P12x en AP a00 Appl Note

8/13/2019 P12x en AP a00 Appl Note

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Application Notes for MiCOM P12xHigh Impedance Protection

April 2004 P12x/EN AP/A00


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HANDLING OF ELECTRONIC EQUIPMENT

A persons normal movements can easily generate electrostatic potentials of severalthousand volts. Discharge of these voltages into semiconductor devices whenhandling circuits can cause serious damage, which often may not be immediatelyapparent but the reliability of the circuit will have been reduced.

The electronic circuits of AREVA T&D UK Ltd - Energy Automation & Informationproducts are immune to the relevant levels of electrostatic discharge when housed intheir cases. Do not expose them to the risk of damage by withdrawing modulesunnecessarily.

Each module incorporates the highest practicable protection for its semiconductordevices. However, if it becomes necessary to withdraw a module, the followingprecautions should be taken to preserve the high reliability and long life for which theequipment has been designed and manufactured.

1.

Before removing a module, ensure that you are a same electrostatic potentialas the equipment by touching the case.

2. Handle the module by its front-plate, frame, or edges of the printed circuitboard. Avoid touching the electronic components, printed circuit track orconnectors.

3. Do not pass the module to any person without first ensuring that you are bothat the same electrostatic potential. Shaking hands achieves equipotential.

4. Place the module on an antistatic surface, or on a conducting surface which isat the same potential as yourself.

5. Store or transport the module in a conductive bag.More information on safe working procedures for all electronic equipment can befound in BS5783 and IEC 60147-0F.

If you are making measurements on the internal electronic circuitry of an equipmentin service, it is preferable that you are earthed to the case with a conductive wriststrap.

Wrist straps should have a resistance to ground between 500k 10M ohms. If awrist strap is not available you should maintain regular contact with the case toprevent the build up of static. Instrumentation which may be used for makingmeasurements should be earthed to the case whenever possible.

AREVA T&D UK Ltd - Energy Automation & Information strongly recommends thatdetailed investigations on the electronic circuitry, or modification work, should becarried out in a Special Handling Area such as described in BS5783 orIEC 60147-0F.


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Application Notes P12x/EN AP/A00

MiCOM P120, P121, P122, P123 Page 1/20

CONTENTS

1. INTRODUCTION 3

2. USE OF METROSIL NON-LINEAR RESISTORS 5

3. MiCOM P12x RANGE 7

3.1 P12x Application Considerations 8

3.2 Restricted Earth Fault (REF) Applications 8

4. APPLYING THE P12x 9

4.1 Advanced application requirements for through fault stability 9

4.2 Transient stability limit 9

4.3 Steady state stability limit 10

5. TYPICAL SETTING EXAMPLES 10

5.1 Restricted earth fault protection 10

5.2 Stability voltage 10

5.3 Stabilising resistor 10

5.4 Current transformer requirements 10

5.5 Metrosil non-linear resistor requirements 11

6. BUSBAR PROTECTION 12

6.1 Stability voltage 12

6.2 Current setting 12

6.3 Discriminating zone 12

6.4 Check zone 13

6.5 Stabilising resistor 13

6.6 Current transformer requirements 13

6.7 Metrosil non-linear resistor requirements 13

6.8 Busbar supervision 14

6.9 Advanced application requirements for through fault stability 14

6.10 Transient stability limit 14

Figure 1: Principle of high impedance protection 3

Figure 2: Double busbar generating station 15

Figure 3: Phase and earth fault differential protection for generators, motors or reactors 15


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P12x/EN AP/A00 pplication Notes

Page 2/20 MiCOM P120, P121, P122, P123

Figure 4: Restricted earth fault protection for 3 phase, 3 wire system-applicable to star connectedgenerators or power transformer windings 16

Figure 5: Balanced or restricted earth fault protection for delta winding of a power transformerwith supply system earthed 16

Figure 6: Restricted earth fault protection for 3 phase, 4 wire system-applicable to star connectedgenerators or power transformer windings with neutral earthed at switchgear 17

Figure 7: Restricted earth fault protection for 3 phase, 4 wire system-applicable to star connectedgenerators or power transformer windings earthed directly at the star point 17

Figure 8: Phase and earth fault differential protection for an auto-transformer with CTs at theneutral star point 18

Figure 9: Busbar protection simple single zone phase and earth fault scheme 18

Figure 10: Restricted earth fault protection on a power transformer LV winding 19


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MiCOM P120, P121, P122, P123 Page 3/20

1. INTRODUCTION

The application of the P12x numerical overcurrent relays as differential protection formachines, power transformers and busbar installations is based on the high impedancedifferential principle, offering stability for any type of fault occurring outside the protectedzone and satisfactory operation for faults within the zone.

A high impedance relay is defined as a relay or relay circuit whose voltage setting is not lessthan the calculated maximum voltage which can appear across its terminals under theassigned maximum through fault current condition.

It can be seen from Figure 1 that during an external fault the through fault current shouldcirculate between the current transformer secondaries. The only current that can flowthrough the relay circuit is that due to any difference in the current transformer outputs forthe same primary current. Magnetic saturation will reduce the output of a current transformerand the most extreme case for stability will be if one current transformer is completelysaturated and the other unaffected.

Figure 1: Principle of high impedance protection

Calculations based on the above extreme case for stability have become accepted in lieu ofconjunctive scheme testing as being a satisfactory basis for application. At one end thecurrent transformer can be considered fully saturated, with its magnetising impedance Z MBshort circuited while the current transformer at the other end, being unaffected, delivers itsfull current output. This current will then divide between the relay and the saturated currenttransformer. This division will be in the inverse ratio of R RELAY CIRCUIT to (R CTB + 2R L) and, ifRRELAY CIRCUIT is high compared with R CTB + 2R L, the relay will be prevented fromundesirable operation, as most of the current will pass through the saturated currenttransformer.

To achieve stability for external faults, the stability voltage for the protection (V s) must bedetermined in accordance with formula 1. The setting will be dependent upon the maximumcurrent transformer secondary current for an external fault ( I f) and also on the highest loopresistance value from the relaying point (R CT + 2 RL). The stability of the scheme is alsoaffected by the characteristics of the differential relay and the application (e.g. restrictedearth fault, busbar etc). The value of K in the expression takes account of both of theseconsiderations. One particular characteristic that affects the stability of the scheme is theoperating time of the differential relay. The slower the relay operates the longer the spillcurrent can exceed its setting before operation occurs and the higher the spill current thatcan be tolerated.


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Vs > KI f (RCT + 2R L) (1)

Where:

RCT = current transformer secondary winding resistance

RL = maximum lead resistance from the current transformer to the relaying point

I f = maximum secondary external fault current

K = a constant affected by the dynamic response of the relay and the application

Note: When high impedance differential protection is applied to motors orshunt reactors, there is no external fault current. Therefore, the lockedrotor current or starting current of the motor, or reactor inrush current,should be used in place of the external fault current.

To obtain high speed operation for internal faults, the knee point voltage, V K, of the CTsmust be significantly higher than the stability voltage, V s . This is essential so that theoperating current through the relay is a sufficient multiple of the applied current setting.

Ideally a ratio of V K 4Vs would be appropriate for most P12x high impedance applications,but where this is not possible refer to the Advanced Application Requirements for ThroughFault Stability. This describes an alternative method whereby lower values of V s may beobtained.

Typical operating times for different V K /Vs ratios are shown in the following tables:

VK /Vs 2 4 8 16

Typical operating time (ms) 48 37 33 18

Table 1: P122/P123 I >>> element

VK /Vs 2 4 8 16

Typical operating time (ms) 49 38 34 32

Table 2: P120/P121 I >, I >>, I >>> & P122/P123 I >, I >> elements

These times are representative of system X/R ratios up to 120 and a fault level of 5 I s orgreater. This is with the exception of the P120 and P121 elements where the operating timeshown are representative of a X/R ratio up to 40. For X/R ratios greater than 40 it is stronglyrecommended that the P122/P123 I >>> element is used.

The kneepoint voltage of a current transformer marks the upper limit of the roughly linearportion of the secondary winding excitation characteristic. This is defined exactly in the IECstandards as that point on the excitation curve where a 10% increase in exciting voltageproduces a 50% increase in exciting current.

The current transformers should be of equal ratio, of similar magnetising characteristics andof low reactance construction. In cases where low reactance current transformers are notavailable and high reactance ones must be used, it is essential to use the reactance of thecurrent transformer in the calculations for the voltage setting. Thus, the current transformerimpedance is expressed as a complex number in the formRCT + jXCT. It is also necessary to ensure that the exciting impedance of the currenttransformer is large in comparison with its secondary ohmic impedance at the relay settingvoltage.

In the case of the high impedance relay, the operating current is adjustable in discrete steps.The primary operating current ( I op ) will be a function of the current transformer ratio, therelay operating current ( I r), the number of current transformers in parallel with a relayelement (n) and the magnetising current of each current transformer ( I e) at the stabilityvoltage (V s). This relationship can be expressed as follows:

I op = (CT ratio) x ( I r + n I e) (2)


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MiCOM P120, P121, P122, P123 Page 5/20

In order to achieve the required primary operating current with the current transformers thatare used, a current setting ( I r) must be selected for the high impedance relay, as detailedabove. The setting of the stabilising resistor (R ST ) must be calculated in the followingmanner, where the setting is a function of the relay ohmic impedance at setting (R r), therequired stability voltage setting (V s) and the relay current setting ( I r).

RST =VsI r

- R r (3)

Note: The ohmic impedance of auxiliary powered P12x relays is extremelysmall and so can be ignored. Therefore:

RST =VsIr

(4)

2. USE OF METROSIL NON-LINEAR RESISTORS

When the maximum through fault current is limited by the protected circuit impedance, suchas in the case of generator differential and power transformer restricted earth fault

protection, it is generally found unnecessary to use non-linear voltage limiting resistors(Metrosils). However, when the maximum through fault current is high, such as in busbarprotection, it is more common to use a non-linear resistor (Metrosil) across the relay circuit(relay and stabilising resistor). Metrosils are used to limit the peak voltage developed by thecurrent transformers, under internal fault conditions, to a value below the insulation level ofthe current transformers, relay and interconnecting leads, which are able to withstand 3000Vpeak.

The following formulae should be used to estimate the peak transient voltage that could beproduced for an internal fault. This voltage is a function of the current transformer kneepointvoltage and the prospective voltage that would be produced for an internal fault if currenttransformer saturation did not occur. Note, the internal fault level, I f , can be significantlyhigher than the external fault level, I f , on generators where current can be fed from thesupply system and the generator.

Vp = 2 2 V K (Vf - VK) (5)

Vf = I f (R CT + 2 RL + R ST + R r) (6)

Where:

Vp = peak voltage developed by the CT under internal fault conditions.

VK = current transformer knee-point voltage.

Vf = maximum voltage that would be produced if CT saturation did not occur.

I f = maximum internal secondary fault current.

RCT = current transformer secondary winding resistance.

RL = maximum lead burden from current transformer to relay.

RST = relay stabilising resistor.

R r = Relay ohmic impedance at setting.

When the value of V p is greater than 3000V peak, non-linear resistors (Metrosils) should beapplied. These Metrosils are effectively connected across the relay circuit, or phase toneutral of the ac buswires, and serve the purpose of shunting the secondary current outputof the current transformer from the relay circuit in order to prevent very high secondaryvoltages.

These Metrosils are externally mounted and take the form of annular discs, of 152mmdiameter and approximately 10mm thickness. Their operating characteristics follow theexpression:

V = C I 0.25 (7)


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Where:

V = Instantaneous voltage applied to the non-linear resistor (Metrosil)

C = constant of the non-linear resistor (Metrosil)

I = instantaneous current through the non-linear resistor (Metrosil)

With a sinusoidal voltage applied across the Metrosil, the RMS current would beapproximately 0.52x the peak current. This current value can be calculated as follows:

I (rms) = 0.52VS (rms) x 2

C 4 (8)

Where:

Vs (rms) = rms value of the sinusoidal voltage applied across the Metrosil.

This is due to the fact that the current waveform through the Metrosil is not sinusoidal butappreciably distorted.

For satisfactory application of a non-linear resistor (Metrosil), its characteristic should besuch that it complies with the following requirements:

At the relay voltage setting, the non-linear resistor (Metrosil) current should be as low aspossible, but no greater than approximately 30mA rms for 1A current transformers andapproximately 100mA rms for 5A current transformers.

The Metrosil units normally recommended for use with 1A CTs are as follows:

Stability voltage Recommended Metrosil type

Vs (V) rms Single pole Triple pole

Up to 125V 600A/S1/S256

C = 450

600A/S3/I/S802

C = 450125-300V 600A/S1/S1088

C = 900

600A/S3/I/S1195

C = 900

The Metrosil units normally recommended for use with 5A CTs and single pole relays are asfollows:

Recommended Metrosil type

Relay stability voltage, V s (V) rms

Secondaryinternal fault

Current(A) rms Up to 200V 250V 275V 300V

50A 600A/S1/S1213

C = 540/640

600A/S1/S1214

C = 670/800

600A/S1/S1214

C = 670/800

600A/S1/S1223

C = 740/870

100A 600A/S2/P/S1217

C = 470/540

600A/S2/P/S1215

C = 570/670

600A/S2/P/S1215

C = 570/670

600A/S2/P/S1196

C = 620/740

150A 600A/S3/P/S1219

C = 430/500

600A/S3/P/S1220

C = 520/620

600A/S3/P/S1221

C = 570/670

600A/S3/P/S1222

C = 620/740

The single pole Metrosil units recommended for use with 5A CTs can also be used with triplepole relays and consist of three single pole units mounted on the same central stud butelectrically insulated from each other. A triple pole Metrosil type and the reference shouldbe specified when ordering. Metrosil units for higher stability voltage settings and faultcurrents can be supplied if required.


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MiCOM P120, P121, P122, P123 Page 7/20

3. MiCOM P12x RANGE

The P12x range are a numerical phase overcurrent and earth fault relays with 3 stages ofphase and/or earth fault protection, I >/ I e>, I >>/ I e>> and I >>>/ I e>>> which can be used for3 phase differential protection or restricted earth fault (REF) protection. The P120 is anumerical single phase overcurrent and earth fault relay with the same 3 stages of phaseand earth fault protection, which can be used for REF protection only.

The protection element used for a specific application depends upon the type of P12x relaychosen. In the case f the P120 and P121 relays the choice of element is irrelevant as thefourier algorithm they employ is identical. This is not the case with the P122 and P123 whichhave an I >>> element that is peak measuring. The peak measuring algorithm gives asignificant improvement in operating time over the fourier algorithm employed by the otherprotection elements. In all cases the time delay characteristic should be and with a setting ofzero seconds.

The Trip Commands menu (AUTMAT.CTRL) should be used to allocate the chosenprotection elements (e.g. t I >>> etc) to the trip relay RL1. Any elements allocated in the tripcommands menu will cause RL1 to pulse for 100ms. For applications where a 100ms pulseis insufficient, the pulse time can be extended up to 5 seconds in the tOpen pulse cell(AUTOMAT.CTRL/CB Supervision).

This feature is not available in the P120 and P121 models. An alternative approach, whichcan be achieved in all P12x relays, is to latch the trip output following a relay operation. Thiscan be done by selecting the appropriate protection element (e.g. t I > etc) in the LatchFunctions menu (AUTOMAT.CTRL).

Any elements not being used should be disabled by selecting No in the appropriatelocation. Setting ranges of P12x elements are:

Phase overcurrent

I > 0.1 25 I n

I > > 0.5 40 I n

I >> > 0.5 40 I n

Earth Fault

Range: 0.002 1 I en

I e> 0.002 1 I en

I e>> 0.002 - 1 I en

Ie>>> 0.002 - 1 I en

Range: 0.01 - 8 I en

I e> 0.01 - 8 I en

I e>> 0.01 - 8 I en

I e>>> 0.01 - 8 I en

Range: 0.1 - 40 I en

I e> 0.01 - 25 I en

I e>> 0.01 - 40 I enI e>>> 0.05 - 40 I en

The ohmic impedance (Rr) of the auxiliary powered KCGG over the whole setting range is0.08 for 1A inputs and 0.008 for 5A relays i.e. independent of current. To comply with the


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definition for a high impedance relay, it is necessary, in most applications, to utilise anexternally mounted stabilising resistor in series with the relay.

The standard values of the stabilising resistors normally supplied with the relay, on request,are 220 and 47 for 1A and 5A relay ratings respectively. In applications such as busbarprotection, where higher values of stabilising resistor are often required to obtain the desired

relay voltage setting, non-standard resistor values can be supplied. The standard resistorsare wire wound, continuously adjustable and have a continuous rating of 145W.

3.1 P12x Application Considerations

Since the performance of the P12x relay varies depending upon application, considerationmust be given to which relay and protection element to use. The following data should beused as a guide in order to obtain the best performance from the relay.

3 phase applications (e.g. Busbars, Generators, Motors etc.)

Relay Type Recommended o/c element

P120* Any

P121* AnyP122 I >>>

P123 I >>>

* suitable for applications where the system X/R ratio does not exceed 40. Higher X/Rratios will have a detrimental effect on relay operating time.

Constants for 3 phase applications:

- K-factor = 1.4

- VK /VS ratio = 4

3.2 Restricted Earth Fault (REF) Applications

Relay Type Recommended o/c element

P120 Any

P121 Any

P122 I e>>>

P123 I e>>>

It is strongly recommended that the 0.01 to 8 I en earth fault board be used for REFapplications. For advice on applying the other earth fault boards (0.002 to 1 I en and 0.1 to

40 I en) contact AREVA T&D UK Ltd Automation and Information.

Constants for REF applications:

- K-factor = 1

- VK /VS ratio = 4


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MiCOM P120, P121, P122, P123 Page 9/20

4. APPLYING THE P12x

The recommended relay current setting for restricted earth fault protection is usuallydetermined by the minimum fault current available for operation of the relay and wheneverpossible it should not be greater than 30% of the minimum fault level. For busbar protection,it is considered good practice by some utilities to set the minimum primary operating currentin excess of the rated load. Thus, if one of the current transformers becomes open circuit thehigh impedance relay does not maloperate.

The I e> earth fault element with its low current settings can be used for busbar supervision.When a CT or the buswires become open circuited the 3 phase currents will becomeunbalanced and residual current will flow. Hence, the I e> earth fault element should give analarm for open circuit conditions but will not stop a maloperation of the differential element ifthe relay is set below rated load. Whenever possible the supervision primary operatingcurrent should not be more than 25 amps or 10% of the smallest circuit rating, whichever isthe greater. The earth fault element ( I e>) should be connected at the star point of thestabilising resistors, as shown in Figure 9. The time delay setting for the supervisionelements (t I e>) should be at least 3 seconds to ensure that spurious operation does not

occur during any through fault. This earth fault element will operate for an open circuit CT onany one phase, or two phases, but not necessarily for a fault on all three when the currentsmay summate to zero. The supervision may be supplemented with a spare phase protectionstage ( I e >) set to the same setting as the I o> element or its lowest setting, 0.1 I n, if the I e>supervision setting is less than 0.1 I n. Note that the I N current should be checked when thebusbar is under load. This can be viewed in the Measurements menu in the relay. It isimportant that the I N> threshold is set above any standing I e unbalance current.The supervision element should be used to energise an auxiliary relay with hand resetcontacts connected to short circuit the buswires. This renders the busbar zone protectioninoperative and prevents thermal damage to the Metrosil. Contacts may also be required forbusbar supervision alarm purposes.

It is recommended that the dual powered MiCOM P124 relay is not used for differentialprotection because of the start-up time delay when powered from the CTs alone. Also, theminimum setting of the phase overcurrent elements, 0.2 I n, would limit its application fordifferential protection.

Figures 3 to 9 show how high impedance relays can be applied in a number of differentsituations.

4.1 Advanced application requirements for through fault stability

When V s from formula 2 becomes too restrictive for the application, the following notesshould be considered. The information is based on the transient and steady state stabilitylimits derived from conjunctive testing of the relay. Using this information will allow a lowerstability voltage to be applied to the relay, but the calculations become a little more involved.

There are two factors to be considered that affect the stability of the scheme. The first issaturation of the current transformers caused by the dc transient component of the faultcurrent and the second is steady state saturation caused by the symmetrical ac componentof fault current only.

4.2 Transient stability limit

To ensure through fault stability with a transient offset in the fault current the required voltagesetting is given by:

Vs = 40 + 0.05R ST +0.04 I f (R CT + 2 RL) (9)

If this value is lower than that given by formula 2 then it should be used instead.

Vs and R ST are unknowns in equation (10). However, for a relay current setting I r, the valueof R ST can be calculated by substituting for V s using equation (5), V s = I r RST .


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RST I r = 40 + 0.05R ST +0.04 I f (RCT+ 2 RL) (10)

4.3 Steady state stability limit

To ensure through fault stability with non offset currents:

(R CT+ 2 RL) must not exceed(VK + V s)/If. (11)

5. TYPICAL SETTING EXAMPLES

5.1 Restricted earth fault protection

The correct application of the P12x as a high impedance relay can best be illustrated bytaking the case of the 11000/415V, 1000kVA, X = 5%, power transformer shown in Figure10, for which restricted earth fault protection is required on the LV winding. CT ratio is

1500/5A.5.2 Stability voltage

The power transformer full load current

=1000 x 10 3

3 x 415

= 1391A

Maximum through fault level (ignoring source impedance)

=100

5 x 1391

= 27820A

Required relay stability voltage (assuming one CT saturated)

= KI f (R CT + 2 RL)

= 1.0 x 27820

x5

1500 (0.3 + 0.08)

= 35.2

5.3 Stabilising resistor

Assuming that the relay effective setting for a solidly earthed power transformer isapproximately 30% of full load current, we can choose a relay current setting, I N> = 20% of5A i.e. 1A. On this basis the required value of stabilising resistor is:

For application where the 5A inputs are used, such as this, a 47 resistor can be suppliedon request. The resistor is continuously adjustable between 0 and 47 . Thus a value of35.2 can be set.

5.4 Current transformer requirements

To ensure that internal faults are cleared in the shortest possible time the knee point voltageof the current transformers should be at least 5 times the stability voltage, V s .

VK = 4V s

= 4 x 35.2V

= 141V


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The exciting current to be drawn by the current transformers at the relay stability voltage, Vs,will be:

I e ) = relay setting

= 1A

n = number of current transformers in parallel with the relay

= 4

I e @ 35.2V element should be set to 0s.

The elements not used should be disabled. Note, the phase overcurrent elements not usedfor restricted earth fault protection could be used to provide normal overcurrent protection.

5.5 Metrosil non-linear resistor requirements

If the peak voltage appearing across the relay circuit under maximum internal fault conditionsexceeds 3000V peak then a suitable non-linear resistor (Metrosil), externally mounted,should be connected across the relay and stabilising resistor, in order to protect theinsulation of the current transformers, relay and interconnecting leads. In the present casethe peak voltage can be estimated by the formula:

Vp = 2 2 V K (Vf - VK)

Where:

VK = 141V (In practice this should be the actual current transformer kneepointvoltage, obtained from the current transformer magnetisation curve).

Vf = If (RCT + 2 RL R ST + R r)

= 27820 x5

1500 x

(0.3 + 0.08 + 35.2)

= 92.73 x 35.58

= 3299V

Therefore substituting these values for V K and V f into the main formula, it can be seen thatthe peak voltage developed by the current transformer is:

Vp = 2 2V K (Vf - VK)

= 2 2 x 141 x 3299 - 141


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= 3158V

This value is above the maximum of 3000V peaks and therefore a non-linear resistor(Metrosil) would have to be connected across the relay and the stabilising resistor. Therecommended non-linear resistor type would have to be chosen in accordance with theinternal fault current and the voltage setting.

6. BUSBAR PROTECTION

A typical 132kV double bus generating station is made up of two 100MVA generators andassociated step-up transformers, providing power to the high voltage system, by means offour overhead transmission lines, shown in Figure 2. The main and reserve busbars aresectionalised with bus section circuit breakers. The application for a high impedancecirculating current scheme having 4 zones and an overall check feature, is as follows:

The switchgear rating is 3500MVA, the system voltage is 132kV solidly earthed and themaximum loop lead resistance is 2 ohms. The current transformers are of ratio 500/1 ampand have a secondary resistance of 0.7 ohms. The system has an X/R ratio of 20.

6.1 Stability voltage

The stability level of the busbar protection is governed by the maximum through fault levelwhich is assumed to be the switchgear rating. Using the switchgear rating allows for anyfuture system expansion.

=3500 x 10 6

3 x 132 x 10 3

Required relay stability voltage (assuming one CT is saturated)

= K If (RCT + 2RL)

=

1.4 x 15300

500 (0.7 + 2)

= 116V

6.2 Current setting

The primary operating current of busbar protection is normally set to less than 30% of theminimum fault level. It is also considered good practice by some utilities to set the minimumprimary operating current in excess of the rated load. Thus, if one of the CTs becomes opencircuit the high impedance relay does not maloperate.

The primary operating current should be made less than 30% of the minimum fault currentand more than the full load current of one of the incomers. Thus, if one of the incomer CTsbecomes open circuit the differential protection will not maloperate. It is assumed that 30% ofthe minimum fault current is more than the full load current of the largest circuit.

Full load current

=100 x 10 3

3 x 132 = 438A

6.3 Discriminating zone

Magnetising current taken by each CT at 116V = 0.072A

Maximum number of CTs per zone = 5

Relay current setting, I r(I >) = 400A = 0.8 I n


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Relay primary operating current,

I op = CT ratio x ( I r + n I e)

= 500 x (0.8 + (5 x 0.072))

= 500 x 1.16

= 580A (132% full load current)

6.4 Check zone

Magnetising current taken by each CT at 116V = 0.072A

Maximum number of circuits = 6

Relay current setting, I r (I >) = 0.8A

Relay primary operating current,

I op = 500 x (0.8 + (6 x 0.072))

= 500 x 1.232

= 616A(141%- full load current)

Therefore, by setting I r (I >) = 0.8A, the primary operating current of the busbar protectionmeets the requirements stated earlier.

6.5 Stabilising resistor

The required value of the stabilising resistor is:

RST =VSI r

=1160.8

= 145

Therefore the standard 220 variable resistor can be used.

6.6 Current transformer requirements

To ensure that internal faults are cleared in the shortest possible time the knee point voltage

of the current transformers should be at least 4 times the stability voltage, Vs.Vk /Vs = 4

Vk = 464V

6.7 Metrosil non-linear resistor requirements

If the peak voltage appearing across the relay circuit under maximum internal fault conditionsexceeds 3000V peak then a suitable non-linear resistor (Metrosil), externally mounted,should be connected across the relay and stabilising resistor, in order to protect theinsulation of the current transformers, relay and interconnecting leads. In the present casethe peak voltage can be estimated by the formula:

VP = 2 2V K (Vf - VK)

where V K = 464V (In practice this should be the actual current transformer kneepoint voltage,obtained from the current transformer magnetisation curve).


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Vr = I f (RCT + 2 RL + R ST + R r)

= 15300 x1

500 x (0.7 + 2 + 145)

= 30.6 x 147.7

= 4520V

Therefore substituting these values for V K and V f into the main formula, it can be seen thatthe peak voltage developed by the current transformer is:

Vp = 2 2V K (Vf - VK)

= 2 2 x 464 x (4520 - 464)

= 3880V

This value is above the maximum of 3000V peak and therefore a non-linear resistor(Metrosil) would have to be connected across the relay and the stabilising resistor. Therecommended non-linear resistor type would have to be chosen in accordance with themaximum secondary internal fault current and voltage setting.

6.8 Busbar supervision

Whenever possible the supervision primary operating current should not be more than 25amps or 10% of the smallest circuit, whichever is the greater.

The I e> earth fault element in the P12x range with its low current settings can be used forbusbar supervision.

Assuming that 25A is greater than 10% of the smallest circuit current.

I e> = 25/500 = 0.05In

The time delay setting of the t I e> element, used for busbar supervision, is 3s.

Any elements not used should be disabled.

6.9 Advanced application requirements for through fault stability

The previous busbar protection example is used here to demonstrate the use of theadvanced application requirements for through stability.

To ensure through fault stability with a transient offset in the fault current the required voltagesetting is given by:

Vs = (0.007 X/R + 1.05) x I f (2 RL + R CT)

To be used when X/R is less or equal to 40. The standard equation should be used for X/Rratios greater than 40.

If the calculated value is lower than that given by equation 1 (with K=1.4) then it should beused instead.

6.10 Transient stability limit

Vs = 0.007 X/R + 1.05) x15300

500 x (0.7 + 2)

Vs = 1.19 x 30.6 x 2.7

Vs = 98V


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MiCOM P120, P121, P122, P123 Page 15/20

The relay current setting, I r = 0.8 I n

RST =VsI r

RST =980.8 = 123

Assuming V K = 4V s

VK = 4V s = 392V

Using the advanced application method the knee point voltage requirement has beenreduced to 392V compared to the conventional method where the knee point voltage wascalculated to be 464V.

Figure 2: Double busbar generating station

Figure 3: Phase and earth fault differential protection for generators, motors or

reactors


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Page 16/20 MiCOM P120, P121, P122, P123

Figure 4: Restricted earth fault protection for 3 phase, 3 wire system-applicable tostar connected generators or power transformer windings

Figure 5: Balanced or restricted earth fault protection for delta winding of a powertransformer with supply system earthed


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MiCOM P120, P121, P122, P123 Page 17/20

Figure 6: Restricted earth fault protection for 3 phase, 4 wire system-applicable tostar connected generators or power transformer windings with neutralearthed at switchgear

Figure 7: Restricted earth fault protection for 3 phase, 4 wire system-applicable tostar connected generators or power transformer windings earthed directlyat the star point


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Page 18/20 MiCOM P120, P121, P122, P123

Figure 8: Phase and earth fault differential protection for an auto-transformer withCTs at the neutral star point

Figure 9: Busbar protection simple single zone phase and earth fault scheme


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MiCOM P120, P121, P122, P123 Page 19/20

Figure 10: Restricted earth fault protection on a power transformer LV winding


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AREVA T&D UK Ltd - Automation & Information St Leonards Works, Stafford, ST17 4LX England

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P12x en AP a00 Appl Note

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