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8.Commissioning Tests

8.Commissioning TestsCT polarity tests:

In many protection schemes the relative polarity of CT is more important and therefore the correct polarity must be ensured before they are connected. The normal current flow convention of CT is when the primary flows from P1 toP2, the secondary current flows from S1to S2 in the external circuit as shown in the fig 8.1

fig 8.1 fig 8.2A simple way to check the polarity is flick test which uses a

battery, center zero ammeter, and a push button connected as shown in the fig8.2. When the push button is pressed the ammeter makes a positive flick and a negative flick when the push button is released, if the assumed polarity is correct.

CT ratio check :The polarity and ratio check can be carried out by the primary

injection tests described below:The circuit for the test is shown in the fig 8.3. A short

circuit is placed across the phases on one side of the CT’s and single phase injection is carried out on the other side. One ammeter A2 is provided in the phase side of the CT circuit and another ammeter A3 is provided in the residual circuit of the CT. current is injected through the primary conductor and measured on test set ammeter A1. The secondary current is measured on the ammeter A2. The ratio of the

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value on A1 to that on A2 should approximate to the ratio marked on the CT nameplate.

fig 8.3

The ammeter A3 in the residual circuit will read few milliamps if the CT polarity is correct. The reading A3 twice that of A2 shows wrong polarity. Single phase injection should be carried out for each pair of phases.

If an earth fault relay with low setting is provided in the residual circuit its operating should be temporarily short circuited during the test to avoid overheating.

Polarity check using series injection :The test circuit is as shown in the fig8.4 & 8.5 below.

Inject current approximate to rated current. The ammeter A3 will read three times A2 if the polarity is correct. If the reading of ammeter A3 is equal to A2,the relative polarity of one of the CT is wrong.

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Test circuit for bar primary CTs fig 8.4

Test circuit for ring type CTsfig 8.5

CT magnetizing curve:A variable voltage supply is connected across the secondary

terminals of the CT as shown in the fig 8.6. If the knee point voltage is higher than local mains supply use a step up interposing transformer.

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Fig 8.6The voltmeter is connected in such away that the ammeter

doesn’t read the voltmeter current. It will be useful to find roughly the voltage at which saturation starts by increasing the voltage until there is a large increase in current for a small change in voltage. From this it can be decided at what values to take readings to give sufficient point to plot the curve. Initially take readings in large steps (say 20 or 30 volts) and smaller steps (say 10 or 5 volts) when saturation starts. Tabulate and plot the readings.

Voltage 0 20 40 60 80 90 100 110 120current in

mA0 1 16 23 31 37 46 65 105

The knee point voltage is the voltage at which an increase of

10% voltage will result in 50% increase in magnetizing current .From the above reading

For 100vols Ie = 46 mAFor (100 +10 ) % i.e. 100 volts Ie = 65mA

65 46 i.e. increase = 100 = 41%

46For 100vols Ie = 65 mAFor (110 +10 ) % i.e. 110 volts Ie = 105 mA

105 65 i.e. increase = 100 = 61%

65Hence knee point voltage is between 100 and 110 volts.

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Voltage transformer polarity check :The polarity of the voltage transformer can be checked with

the test described for current transformer. While checking, the battery should be connected to the primary winding and the polarity winding connected to the secondary winding. If the VT is of capacitor type then the polarity of the transformer at the bottom of the capacitor should be checked.

Voltage transformer ratio check :the check can be carried-out when the main circuit is first

made alive. The secondary winding voltage of the VT is compared with secondary winding voltage of a VT already connected to the same bus-bars.

Voltage transformer phasing check :The secondary connection of a three phase voltage

transformer or a bank of three single phase transformers should be checked for correct polarity as detailed below:

fig 8.7With the main circuit alive the secondary voltage of the VT

between phase to phase and phase to neutral should be measured for correct magnitude. If an existing proven VT is available in the same primary system and the secondary earthing is employed then the correct phasing can be proved by measuring the voltage between the

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respective phases of both the VT’s. This voltage should be normally little or no voltage if the phasing is correct.

If the VT has a broken delta territory winding then a check should be made for the voltage across the broken terminals Vn and VL, when a rated balanced three phase voltage applied to the primary of the VT. The broken delta voltage should be below 5volts when rated burden connected.

Secondary injection tests:Over current and earth fault relays:

fig 8.8The over current relays can be checked using an over current

test set. Alternatively a test setup as shown in the fig 8.8 may be used. While checking over current relays care should be taken to avoid excessive currents flowing through the coil for a long time to avoid excessive heating.

Instantaneous over current relays are checked for minimum current required for operation (pick-up) of the relay and maximum current at which the relay resets (drop-off) for each current setting of the relay.

Definite time over current relays are first checked for pick-up and drop-off as detailed above with timer setting at zero. Then the time delay should be checked at 1.3 times (approx.) the current setting.

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For inverse time over current relays the following checks should be performed.

1) Check that starting current ( i.e. the current at which the disc just begins to move but does not completes its travel to close the contact.) is within specified limits.

2) Check that the closing current ( i.e. the minimum current at which at which the disc completes its travel to close its contact) is within its limits.

3) Check the operating time of the relay at 2,5,10 times the current setting (plug setting ). These times should be within the tolerance limits. (For electronic and numerical relays the checking of starting and closing current does not arise.

Differential relays :The sensitivity of the differential relay can be checked using a

over current test set as described below:

fig 8.9 fig 8.10While checking unbiased differential relay the current setting

should be checked by slowly raising the current until the relay pickup. This should be checked for all the setting and finally for the required setting.

The biased differential relay can be checked with the over current test set by injecting current simultaneously to the operating coil and the bias coil as shown in fig8.10 above, at different points of the bias characteristic by adjusting the rheostats R1and R2. Record the operating currents for different values of bias current. The ratio of the

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operating current A1 to the through current A2 is approximate to the value of the bias slope of the relay.

The relative value of the resistor R2 and R1 should roughly the same as the ratio of the bias slope of the relay.

e.g. for bias slope of 30%, R2/R1 = 0.3 To determine the position of the characteristic inject a current

of I2Amps through R2 and adjust R1 until the relay operates.Let the operating current be I1 I2 + ( I1 + I2 ) I1Then the average bias IB = = I2 + 2 2 I1 Then the bias slope of the relay is IBRepeat the test for other values of currents to plot the curve.

Negative phase sequence relays :

fig 8.11The current setting of the negative phase sequence relay is

usually expressed in terms of negative phase sequence currents. The relay can be checked with over current test set. If the relay is provided with an external filter current injection should be made before the filter.

The relay can be checked for phase to earth fault or phase to earth fault simulation. If the relay is checked for phase to earth fault

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simulation 1/3rd of the injected current is negative sequence current. If the relay is checked for phase to phase fault simulation 1/3 times of the injected current is negative sequence current.

For phase to earth fault simulation of a relay rated for 1amps and I2s setting of 10%

10I2s = 1 = 0.1Amps 100In order to obtain 0.1Amps negative phase sequence current a

test current of 3 0.1 Amps should be injected into each phase in turn.Test current = 0.1 3 2 = 0.6 Amps.

The factor 2 is for two times the set current and the factor 3 is for phase to neutral simulation.

For phase to phase fault simulation of a relay rated for 5amps and I2s setting of 7.5%

7.5I2s = 5 = 0.375Amps 100In order to obtain 0.375Amps negative phase sequence

current, a test current of 3 0.375 Amps should be injected into each pair of phase in turn.

Test current = 0.375 3 4 = 2.6 Amps.The factor 4 is for four times the set current and the

factor 3 is for phase to phase simulation.

If the relay has an inverse time characteristic check operating time at different points of the curve.

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Directional relays :

fig 8.12The directional characteristic and maximum torque angle of

many relays can be checked with a phase shifting transformer as shown in the fig8.12. The phase shifting transformer permits the phase angle of the relay voltage to be varied with respect to the relay current. Adjust the current flowing through the relay current coil to correspond to rated current and the applied voltage to the voltage coil corresponds to the rated voltage. Rotate the phase shifter till the phase angle meter reads zero degree (unity PF). Check the relay is in operated position. If not reverse the voltage or current connection and check the operation of the relay at UPF. Rotate the phase shifter in the clockwise direction until the relay contact opens and then rotate back in the anti clockwise direction and note the phase angle meter reading 1,when the relay contact just close. Continue to rotate in the anti clockwise direction till the relay contact open again, then rotate backwards in clockwise direction and note the phase angle meter reading 2, when the contact just close. The maximum torque angle line of the relay is given by the bisector of 1 2 and the angle between this line and the zero degree line (UPF)gives the maximum torque angle of the relay which should be within 4 of the declared maximum torque angle (MTA).

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fig 8.13

Distance relays: Distance relay s are required to measure impedance accurately

over a wide range of current and voltages. The impedance measurement of these relays can be checked using a phase shifting transformer explained earlier for checking directional relays. These equipment measures the impedance at static conditions i.e. the current increased slowly or the voltage is decreased slowly until the relay operates. While testing high speed distance relays, it is important to apply simulated fault conditions suddenly, because during the fault condition the relay voltage falls suddenly from nominal voltage to fault voltage and the current from the load value to fault value accompanied by changes in phase angles. So in order to check the distance relays under dynamic condition most of the manufacturers of relay have developed dynamic test set. The basic principle of operation of the dynamic test set is as detailed below:

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fig 8.14Normal working voltage is applied to the relay through a

voltage auto transformer. When the fault contactor is closed the fault current If is applied to the relay. At the same instant the voltage applied to the relay collapses to the fault voltage VL. The magnitude of the fault voltage VL depends on the ratio of the source impedance to line impedance (Zs / ZL). The voltage auto transformer has 10%and 1% tapings to allow the line impedance ZL to be matched to the relay ohmic setting. By varying the source impedance ZS the relay ohmic setting can be checked over a wide range of voltage and current magnitudes.

Usually the distance relay ohmic setting is checked at actual line angle. For this a choke and resistance with tapping of the line impedance ZL are provided. Angles other than line impedance angles may be chosen for checking the characteristic of mho or reactance relays. The points usually checked on the mho and reactance relay characteristic are shown in fig 8.15.

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fig 8.15

Primary injection tests :Over current and earth fault relays :

The sensitivity of the over current relays can be checked by phase to phase injection. Phase to phase injection is explained earlier in CT ratio and polarity checks. For checking earth fault relays single phase injection should be used as shown in fig8.16.

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Directional relays :The directional feature of the directional over current, earth

fault, distance and other wattmetric relays can be checked by the use of load current.

The phase fault directional over current relays can be checked by use of load current when the load current is appreciable and the direction is in no doubt. The relay contact should close in the operating direction and open in the reverse direction. The direction of the current to the relay can be reversed by cross connecting the voltage or current leads to the relay through the test plug as shown in fig 8.17.

fig 8.17Earth fault over current relays are usually fed from broken

delta voltage of the three phase voltage transformer and residual current of the main current transformers. Under normal condition the relay is not energized, so it is necessary to simulate the earth fault condition. With one phase of the voltage transformer disconnected and short circuited while the current transformers of the other two phases disconnected and short circuited as shown in fig8.18, simulates a condition of earth fault on the phase from which voltage is disconnected. The relay should operate when the load current flows in the operating direction.

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fig 8.18Mho relays can be made to operate with load current flowing

in the operating direction by removing the restraint voltage from the relay. In most modern distance relays a convenient switch link or plug is provided o the relay to facilitate this test. The removal of the restraint voltage from the mho relay changes the mho circle characteristic into a plain directional characteristic with the same characteristic angle as shown in the fig8.19.

fig 8.19 With lagging current flowing in the operating direction the relay will operate a soon as the restraint voltage is removed. The relay should be then checked for non operation for load flow in the opposite direction. This can be done by inserting a test plug with crossover connection of

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the current transformer leads as shown in the fig 8.20 to reverse the current flow to the relay.

fig 8.20

Generator differential protection :

The sensitivity of this type of differential scheme can be checked by primary injection test as shown in the fig 8.21.

fig 8.21

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Primary current is passed through one of the main CT and slowly raised until the relay operates. This gives the true operating current necessary in the primary to cause operation, that includes the magnetizing current of the CT shunting the relay. the voltage across the relay coil and stabilizing resistance should also be measured at the time of operation of the relay to check the voltage developed by the main CT that causes the relay operation. The sensitivity check should be carried out by injecting through the CT primary of each phases in turn to check all units of the relay.

Another method of checking the sensitivity is by use of machine itself to supply the primary current by carefully controlling the machine excitation from a low value to the required primary current. For this the main circuit breaker is left open and a three phase short circuit is provided in the generator link cubicle to simulate an internal fault as shown in fig 8.22.

fig 8.22 The machine is then run up to full speed and the excitation is raised slowly until the three elements of the relay operates. The suitability of this type of checking depends on the type of excitation and degree of control available.

The stability of the generator differential protection can be checked by providing a short circuit at the bus side of the generator circuit breaker as shown in fig8.23.

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fig 8.23The circuit breaker should be closed and excitation of the machine raised until full load current is circulated in the primary circuit. The operating coil of the relay should be short circuited and an ammeter of twice the rated current of the CT should be used initially until the correct polarity of the CT is confirmed. Wrongly connected or open circuited CT will produce higher spill current reading in the ammeter. If the ammeter reads very little current when circulating full load in the primary circuit, the ammeter may replaced with a higher sensitive meter to measure the spill current which will in the order of milli-amperes.

The stability of motor and generator protection can be checked by primary injection as detailed below :

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fig 8.23(a)

Current injected through R & y phases as shown in fig 8.23(a) keeping the R & Y windings short circuited. While primary current is flowing, the relay should maintain stability and the reading of the ammeters should read few milli-amps only. Wrongly connected or open circuited CT will produce higher spill currents. It is advisable to short circuit the relay coils and stabilizing resistors throughout the test. Repeat the test for Y & B and B & R phases to confirm the stability in all the phases.

Trans former biased differential protection :The sensitivity of the relay can be checked with a single phase

primary injection test set as described for generator differential protection. The test can also be carried out by injecting current through one CT to simulate earth fault or two CTs to simulate phase fault as shown in fig 8.24 & 8.25.

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Earth fault simulation fig 8.24

Phase fault simulationfig 8.25

The stability of the transformer differential protection can be checked by circulating full load current through the main CT’s primary windings. The best source of power for this test is a generator of adequate current rating. Full load current can be circulated through the by putting a shot circuit at one side of the transformer external to the protection. Connect the terminals of the generator to the other side of

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the transformer and slowly excite the generator at rated speed until full load current flows through the primary winding of the transformer. Approximately 12% of the transformer winding rated voltage will have to be raised to circulate full load current.

fig 8.26If there is no machine available to circulate full load current

through the transformer, connect the transformer to the local bus bars that have voltage rating of approximately 5% to 12% of one of the winding. The transformer short circuit impedance must be known in order to calculate the bus-bar voltage required to circulate full load current. If both the above possibility is not available of protection can be checked when the transformer is first put in load.

When the load current flowing through the transformer winding the protective relay should remain stable and the spill current in the operating coil of the relay should be very small, provided the CT ratios associated with each winding of the transformer are chosen correctly and the transformer tap changer is at the nominal tap. Measure the spill current through the relay during the load test with the tap changer at its maximum and minimum tap. This spill current expressed as a percentage of load current indicates the minimum amount of bias of the relay required to maintain stability for through faults. If the relay operates on load with the bias set correctly, the circuit diagram should be checked for correct CT connection according to the vector group of the transformer being protected. The possible

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errors in diagram or actual wiring and the respective spill current are tabulated below.

POSSIBLE ERRORS PILOT CURRENT

S

OPERATING CURRENTS

R Y B R Y B1 One set of CT reversed I I I 2I 2I 2I2 Three pilots transposed I I I 3I 3I 3I3 Y&B pilots transposed I I I - 3I 3I4 One set of CT reversed and

three pilots transposedI I I I I I

5 One set of CT reversed and Y&B pilots transposed

I I I 2I I I

Restricted earth fault relay :The sensitivity of the restricted earth fault (REF) protection

can be checked by injecting current using a single phase test set through each of the main CT in turn as shown in fig 8.27.

Sensitivity checkfig 8.27

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Stability checkfig 8.28

While carrying out this test measure the voltage developed across the relay coil and stabilizing resistance so as to check the voltage by the main CT to relay operation.

The stability of the REF protection can be checked by injecting current through the neutral CT and each phase CT in turn as shown in fig8.28. If the protection is combined with differential protection the connection to the differential relay should be short circuited. While primary current is flowing the relay should maintain stability and the reading in ammeter connected in series with the relay coil should be few milliamperes only. It is advisable to short circuit the relay coil and the stabilizing through the test.

Negative phase sequence relays:The sensitivity of the NPS relay can be checked by injecting

current using a single phase test set through each pair of phases the in turn as shown in fig 8.29.

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fig 8.29NPS relays are usually calibrated in NPS currents only. To check the relay for phase to phase fault simulation, inject 3 times the setting current i.e. 1/3 times the injected current is the negative phase sequence current.

To test the relay at four times the set current for a NPS setting of 7.5% and a CT ratio of 1000/5A with phase to phase fault simulation, inject a primary current of

7.5 1000 3 4 i.e. 520 Amps.

100For phase to earth fault simulation the same test current should be 7.5

1000 3 4 i.e. 900 Amps. 100The relay should also be checked for load test when it is first put on service. When the load is balanced and phase sequence of the current supplied to the relay is correct, the relay should not operate. To confirm the relay for operation on NPS current, two of the main CT inputs to the relay should be transposed. This can be done by either using a test plug inserted into the relay or by reconnecting the leads too the terminals of the relay. in this case all the phase current entering the relay is NPS current and the relay will operate if the current is above the set current.

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Bus-bar protection :In bus-bar protection scheme all the current transformers are

connected to form a circulating current scheme. So the ratios of al the CTs should be same and connected with correct polarity. The polarity and ratio of the bus-bar protection scheme can be checked by choosing one CT circuit as reference after checking its ratio and polarity and use it to check the remaining CT circuits.

To check the polarity and ratio of the reference CT provide a short in the bus-bars as shown in fig 8.30.

fig 8.30Inject current through R and Y phase of the bus-bars. The

ratio of the ammeter reading A1 to that of A2 should approximate to the ratio marked on the CT name plate. The ammeter A3 connected in the neutral test link will read a few milliampere if the CT polarity is correct. Reading A3 twice that of A2 shows wrong polarity. Repeat the test with current injection in Y and B phases bus-bars. During the test the operating coil of the relay and stabilizing resistance should be short circuited to avoid overheating as they are not continuously rated. A convenient way to do this is by manually operating the supervision relay which in turn short circuit the bus- wires. If the scheme of duplicate bus bar type in the CTs are switched between the main and

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reserve bus wires by isolator auxiliary switches, then it is advisable to carryout the test with bus bar isolators in both the position in order to check the wiring.

After checking the ratio and polarity of the reference CTs, the other CTs can be checked against the reference CT. This is carried out by providing a temporary short circuit on the test circuit and injecting a primary current in R & Y phases of the reference circuit as shown in the fig 8.31

fig 8.31The ammeter A2 provide the ratio check and A3 check the polarity as explained earlier. Repeat the test with current injection in Y & B phases. If the scheme is a duplicate bus bar type the test should be carried out with the bus bar isolators in both the positions to check the wiring.

The sensitivity of the bus bar protection relay can be checked by passing single phase primary current through one CT only as shown in fig 8.32 and measuring the current necessary to cause operation of the

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fig8.32 circulating current relay with maximum number of CTs in idle shunt. If the scheme provided with an overall check feature, the sensitivity of this can be checked in the same manner and at the same time. The sensitivity check should be carried out for each discriminating zone. A voltmeter should be connected across the relay coil and stabilizing resistor in order to check the correctness of value of stabilizing resistor.

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