. The breaker uses SF6 (Sulpher Hexa

fluoride) gas for arc extinction purpose. This gas has excellent current interrupting and insulating

properties, chemically, it is one of the most stable compound in the pure state and under normal

condition it is physically inert, non-flammable, non toxic and odorless and there is no danger te

personnel and fire hazard. It's density is about. 5 times that of air insulating strength is about 2-3

times that of air and exceeds that of oil at 3 Kg/Cm pressure.

SF6 breaker called as maintenance free breaker, has simple construction with few moving parts:

The fission products created during breaking and not fully recombined are, either precipitated as

metallic fluoride or absorbed by a static filter which also absorbs the residual moisture.

Since no gas is exhausted from the breaker and very little compressed air is required for

operation, noise during the operation is also very Jess.

Since SF6 gas is inert and stable at normal temperature, contacts do not settler from oxidization or

other chemical reactions, whereas in air or oil type breakers oxidation of contacts would cause

high temperature rise. SF6 gas circuit breakers, designed to conform to the same standards as air

or oil breakers, but in operation it is possible to get better service even at higher fault levels.

Sulphur hexafluoride gas is prepared by burning coarsely crushed roll sulphur in the fluorine gas,

in a steel box, provided with staggered horizontal shelves, each bearing about 4 kg of sulphur. The

steel box is made gas tight. The gas thus obtained contains other fluorides such as S2F10, SF4

and must be purified further SF6 gas generally supplier by chemical firms. The cost of gas is low if

manufactured in large scale.

During the arcing period SF6 gas is blown axially along the arc. The gas removes the heat from the arc by axial convection and radial dissipation. As a result, the arc diameter reduces during the decreasing mode of the current wave. The diameter becomes small during the current zero and the arc is extinguished. Due to its electro negativity, and low arc time constant, the SF6 gas regains its dielectric strength rapidly after the current zero, the rate of rise of dielectric strength is very high and the time constant is very small.

Fig: SF6 circuit breaker.

Gas circuit breaker: high voltage side

Type 220-SFM-20B

Voltage rating: 220kv

Rated lightening impulse withstand voltage: 1050 kVp

Rated short circuit breaker current: 40 kV

Rated operating pressure: 16.5 kg/ cm2g

First pole to clear factor 1.3

Rated duration of short circuit current is 40 kA for 30 sec.

Rated ling charging breaker breaking current 125 Amp

Rated voltage 245 kV

Rated frequency 50 Hz

Rated normal current 1600 Amp

Rated closing voltage: 220 V dc

Rated opening voltage 220 V dc

Main parts:

(a) Power circuit(b) Control circuit

Gas circuit breaker: low voltage side

Type 120-SFM-32A

Voltage rating: 220kv

Rated lightening impulse withstand voltage: 650 kVp

Rated short circuit breaker current: 31.5 kV

Rated operating pressure: 15.5 kg/ cm2g

Lightning arrester:

High Voltage Power System experiences overvoltages that arise due to natural lightning or the

inevitable switching operations. Under these overvoltage conditions, the insulation of the power

system equipment are subjected to electrical stress which may lead to catastrophic failure.

Broadly, three types of overvoltages occur in power systems: (i) temporary over-voltages,(ii)

switching overvoltages and(iii) lightning overvoltages.

The duration of these overvoltages vary in the ranges of microseconds to sec depending upon the

type and nature of overvoltages. Hence, the power system calls for overvoltage protective devices

to ensure the reliability.

Conventionally, the overvoltage protection is

obtained by the use of lightning / surge arresters .

Under normal operating voltages, the impedance

of lightning arrester, placed in parallel to the

equipment to be protected, is very high and allow

the equipment to perform its respective function.

Whenever the overvoltage appears across the

terminals, the impedance of the arrester

collapses in such a way that the power system

equipment would not experience the overvoltage.

As soon as the overvoltage disappears, the arrester recovers its impedance back. Thus the

arrester protects the equipment from overvoltages.

The technology of lightning arresters has undergone major transitions during this century. In the

early part of the century, spark gaps were used to suppress these overvoltages. The silicon

carbide gapped arresters replaced the spark gaps in 1930 and reigned supreme till 1970. During

the mid 1970s, zinc oxide (ZnO) gapless arresters, possessing superior protection characteristics,

replaced the silicon carbide gapped arresters. Usage of ZnO arresters have increased the

reliability of power systems many fold.

Current transformer:

Current Transformers (CT’s) are instrument transformers that are used to supply a reduced value of current to meters,protective relays, and other instruments. CT’s provide isolation from the high voltage primary, permitgrounding of the secondary for safety, and step-down the magnitude of the measured current to a value that can besafely handled by the instruments.

TECHNICAL SPECIFICATION FOR CURRENT TRANSFORMERS

1.0 GENERAL

1.1 This specification covers manufacture, test, & supply of LT Current transformers of class 0.5 accuracy.

1.2 The CTs shall be suitable for metering purpose.

2.0 TYPE:

2.1 The CTs shall be of ring type or window type (bar type or bus-bar type CT’s shall not be accepted).

2.2 The secondary leads shall be terminated with Tinned Cooper rose contact terminals with arrangements for sealing purposes.

2.3 Polarity (both for primary and second leads) shall be marked.

2.4 The CTs shall be varnished, fiberglass tape insulated or cast resin, air-cooled type. Only super enameled electrolytic grade copper wires shall be used.

2.5 The CTs shall conform to IS 2705:Part-I & II/IEC:185 with latest amendments.

3.0 TECHNICAL DETAILS:

3.1 Technical details shall be as given below:

1. Class of Accuracy 0.52. Rated Burden 5.00 VA3. Power Frequency Withstand

Voltage 3KV

4. Highest System Voltage 433 V5. Nominal System Voltage 400 V6. Frequency 50 Hz7. Supply System 3 Ph. Solidly grounded Neutral

System

3.2 Transformation ratio shall be specified from the following standard ratings as per requirement :

Ratio 50/5 150/5 300/5 400/5 1000/5

(Secondary with 1 A may be specified by the utility incase the same is desired.)

3.3 Bore diameter of the CT shall not be less than 40 mm. Ring type CTs shall have suitable

clamp to fix the CT to panel Board, wherever required.

3.4 The limits of current error and phase angle displacement as per IS:2705 at several defined

percentage of rated current are:

Accuracy

Class

% Ratio error at % of

rated current

Phase displacement in minutes

at % of rated current

5 20 100 120 5 20 100 120

0.5 1.5 0.75 0.5 0.5 90 45 30 30

Note : Current error and phase displacement at rated frequency is required to be as above when

the secondary burden from 25% to 100% of the rated burden i.e. 50 V A.

3.5 Rated extended primary current shall be 120% of rated primary Current in accordance with

IS:2705 Pt-II.

3.6 Rated ISF (Instrument Security Factor) shall be declared by the manufacturer & marked on the

CT.

3.7CT’s shall be made with good engineering practices. Core winding shall evenly spread stress &

avoid stress concentration at any one point. Cast resin CT’s sha;; be processed by hot curing

method under controlled vacuum conditions.

3.8The base shall be of hot dip galvanized steel.

4.0 TESTS:

4.1. TYPE TESTS:

4.1.1 Copies of all type tests as per IS.2705 Part-I and II including short time current &

temperature rise tests in NABL accredited laboratory shall be submitted and got approved before

commencement of supply.

4.2 ROUTINE TESTS:

4.2.1 The supplier shall conduct all the routine tests such as Ratio test, phase angle error test for

0.5 accuracy class as per IS 2705 Part I & II.

4.3 Commissioning test :

4.3.1 In accordance with IS:2705, Power frequency test on primary winding shall be carried out

after erection on site on sample basis.

5.0 Marking :

5.1 The CTs shall have marking and nameplate as per IS 2705 in addition to class of insulation &

ISF. The markings shall be indellible. The nameplate shall be securely fixed to the body of the CT.

6.0 PACKING:

6.1 Each CT shall be securely packed so as to withstand rough handling during transit and

storage.

7.0 QUALITY ASSURANCE PLAN:

7.1 The requirements of clause 29.0 of Section – I of

main specification for Energy Meters shall apply.

Wave trap

Current transformer

Current transformer rating table for all cores:

CT Naming

Ratio BurdenVA

Knee pointVoltage V(min.)

Mag. Current

atKpv

mA(max)

Class Sec.resistance Ω

Purpose

1CT 1N 500/1 - 300 40AT Vk/2 PS 5 REF

1CT 1U11CT 1V11CT

1W1

500/1 - 300 40AT Vk/2 PS 5 REF

1CT 2U11CT 2V11CT 2W1

500/1 - 300 40AT Vk/2 PS 5 REF

1CT 3U11CT 3V11CT3W1

2000/1 - 600 30AT Vk/2 PS 5 Differenti

al

2CT 3U12CT 3V12CT 3W1

2000/1 - 300 40AT Vk/2 PS 5 Spare

3CT 3U13CT 3V13CT 3W1

2000/1 30VA - - 1.0 5 Metering

WCT

1U2

198/3.0

-2.5

1.7VA - - 5 - WTT+RT

D

Capacitor voltage transformer:

In high and extra high voltage transmission systems, capacitor voltage transformers (CVTs) are

used to provide potential outputs to metering instruments and protective relays. In addition, when

equipped with carrier accessories, CVTs can be used for power line carrier (PLC) coupling.

A capacitor voltage transformer (CVT) is a transformer used in power systems to step-down extra

high voltage signals and provide low voltage signals either for measurement or to operate a

protective relay. In its most basic form the device consists of three parts: two capacitors across

which the voltage signal is split, an inductive element used to tune the device to the supply

frequency and a transformer used to isolate and further step-down the voltage for the

instrumentation or protective relay. The device has at least four terminals, a high-voltage terminal

for connection to the high voltage signal, a ground terminal and at least one set of secondary

terminals for connection to the instrumentation or protective relay. CVTs are typically single-phase

devices used for measuring voltages in excess of one hundred kilovolts where the use of voltage

transformers would be uneconomical. In practice the first capacitor, C1, is often replaced by a

stack of capacitors connected in series. This results in a large voltage drop across the stack of

capacitors that replaced the first capacitor and a comparatively small voltage drop across the

second capacitor, C2, and hence the secondary terminals.

INSULATING SYSTEMS:

The external insulation is provided by the porcelain housing and coordinated with the capacitor

stack, consisting of virtually identical elements so that the axial voltage distribution from the line

terminal to ground is essentially uniform. The capacitor elements have a mixed dielectric material

consisting of alternating layers of polypropylene film and Kraft paper. The Kraft paper layers serve

as a wicking agent to ensure homogenous synthetic oil impregnation. The electromagnetic unit

(EMU) is housed in an oil-filled tank at the base of the capacitor stack. Mineral oil is employed as

the insulating medium instead of air because of its superior insulating and heat transfer properties.

The use of an oil-filled base tank removes the need for space heaters in the secondary terminal

box as this area is warmed by heat transfer from the insulating oil. This results in a more reliable

and cost effective design.

INSULATING OIL:

We use insulating oils with excellent dielectric strength, aging, and gas absorbing properties. The

synthetic oil used for the capacitor units possesses superior gas absorption properties resulting in

exceptionally low partial discharge with high inception/extinction voltage ratings. The oil used for

the EMU is premium naphthenic mineral oil. The oil is filtered, vacuum dried and degassed within

house processing. It contains no PCB.

CAPACITOR STACK:

The capacitor stack is a voltage divider which provides a reduced voltage at the intermediate

voltage bushing for a given voltage applied at the primary terminal. The capacitor stack is a multi-

capacitor-unit assembly. Each unit is housed in an individual insulator. A cast aluminum

cover is on top of the upper capacitor assembly and is fitted

with an aluminum terminal. An adapter for mounting a line trap

on top of the CVT can be provided with an optional (and

removable) HV terminal.

The capacitor units are mechanically coupled together by

means of stainless steel hardware passing through the

corrosion resistant cast aluminum housing. The mechanical

connection also establishes the electrical connection between

capacitor units. This facilitates field assembly of the CVT

.

1 - Primary terminal

2 - Cast aluminum bellow housing

3 - Stainless steel expansion bellow

4 - Compression spring

5 - Insulated voltage connection

6 - Capacitor elements

7 - Insulator (porcelain or composite)

8 - Voltage divider tap connection

9 - Cast-epoxy bushing

10 - HF terminal connection

11 - Ferro-resonance suppression device

12 - Secondary terminals

13 - Oil level sight-glass

14 - Aluminum terminal box

15 - Intermediate transformer

16 - Oil/air block

17 - Oil sampling device

18 - Compensating reactor

19 - Aluminum cover plate

1 - Primary terminal

2 - Cast aluminum bellow housing

3 - Stainless steel expansion bellow

4 - Compression spring

5 - Insulated voltage connection

6 - Capacitor elements

7 - Insulator (porcelain or composite)

8 - Voltage divider tap connection

9 - Cast-epoxy bushing

10 - HF terminal connection

11 - Ferro-resonance suppression device

12 - Secondary terminals

13 - Oil level sight-glass

14 - Aluminum terminal box

15 - Intermediate transformer

16 - Oil/air block

17 - Oil sampling device

18 - Compensating reactor

19 - Aluminum cover plate

PRINCIPLE CIRCUIT DIAGRAM

Wave trap:

1. High Voltage terminal2. Compensation reactor3. Intermediate voltage transformer4. Ground terminal5. Ferro-resonance suppression device6. Damping resistor7. Carrier (HF) terminal (optional)8. Overvoltage protective device9. Secondary terminals10. Link, to be opened for test purposes

Line trap also is known as Wave trap. What it does is trapping the high frequency communication

signals sent on the line from the remote substation and diverting them to the telecom/teleportation

panel in the substation control room (through coupling capacitor and LMU).

This is relevant in Power Line Carrier Communication (PLCC) systems for communication among

various substations without dependence on the telecom company network. The signals are

primarily teleportation signals and in addition, voice and data communication signals.

The Line trap offers high impedance to the high frequency communication signals thus obstructs

the flow of these signals in to the substation busbars. If there were not to be there, then signal loss

is more and communication will be ineffective/probably impossible.

Construction:

1. Main coil:

The main coil winding are encapsulated by winding continuous filament fiberglass

That has been impregnated with a specially selected epoxy resin harden system. The epoxy resin

fiberglass composite is then curved according to a programmed temperature Schedule.

2. Tuning pack:

Tuning pack is connected in parallel with the main coil to provide a high

impedance to the desired carrier frequency.

3. Lighting arresters:

The line traps are protected by a lighting arrestors against high voltage surges

caused by atmospheric effects or switching operations.

Protective relays:

Protective relaying is one of the several features of power system design.

Every part of the power system is protected. The protective relaying is used to give an alarm or to

cause prompt removal of any element of power system from service when hat element behave

abnormally.

The relays are compact and self contained devices which can sense abnormal conditions.

Whenever abnormal condition occur , the relays contacts get closed. This in turn closes the trip

circuit of a circuit breaker.

For switchyard protections following type relays are used:

1. Overcurrent relay

2. Earth fault relay

3. REF relay

4. Differential relay

5. Directional relay

6. Over flux relay

7. Buchoolz relay

8. IDMT relay

Differential relay

O/F protection +FFR

Group A trip relay

CB trouble relay

Group B trip relay

DR earth switch relay

O/C protection relays

Breaker failure relay

RES E/F +O/LProtection relay

Restricted earth fault protection relay:

The REF protection method is a type of "unit protection" applied to transformers or generators and

is more sensitive than the method known as differential protection.

An REF relay works by measuring the actual current flowing to earth from the frame of the unit. If

that current exceeds a certain preset maximum value of milliamps (mA) then the relay will trip to

cut off the power supply to the unit.

Differential protection can also be used to protect the windings of a transformer by comparing the

current in the power supply's neutral wire with the current in the phase wire. If the currents are

equal then the differential protection relay will not operate. If there is a current imbalance then the

differential protection relay operates.

REF protection is applied on transformers in order to detect ground faults on a given winding more

sensitively than differential protection.

Directional relay:

Directional relays have protection zones that include all of the power system situated in only one

direction from the relay location. (This is in contrast to magnitude relays which are not directional,

i.e., they trip based simply on the magnitude of the relay.

Consider the one-line diagram in Fig. 1.

Fig. 1

If the relays R1 and R2 in Fig. 1 are directional relays, then

- R1 “looks” to the left but not to the right, and

- R2 “looks” to the right but not to the left.

In order to understand how the directional relay works, first, consider that R2 measures the

phasors V2 and I23. Now define the following parameters associated with Fig. 1:

L23: length of circuit 2-3.

x: distance from R2 to a fault on circuit 2-3.

λx=x/L23: the fraction of the circuit length between the relay R2 and the fault at point x.

I23: the current in circuit 2-3 resulting from the fault x on circuit 2-3 (a phasor).

V2: the bus 2 voltage (a phasor).

Z23: total series impedance of circuit 2-3.

If a fault occurs on circuit 2-3, at point x, then the fraction of total circuit length is λx. If the circuit

has uniform impedance per unit length, then the impedance between the relay R2 and the fault

point is λxZ23, and with the bus 2 voltage being V2, the current flowing into circuit 2-3 from bus 2 is:

(1)

But recall that for transmission lines, it is generally the case that R<<X, and therefore

(2)

In that case, eq. (1) becomes:

(3)

Recognizing that 1/j=-90°, eq. (3) becomes:

(4)

Therefore I23 lags V2 by 90°.

Now if the fault occurs on circuit 1-2, at point y in Fig. 1, we can repeat the same analysis as eqs.

(1)-(4), except for point y, where we use λy=y/L12, The result will be

(5)

But R2 measures I23, not I21. Reference to Fig. 1 results in the conclusion that

(6)

Therefore, in this case, I23 leads V2 by 90°.

From this simple analysis, we can establish a logic for the directional relay R1.

Define θ23 as the angle of the phasor I23, i.e.,

(7)

Then if we

trip when current exceeds pickup & θ23=-90°and

block if θ23=+90°,

the relay will be directional.

In reality, of course, the circuits do have resistance, and so eq. (3), for a fault at point x, should be

(8)

And eq. (6), for a fault at point y, should be:

(9)

The current-plane representing the associated relay logic is in Fig. 2:

Fig. 2

The tripping logic can be stated for R2 as

-180<θ23<0, and |I23|>Ip Trip

0<θ23<180, or |I23|<Ip Block

where Ip is the pickup.

Buchholz relay:

In the field of electric power distribution and transmission, a Buchholz relay, also called a gas relay or a sudden pressure relay, is a safety device mounted on some oil-filled power transformers and reactors, equipped with an external overhead oil reservoir called a conservator. The Buchholz Relay is used as a protective device sensitive to the effects of dielectric failure inside the equipment.

The relay has two different detection modes. On a slow accumulation of gas, due perhaps to slight overload, gas produced by decomposition of insulating oil accumulates in the top of the relay and forces the oil level down. A float switch in the relay is used to initiate an alarm signal that also serves to detect slow oil leaks.

If an arc forms, gas accumulation is rapid, and oil flows rapidly into the conservator. This flow of oil operates a switch attached to a vane located in the path of the moving oil. This switch normally will operate a circuit breaker to isolate the apparatus before the fault causes additional damage. Buchholz relays have a test port to allow the accumulated gas to be withdrawn for testing.

IDMT relay:

The IDMT relay work on the induction principle, where an aluminum or copper disc rotates

between the poles of electromagnet and damping magnet. The fluxes induce eddy current in the

disc which interact and produce rotational torque. The disc rotates to a point where it operates a

pair of contact that breaks the circuit and removes the fault condition.

Shunt reactor for bus voltage:

In EHV substations, it is a common practice to use breaker switched bus reactors to maintain the

bus voltage within permissible limits under varying load conditions. With the development of

Controlled Shunt Reactor (CSR) which is a thyristor controlled high impedance transformer, a

stable bus voltage can be maintained by providing variable reactive power based on the bus

voltage deviations due to the load variations. The high impedance transformer which is also known

as reactor transformer (RT) can be made to any size without any limitation unlike gapped core

shunt reactors. As a single CSR of large capacity can be realized with suitable control mechanism,

this approach proves to be technically superior and economical compared to the existing practice

of breaker switched bus reactors.

A CSR with a detailed control system is modeled along with a typical EHV system in

PSCAD/EMTDC environment. The study includes the effectiveness of filters introduced in the

tertiary of the reactor transformer in controlling the harmonics generated during partial conduction

of thyristors. The transient and steady state performance of the CSR system for varying system

conditions is studied and the same is compared with the conventional practice. The paper

presents and discusses the results of the study.

Keywords: High impedance transformer, shunt reactor, reactive power, compensation, EHV

systems, voltage control, thyristors. Shunt reactors which are meant to be used for controlling the

bus voltage of sub station are known as bus reactors. These are always connected through a

circuit breaker and switched on or off, based on the voltage variations. In large switching

substations, it is not uncommon to find multiple bus reactors when the total reactor capacity

required is large. Due to limited standard ratings of gapped core shunt reactors, it is necessary to

provide in multiples of standard ratings along with associated bay equipment and space for

accommodating the same. The CSR mentioned above is based on a high impedance transformer

known as Reactor Transformer (RT) with a provision to control from the secondary side through

thyristor valves. As RT of any large capacity can be realized as a single three phase unit or three

single phase units, it is possible to provide variable reactive power support by controlling the firing

angle of the thyristor valves. This continuously variable CSR as bus reactor offers following

advantages.

1. Continuously variable reactive power based on the voltage variation.

2. Fast Response to dynamic conditions like load throw off

3. Reduced losses with optimized reactive power support.

4. Better economy in terms of substation space and auxiliary equipment.

Figure: shunt reactor

Shunt capacitor bank:

Shunt capacitor banks are used to improve the quality of the electrical supply and the efficient

operation of the power system. Studies show that a flat voltage profile on the system can

significantly reduce line losses. Shunt capacitor banks are relatively inexpensive and can be

easily installed anywhere on the network. Shunt capacitor banks (SCB) are mainly installed to

provide capacitive reactive compensation/

Power factor correction. The use of SCBs has increased because they are relatively inexpensive,

easy and quick to install and can be deployed virtually anywhere in the network. Its installation

has other beneficial effects on the system such as: improvement of the voltage at the load, better

voltage regulation (if they were adequately designed), reduction of losses and reduction or

postponement of investments in transmission.

The main disadvantage of SCB is that its reactive power output is proportional to the square of

the voltage and consequently when the voltage is low and the system need them most, they are

the least efficient.

Figure

Clearances At A Glance:

Various clearances required to be maintained as per Indian Electricity Rules and Code of practice etc. during construction of a transmission line are given at appropriate places in various chapters. However, for convenience, the various clearances required to be maintained in the construction of a transmission line at a glance are given in the following table:

TABLE

SI. No.

Particulars Units Clearance required to be maintained for

132 KV 220 KV 400 KV

1. Live Metal Clearance

(a) Suspension Towers mm 1525(0°- 25° swing) 1075(25° - 45° swing)

2130 (0° - 20°) 1675 (20° -50")

2600 (V - string)

(b) Tension towers mm 1525 2130 2600

2. Ground Clearance m 6.1 7.0 8.84

3. Mid Span Clearance in 6.1 8.5 9.0

4. Phase to Phase Clearance

mm 3900 5130 7000

5. Maximum Shielding Angle

30° 30° 30° 20°

6. Power Line Crossing Clearance between Lines

m 3.05 (from other lines of 11 KV to 132KV)

4.58 (from other lines of 11 KV to 220 KV)

6.10 (front other lines of 11-KV to 400KV)

7. Clearance Between lines and Tramway Crossing

m 3.05 3.05 3.05

8. Clearance from Railway^ Track

m 14.60 15.40 17.90

9. Prescribed Corridor for Forest clearance etc.

m 27.60 35.00 52.00

10. Minimum Clearances from Trees

m 4.0 4.6 5.5

11. Clearance over Rivers from HFL

(i) Non Navigable River m 6.1 7.00 8.84

(ii) Navigable River m To be maintained in relation to tallest mast in consolation with navigation authorities

12. Clearances from Buildings

(a) Horizontal m 2.744 3.659 5.489

(b) Vertical m 4.573 5.488 7.318

13. Clearance over Telecommunication lines

m 2.745 3.050 4.880

Power line communication & SCADA system of UPPTCL:

Uttar Pradesh Power Transmission Corporation Ltd. (UPPTCL) has a very large network of high

voltage transmission lines in whole UP (about 24,000Km). Transmission lines transfer power from

power houses to substations and from one substation to many other substations or vice versa.

Power is generated at low Voltage (of the order of 3.3KV to 25KV) and is stepped-up to high

voltage (765KV, 400KV, 220KV & 132KV) for evacuating power into the grid network through

transmission lines.

Transmission of Data

Below in Figure 1, main equipment from substation/power house to its subLDC has been shown in a very simple form.

Figure 1: Transmission of Data from substation/Power house to subLDC

Current Transformers (CTs) and Potential Transformers (PTs), installed on transmission lines, provide inputs to transducers of SIC (Supervisory Interface & Control) & RTU (Remote Terminal Unit) panel. Circuit breakers & isolators' status are extended up to SIC panel. If for such extension extra potential free contacts are not available in the Control Panels, Contact Multiplying Relays (CMRs) are used to provide potential free contacts. The output of RTU is connected to the communication equipment, through Modem. In between substation & subLDC, a communication link has been shown. Telephone exchanges are connected with the communication equipment. Such communication links can be of any type. UPPTCL has got its own three different type of communication systems, i.e. PLCC (Power Line Carrier Communication), microwave and fibre-optic. PLCC system is more prevalent in UPPTCL. Modem output at receive side is connected with the CFE (Communication End Frame). Its output is connected with data takes over. Each RTU is automatically polled by Server of subLDC to obtain each data of repeats at least once in 10 sec and is stored in the database of subLDC. This data is processed in database formats and is retrieved for different applications. These formats or graphics are displayed or printed as per requirement. At subLDC, System Control Officers use this data to monitor and analyze position of the grid.

Below in Figure 2, main equipment from subLDC to SLDC, Lucknow has been shown in a very simple form.

A systematically combined/processed data of all RTUs, in server of subLDC, is transmitted to

SLDC Lucknow. This data in the form of 64Kb/s signal is sent through multiple paths/channels.

Presently four channels are used. For this purpose 'Routers' are used. Routers basically work as

modem but is has multiple paths for LAN, WAN or internet, etc. In UPPTCL, for transmission of

data, from subLDC to SLDC, only wideband communication system (microwave or fibre-optic

links) is being used. In SLDC, data from all other subLDCs is also received simultaneously and are

processed for different purposes and applications. From Inter-Control Centre Communications

Protocol (ICCP) Servers of SLDC, complete data of all subLDCs is sent to NRLDC, New Delhi

through wideband communication system. This way communication plays a major role in grid

management.

Communication for Power System

Following are mainly three inter-related areas of functions in UPPTCL for management of power

system:

A) Telecommunication

B) SCADA- Supervisory Control and Data Acquisition System.

C) EMS- Energy Management System

A) TELECOMMUNICATION

There are three different types of telecommunication systems in UPPTCL i.e.

i. Microwave Communication System,

ii. Fibre-optic Communication System,

iii. PLCC-Power Line Carrier Communication.

Voice Frequency (VF) channels of all these systems have been integrated/interconnected to make

a hybrid communication system. Microwave & Fibre Optic are multi-channels communication

systems and are also called 'Wideband communication system'. PLCC is single channel

communication system.

SCADA SYSTEM

In SCADA system measured values, i.e. analogue (measured value) data (MW, MVAR, V, Hz

Transformer tap position), and Open/Closed status information, i.e. digital data (Circuit

Breakers/Isolators position i.e. on/off status), are transmitted through telecommunication channels

to respective sub-LDCs. For this purpose Remote Terminal Units (RTUs) at 400KV, 220KV and

few important 132KV sub-stations have been installed. System values & status information below

132 KV have not been picked up for data transmission, except for 33KV Bus isolator position and

LV side of generators. Secondary side of Current Transformers (CT) and Potential Transformer

(PT) are connected with 'Transducers'. The output of transducers is available in dc current form (in

the range of 4mA to 20mA). Analogue to digital converter converts this current into binary pulses.

Different inputs are interleaved in a sequential form and are fed into the CPU of the RTU. The

output of RTU, containing information in the form of digital pulses, is sent to subLDC through

communication links. Depending upon the type of communication link, the output of RTU is

connected, directly or through Modem, with the communication equipment. At subLDC end, data

received from RTU is fed into the data servers. In general, a SCADA system consists of a

database, displays and supporting programmes. In UPPTCL, subLDCs use all major functional

areas of SCADA except the 'Supervisory Control/Command' function. The brief overview of major

'functional areas' of SCADA system is as below:

1. Communications - Sub-LDC's computer communicates with all RTU stations under its

control, through a communication system. RTU polling, message formatting, polynomial

checking and message retransmission on failure are the activities of 'Communications'

functional area.

2. Data Processing - After receipt of data through communication system it is processed.

Data process function has three sub-functions i.e. (i) Measurements, (ii) Counters and (iii)

Indications.

'Measurements' retrieved from a RTU are converted to engineering units and linearised, if

necessary. The measurement are then placed in database and are checked against various

limits which if exceeded generate high or low limit alarms.

The system has been set-up to collect 'Counters' at regular intervals: typically 5 or 10

minutes. At the end of the hour the units is transferred into appropriate hour slot in a 24-

hour archive/history.

'Indications' are associated with status changes and protection. For those statuses that are

not classified as 'alarms', logs the change on the appropriate printer and also enter it into a

cyclic event list. For those statuses, which are defined as an 'alarms' and the indication

goes into alarm, an entry is made into the appropriate alarm list, as well as in the event list

and an audible alarm is generated in the sub-LDC.

3. Alarm/Event Logging - The alarm and event logging facilities are used by SCADA data

processing system. Alarms are grouped into different categories and are given different

priorities. Quality codes are assigned to the recently received data for any 'limit violation'

and 'status changes'. Alarms are acknowledged from single line diagram (or alarm lists) on

display terminal in LDCs.

4. Manual Entry - There is a provision of manual entry of measured values, counters and

indications for the important sub-station/powerhouse, which are uncovered by an RTU or

some problem is going on in its RTU, equipment, communication path, etc.

5. Averaging of Measured Values - As an option, the SCADA system supports averaging of

all analogue measurements. Typically, the averaging of measured values over a period of

15 minutes is stored to provide 24 hours trend.

6. Historical Data Recording (HDR) - The HDR, i.e. 'archive', subsystem maintains a history

of selected system parameters over a period of time. These are sampled at a pre-selected

interval and are placed in historical database. At the end of the day, the data is saved for

later analysis and for report generation.

7. Interactive Database Generation - Facilities have been provided in such a way that an off-

line copy of the SCADA database can be modified allowing the addition of new RTUs,

pickup points and communication channels.

8. Supervisory Control/Remote Command - This function enables the issue of 'remote

control' commands to the sub-station/powerhouse equipment e.g. circuit breaker trip

command. Though, there is provision of this function in this system, yet it is not used in U.P.

As such, related/associated equipment have not been ordered.

9. Fail-over - A 'Fail-over' subsystem is also provided to secure and maintain a database of

devices and their backups. The state of the device is maintained indicating whether it is 'on-

line' or 'failed'. There is a 'backup' system, which maintains database on a backup computer

and the system is duplicated.

SLDC Lucknow has a large and active 'Mimic Board' in its Control room. This mimic board

displays single line diagram of intra State transmission system i.e. grid network of 400KV, 220KV

and important 132KV sub-stations, transmission lines, thermal & hydro powerhouses. Outgoing

feeders, shown in the mimic board, have 'achieve' (LED display) colored indications, of three

different colors, to show the range of power flow at any moment i.e. 'Normal', 'Nominal' or

'Maximum' of its line capacity. UPPTCL's transmission network is expanding rapidly and thereby

number of RTUs is also increasing. For new substations and lines, displays in active and passive

forms are required to be made in the Mimic diagram. But, Mimic Board has a limitation that it

cannot incorporate/add large volume of displays for substations/power houses/transmission lines

in 'active' form due to space constraint and congestion. Due to this Mimic Board is going to be

supplemented with a Video Projection System (VPS) at SLDC, Lucknow in near future. Also in

SLDC & subLDCs, displays of single line diagrams of RTU sub-stations/power house are viewed

on VDUs of large size (21").

Other definitions and terms:

What is OLTC in a transformer:

Onload Tap Changer (OLTC) is used with higher capacity transformers where HT side voltage

variation is frequent and a nearly constant LT is required. OLTC is fitted with

the transformer itself. Multiple tappings from HV windings are brought to the OLTC chamber and

conacted to fixed contacts. Moving contacts rotates with the help of rotating

mechanism having a spindle. This spindle can be rotated manually as well as electrically with a

motor. Motor is connected in such a way that it can rotate in both the directions so as to rotate the

OLTC contacts in clockwise and anticlock-wise direction. Two push buttons are fitted on the LCP

(local control panel) to rotate the motor and hence the OLTC contacts in clockwise and anti-

clockwise direction. This movement of contacts thus controls the

output LV voltage of the transformer. So rotating of OLTC contacts with spindle or push buttons in

this way is a manuall process. In case this process of rotating the OLTC

contacts and hence controlling the LV side voltage is to be done automatically then a RTCC

(Remote Tap Changer Controller) is installed with the transformer HT Panel. The RTCC sends

signals to LCP and LCP in turn rotates the motor as per the signals received from the the RTCC.

Interposing CT:

Transformer differential relays compare the phase and magnitude of the current entering one

winding of the transformer with that leaving via the other winding(s). Any difference in

Phase or magnitude between the measured quantities will cause current to flow through the

operate winding of the relay. If this current exceeds the relay setting, tripping of the

Transformer circuit breakers will be initiated. To enable a comparison to be made, the differential

scheme should be arranged so that the relay will see rated current when the full load current flows

in the protected circuit. In order to achieve this, the line current transformers must be matched to

the normal full load current of the transformer. Where this is not the case it is necessary to use an

auxiliary interposing current transformer to

Provide amplitude correction. The connection of the line CTs should compensate for any phase

shift arising across the transformer. Alternatively the necessary phase correction may also be

provided by the use of an interposing CT.

Local backup protection:

The primary objective of back-up protection is to open all sources of generation to an unclearedfault on the system. To accomplish this objective, an adequate back-up protective system must meet the following functional requirements:

1. It must recognize the existence of all faults which occur within its prescribed zone of protection.2. It must detect the failure of the primary protection to clear any fault as planned.3. In clearing the fault from the system, it must a. Initiate the tripping of the minimum number of circuit breakers.b. Operate fast enough (consistent with coordination requirements) to maintain system stability, prevent excessive equipment damage, and maintain a prescribed degree of service continuity.

Insulators:

Table for insulators string:

Line voltage Single suspension

Single tension Double

suspension

Double tension

132 KV 9 10 2*9 2*10

220KV 14 16 2*14 2*16

400KV 2*21 2*21

Corona ring:

A corona ring, also called anti-corona ring, is a toroid of (typically) conductive material located in

the vicinity of a terminal of a high voltage device. It is electrically insulated. Stacks of more spaced

rings are often used. The role of the corona ring is to distribute the electric field gradient and lower

its maximum values below the corona threshold, preventing the corona discharge.

Corona rings are typically installed on very high voltage power line insulators. Manufacturers

suggest a corona ring on the line end of the insulator for above 230 kV and on both ends for above

500 kV. Corona rings prolong lifetime of insulator surfaces by suppressing the effects of corona

discharge.

Appendix A. Protection diagram .1

B.Protection diagram .2

C.Terminal on tank cover of transformer

References

1 .www.upptcl.com

2 .www.cgl.com

3 .“ Electrical power system” by Ashfaq Hussain

4 .“Switchgear & protection” by U.V.Bakashi