3.15& 5 MVA Power Transformer Testing

8/11/2019 3.15& 5 MVA Power Transformer Testing

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Protection to TransformerTransformer, being the costliest equipment in any sub-station, should be

well protected. The transformer is to be protected from external fault (throughfault) and internal fault.

The best protection technique now and for more than 50 years for faultwith in the Power transformer is known as differential protection. The differentialrelays are triple pole high speed biased designed to protect large three phasepower transformers against internal faults (Below 40 m sec). Biased differentialrelay is extremely stable during through faults and provides high speed operation

on heavy internal fault. (Below 20 m sec). The relays utilize harmonic restraint toprevent operation by magnetizing in-rush current during the transformer isenergized. In addition, the relays employ fifth harmonics restraint to avoidpossible malfunction under overexcited conditions.

The minimum operating time of Buchholz relay is about 100 millisecondand an average time is 200 millisecond which is somewhat slow. On theother hand, electrical relays (viz differential relays) can be used for heavy faultswhere high speed (20 40 m sec.) is necessary.

Differential relay designs vary with different manufacturers and areconstantly changing, especially as microprocessor technology impacts this area.However, protective relaying applications remain basically the same; relativelyindependent of design and their trends.

In this booklet, the testing of individual differential relay, connecting in theprotective circuit and testing of the entire differential scheme are discussed. Thevarious connections that lead to malfunction, nonoperation, and sluggishoperation are also brought-out for the testing engineers reference.

I hope this booklet will be a supplement to the ones already brought out on

this subject.

2. TERMINOLOGY

2.1 Differential Relay :

A relay which, by its design, is intended to respond to differential current.


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2.2 Biased (or Percentage) Differential Relay :

A differential relay in which the designed response is modified by arestraint current.

2.3 Differential Current :

In a differential relay, a current which is the phasor difference betweenspecified incoming and outgoing currents.

2.4 Restraint Current :

In a differential relay, the combination of incoming and outgoing currents,which restraints operation of the relay.

2.5 Restraint Percentage :

The ratio, expressed as a percentage, between the differential current andthe restraint current(s) up to which the relay does not operate.

2.6 Through Current :

In a differential relay, that portion of the total incoming current which isalso present in the outgoing current.

3. WORKING PRINCIPLE


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Figure shows an explanatory diagram illustrating the principle of thecirculating current system. If the two current transformers have the same ratio,

and are properly connected, their secondary current will merely circulate betweenthe two C.Ts as shown by arrows and no current will flow through the differentialrelay for external faults.

For the internal faults, the operating current through the differential relaywill be proportional to the vectorial difference between the currents entering and

leaving the protected circuit, and if the differential current exceeds the relayspickup value, the relay will operate.

4. DIFFERENTIAL RELAY INPUT SOURCES:-

Input sources are

1) Auxiliary DC supply

2) C.Ts secondary currents

4.1. Auxi liary Supply

Check the relays rated auxiliary voltage on the front panel and connect asuitable D.C. supply (or) station battery supply to relay terminal. The connectionshould be correct and tight. Tightness is very important to avoid non-tripping dueto considerable voltage drop at this point of connection.


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4.3. Typical calculations of C.T. requirements for differential protection :

Transformer particulars

Rating - 10 MVAVoltage Ratio - 110 KV / 11 KV% impedance - 8.46Vector Group - Dyn 11

110 KV Side

Bushing C.T. ratio - 60 /1 A10 x 10

3

110 KIV rated current = ----------- = 52.5 A110 x 3

C.T. secondary pilot 52.5Current = -------- = 0.875 A

60

11 KV Side

If there is no bushing C.T. L.V. breaker C.T. ratio = 600 / 1Acan be used.

10 x 103

Rated current = ----------- = 525.5 A

11 x 3

525.5CT Secondary Current = -------- = 0.875A

600

If connected in delta, the pilot currentin the relay circuit is = 0.875 x3 = 1.516 A

Since the current is more than the rated current of the relay by 50%, ratiomatching (interposing) current transformers are required. When interposingC.T.s are used, connect main C.Ts in star, interposing primary in star andsecondary in delta.

Ratio of interposing C.T.s Primary - 1A

Secondary - 1/3 (or) 0.577A

Actual Current on Secondary = 0.875/ 3 = 0.505 A

Pilot current in relay circuit = 0.505 x3 = 0.875A

The Pilot current in relay circuits both on H.V. and L.V. sides are 0.875A


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Differential Operation during External Fault (Phase to Ground):

Differential Relay : Does not operate

120 A

2 A

2A 2 A

120 A

120 A

2 A2 A

2 A2 A

2 A2 A

2 A

60/1600/0.577A

2078 A

Current in Pr. Tr.

Current in CT

Secondary

Pr. Tr.110/11kV

b

c

a

Differential Connection with ICTDYn 11

ICT


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During External Fault (Phase to Phase) :

Differential Relay : Does not operate

2 A

2 A 2 A

120 A

2 A2 A

2 A2 A

2 A

60/1A

600/0.577A

2078 A

Current in Pr. Tr.

Current in CT Secondary

Pr. Tr. 110/11kV

10MVA

b

c

2078 A

2078 A120 A

120 A

120 A

240 A4 A

2 A

2 A

4 A

4 A4 A


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During Internal Fault (Phase to Phase) :

Differential Relay : Operates

2 A

2 A2 A

120 A

2 A

2 A

60/1A600/0.577A

Current in Pr. Tr.

Current in CT Secondary

Pr. Tr. 110 /11kV

10MVA

b

c

a

2078 A

2078 A120 A

120 A

120 A

240 A4 A

4 A

2 A

2 A

2 A

4 A


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4.4. C.T. Requirement:

Class PS C.T.s are used in differential protection and the C.Ts are

usually provided in the transformer bushing itself with the required ratio based onthe power transformer voltage and power rating. C.T.s associated withdifferential relay should be of low reactance type as per IS 2705 Part IV. Inaddition, C.T.s should develop knee point voltage as given below.

Vk > 2 If(RCT+ 2 RL) for star connected CTs

2 If> ------- (RCT+ 3 RL) for Delta connected CTs

3Where,

If = Maxim um through fault current referred to secondaryWinding of s tar connected C.T. with a 3 phase systemFault.

RCT = Secondary winding resistance of C.T.RL = One way lead resistance between C.T. terminals and

the relay terminals.(or) Vk = 40 I (RCT+ 2 RL)

I = Relay rated current

At the knee point, the C.T becomes saturated and the magnetizing currentis less than 0.03 x I at Vk / 4. The C.T. output being unsymmetrical in the

positive and negative half cycles generate second harmonic content. Unlessextremely large C.T. cores are used this problem cannot be overcome totally.The relay will be sluggish during heavy internal faults with moderately sizedC.T.s.

Please refer manual on pre-commissioning and periodical testing ofelectrical installation TNEB and the The practical guide to differential protectivescheme to power transformer by Er. A.S. Kandasamy M.E., Rtd., Chief Engineer

/ Transmission for testing of bushing CTs in detail.

5. PERCENTAGE (OR) BIASED DIFFERENTIAL RELAYS

In the differential protection for transformers, there will be two groups ofC.Ts each consisting of 3 Nos , one group connected in star with secondary rating1 A and other group connected in delta with secondary rating 0.577A. Thetransformer differential relay compares the currents in the windings of thetransformer through the medium of C.Ts whose ratios are such as to make theirsecondary currents normally equal except for the core magnetizing currents ofthe transformer which are relatively small. Even under normal operatingconditions, small unbalance current in milli amps (spill current) may appear.


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At high through fault current, the unbalance between C.Ts secondaryincreases with increase of through currents which is sufficient to actuate thedifferential relay. Bias relay is used to overcome this problem for external faults.

The spill current required to operate the relay is usually expressed as apercentage of the through currents in the restraining coils and the ratio isgenerally termed as percentage s lope.

The purpose of % slope characteristics is to prevent undesired relay operationbecause of unbalance between C.T. s during external fault arising from anaccumulation of unbalances for the following reasons.

1) Tap changing in the power transformer2) The difference between the errors of the C.Ts on either side of the

power transformer.3) Possible mismatch of ratio among different current transformers.4) Phase displacement between primary and secondary current.5) The difference in the magnetizing characteristic of H.V. & L.V. Side

C.Ts

These unbalances are in the same direction to get the total maximumunbalance. The slope of this line is approximately the total percentagemismatch. Then add at least 5% to this value, and the new total is the minimumpercentage slope that should be used. The bias setting will be ensured to avoidunwanted operation due to spill current at through fault and yet maintain highsensitivity for internal fault.


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6. RELAY TEST SETUP

6.1 Relay Test Setup:

a) 500 V megger - 1 No.b) Supply extension box with RCCB - 1 No.c) DPST Switch - 1 No.d) Variable Voltage Transformer (Variac) - 1 No.e) Rheostart (Non-inductive) 40, 5A - 2 Nos.f) A.C Ammeter range (0 5A) - 1 No.g) A.C Milli Ammeter range (0 1000) - 1 No.h) Diode IN 4007 / 5 Amps - 1 No.i) A.C. Ammeter range (0 20 A) - 1 No.

j) D.C. Ammeter range (0 1A) - 1 No.

k) Tim e interval meter (Digital) - 1 No.l) Digital multimeter - 1 No.

6.2 Insulation Test:

Before testing of individual relays, the following items are to be measuredwith a 500V megger.

a) IR values of D.C. Circuit to earth.b) IR values of main current transformer secondary circuit to earth.c) IR values of main current transformer secondary circuit to D.C. Circuit.

d) IR values between main current transformer secondary circuits.

The insulation resistance value has relation to the type of wiring and it is tobe considered for getting satisfactory results. However, the values of insulationresistances are to be noted in pre - commissioning test report and measuredfrom time to time to check deterioration of insulation during operation.

7. TESTING OF THE BIASED DIFFERENTIAL RELAY

The individual relay may be tested for the following characteristics inaccordance with the manufactures testing manual.

Pickup test and operating time measurement test Bias characteristic test 2nd Harmonic Restraint test 5thHarmonic Restraint test High set test


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7.1 Pickup test and operating time measurement test:

Circuit Diagram:

Ammeter Range - 0 to 500 mA HV - HV C.T. Current Input Terminal

RCCB Rating - In 16A, In 30 mA LV - LV C.T. Current Input Term inalSpil l Unbalanced Current Terminal

The Pick up Value indicates the minimum current in terms of percentageof rated current of the relay at which the relay would operate.

Actual Relay Pick-up(Amps)Phase

RelayPick-upset at HV LV

A Phase

B Phase

C Phase

Testing Procedure

Pickup Test:

1) Connect over current test set to any one H.V. C.T. input and operatingwinding terminals.

2) Connect the 230V mains to over current tes t set.

Bias Coils

Operating

Coil


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3) Switch on Auxiliary Supply4) Keep pick-up setting at minimum position (20%) if variable pickup is

available (or) fixed pickup setting.

E.g.: 15% for EE make20% for ER make

5) Switch on over current test set and slowly increase current value till therelay operates.

6) Note down current value, it must be equal to pickup setting (or) within themanufacturers tolerance.

7) Switch off the over current test set8) Reset the relay9) Repeat procedure for other HV and LV C.T. inputs.10) Repeat procedure for different pickup setting if available.

Operating Time Measurement Test:

1) Connect over current test set to any one HV C.T. input.2) Connect trip contact to stop terminal of timer3) Switch on Auxiliary Supply.4) Keep pickup setting on minimum position5) Switch on over current test set and adjust the current value as per

manufacturer recommendation ie. 2.0 x In or 3.0 x In.where In - rated current.

6) Switch off the over current test set and reset timer.7) Switch on the over current tes t and measure relay timing.

8) Tim ing must be less than or equal to manufacturer specification.

7.2 Bias Characteristic Test:

Circuit Diagram:

IbAmmeter Range - 0 to 4 AmpsIdAmmeter Range - 0 to 2 Amps

Ib Bias Current (Circulate Around the Balancing Loop)Id- Differential Current (Out of Balance Current flow through operating coil)

OperatingCoil

Bias Coil s


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Testing Procedure

1) Connect test set up as shown in circuit diagram.

2) Switch on Auxiliary supply.3) Keep bias set switch on required percent position.4) Keep pickup setting at minimum position.5) Adjust bias current Ib= 1 Amp by adjusting the resistance Rb.6) Slowly increase the differential current Id by decreasing resistor Rd

until relay operate and note the value at which it operates.7) Calculate bias by us ing formula (or) compare the Id current limits as

specified by the manufacturer.8) It must be equal or with in the tolerance as specified by the

manufacturer.9) Repeat procedure for other bias setting and all other phases.

10) After completion of testing, the bias current Vs differential currentcurve m ay be plotted and recorded for future reference.

Id% BIAS = ----------------- x 100

Ib+ (0.5 Id)

Setting PhaseOperating Current

Measured Id

(Amps)

Calculated% Bias

IdLimits

A

BIb:

CAB

% Bias : 20 %

Ib:

CABIb:

CA

B

% Bias : 30 %

Ib:CABIb:

CAB

% Bias 40%

Ib:C


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7.3 2nd

Harmonic Restraint Test:

For conducting this test at site, the circuit with rectifier can be made use of

as shown circuit below. In this circuit, the relay is connected to receive both A.Cand half-wave rectified D.C. current. The percentage of second harmonicscurrent component generated in this circuitry is given in the formula.

0.212 I2= -------------------- x 100

0.45 I1+ 0.5 I2

< 20%

Circuit Diagram:

A - A.C. Ammeter Range 0 5 Amps

D.C. Ammeter Range 0 1 Amps

D - Diode IN 4007 / 5 Amps.

Phase IDCSet I2(Amps)

IACmeasuredI1 (Amps)

% Restraintcalculated

HVA Phase

LV

HVB Phase

LV

HVC Phase

LV

Testing Procedure:

1) Connect tes t setup as shown in Circuit Diagram2) Switch on Auxiliary Supply3) Keep I2 (D.C. Current through the test rectifier) 0.467 or 0.8 amps D.C.

constant for 1 Amp and 2.335 Amp for 5 Amps rated relay.4) Observe, the relay should not operate.

Bias Coil s

Operating

Coil

1

2


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current may be measured by using power quality analyzerand recorded foreach power transformer for future reference.

7.5 High Set Test:

The differential relays are provided with a set instantaneous element forhigh speed protection against internal faults in the transformers.

Circuit Diagram:

A - A.C. Ammeter Range 0 20 Amps

Actual Relay Pick-up

(Amps)Phase

Relay

Pick-up

set at HV LV

A Phase

B Phase

C Phase

Testing Procedure

1) Connect test set up as shown in circuit diagram2) Switch on Auxiliary Supply.3) Keep High set at required position.

Bias Coils

Operating

Coil


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4) Switch on over current set and slowly increase current value till relayoperates.

5) Note down current value it must be equal to within +/- 10%

6) Repeat procedure for al l the three phases any one input.

Operating time measurement test:

1) Switch on over current tes t set and adjust the current value to 2 timesof High set value.

2) Switch off the over current test set and reset timer.3) Switch on the over current test set and measure relay tim ing.4) Timing must be less than 20 milli seconds.

8. CLASSIFICATION OF RELAYS AND THEIR TECHNICAL DATA

8.1 Classification of Relays :

Before we discuss the salient features of relays, what are the relaysavailable in board and their technical data are to be studied first.Static relays were mostly used in TNEB. The advanced microprocessor basedstatic relays are also nowadays utilized.

The following make relays are available in board.

Hindustan brown boveri

Easun Reyrolle

Asea brown boveri

Ashida

English Electric

Duobias-M (ER-Make)

All the above relays are classified in the categories of static and

microprocessors based. Duobias-M relay is latest numerical version for

transformer protection suitable for 2 & 3 winding.


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8.2 Comparison of Technical Data:

S.No Description EE HBB ER ABB Ashida Duobias-M

1. Current Rating 1 or 5A 1 or 5A1,1.73,

5A1 or 5 A

1 or 5Amp.

1 or 5 Amp.

Bias Setting 15 45% 10 50% 20 -40% 20 - 50% 20 45%10 70%(in steps of5%)

Pickup Setting 15 % Fixed 20 50%20 %Fixed

20 - 50% 20 -100%10 50%(in steps of5%)

2.

High Set

Setting10 In Fixed - Optional

8,13,20

In2 20 In 4 to 25 In

3.OperatingTime

45m sec.

30 60m sec.

50m sec.

30m sec.

< 35m sec.

30 m sec.

4. Burden 0.39 VA 0.3 VA 1 VA 0.02 VA < 0.2 VA < 0.05VA

5.Aux. supplyD.C.

30, 110 205 V

48, 110,250 V 30 V

24 55 V110-250V

18 40V85 250V

24 to 135V88 to 280V

6.Over loadingcapacity

2 Incontinuously40 In 3 sec

5 x In

Continuously

80 x In1 sec

-

10 x Incontino

usly

40 x Innormalcurrent

3 x In Continuously250 x In1 sec

7. Insulation Test2 KV

50 Hz for1 min

2 KV50 HzFor

1 min

2 KV50 Hz

For1 min

2.5 KV50 HzFor

1 m in

2.0 KV50 HzFor

1 min

2.0 KV50 HzFor1 min

8.ImpulseVoltage Test

5.0 KV / 1.2

/ 50 s5 KV

1.2/50 s-

5.0 KV

1.2/50s5.0 KV

5.0 KV

1.2 / 50 s

9.TemperatureRange -

- 20C to+ 60 c -

- 25C to+ 55C -

-10oto

+ 55oC

10.OperationalIndicator

Flag LED Flag LED LED LED

11.High SetIndication

Nil Nil Nil LED LED LED

12.ICTrequirement

Yes Yes Yes Yes Yes Nil


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8.3 Typical Block Diagrams and Connections:

English Electric

Ashida Relay


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9. COMPARISION OF OLD AND NEW VERSION RELAYS

9.1 Principle and Operation of Old Make ER 4C21 Relay

Schematic Diagram

C.T. Secondary currents circulate through the primary winding of the biastransformer, the rectified output of which is applied to the bias windings of thetransductor via the shunt resistor. Spill current (or) out-of-balance current flowsfrom the centre top of the primary winding of the bias transformer energizing thetranductor input winding and the harmonic bias unit.

Normal Condition / Through Fault Condition

As long as the power transformer is healthy, the transductor bias-windingis energized by full-wave rectified current which is proportional to the load (or)through fault current, this bias current saturates the transductor. The smallamount of out-of-balance currents in the transductor input winding i.e., operatingm.m.f does not exceed the bias m.m.f., resulting change in working flux density issmall and consequently the output to the relay is negligible.


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Internal Fault Condition

If there is an internal fault, the operating m.m.f. produced by the

secondary fault current in the transductor input winding exceeds the bias m.m.f.resulting in a large change in working flux density. This produces acorrespondingly large voltage across the relay winding; the resultant currentoperates the relay.

Magnetizing in-rush conditions

The harmonic bias unit is a simple tuned circuit which responds to thesecond harmonic component of the magnetizing current. When magnetizing in-rush current flows through the relay operating circuit, the rectified output of theharmonic bias unit is injected into the transductor bias winding and restrains the

relay.

9.2 New Version A.B.B. Make RADSB Type

Principles and Operations

Schematic Diagram

The output voltage from biased transformer T1 is rectified and obtainedregulated Negative voltage Ut after pass ing through non-linear circuit. Thetransformer T2 has two secondary winding with suitable adapted load resistors.One of the winding provides the voltage that initiates operation at internal fault.


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The voltage passes through a low-pass fil ter and then rectified in an idealrectifier. Finally the positive voltage Udis obtained. Another winding providesthe voltage to restraint the operation at inrush. The voltage passes through two

band-pass filters tuned for 2nd and 5th harmonics and provides after an idealrectifier a negative voltage Uh is obtained. The bias voltage Ut, harmonicsNegative voltage Uhand differential voltage Udare summed and supplied to leveldetector. The resultant voltage Usis compared with reference voltage Urwhich isselectable in front of Relay panel. The output voltage Ua is constant amplitudeand the time is proportional to variation of voltage with reference voltage. Thevoltage pulses Uaare integrated (Ub) and compared permanently set referencevalue Uzof the level detector. If Ube xceeds Uz, relay drive output trips relay.

Magnetising in-rush Condition: The harmonic voltage Uh is opposite to thevoltage Udand prevents operation.

Internal Fault Condition: The voltage Uh will be lower than voltage Ud, thedifferential relay will therefore operate.

Normal Condition : The voltage U t opposite to Ud, the restraint the relayoperation small at small through currents and large at large through currents.

9.3 NUMERICAL VERSION RELAY :

Duo Bias M (ER Make):

Duo Bias - M is an integrated multi microprocessor relay capable ofproviding all the transformer protection and alarm function for a 2 or 3 windingpower transformers. The protection function provided are current differential


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protection with load bias and second harmonic bias, differential high set overcurrent and restricted earth fault for each transformer winding. For a transformerdifferential protection it is necessary to correct the phase relationship and

magnitude of the C.T. secondary currents resulting from the arrangement of theprimary and secondary power transformer winding. Previously this wasaccomplished using a complicated combination of interposing C.T. s and star /delta arrangements to the current circuit. Duobias M eliminates this for almostall application.

The relay includes internal vector group compensation and currentamplitude correction for each transformer winding. Duo Bias M includes apowerful storage facility of HV and LV waveforms, indication and the status ofeach d.c. plant input and each output relay. The LCD on the front of the relayshows values for operate and restraint currents, and those following amplitude

and vector correction. This feature greatly simplifies commissioning and enablesthe rapid verification of data.

The input I1 represents the current on the HV side of the protectedtransformer after any necessary phase shifts and amplitude modification hasbeen applied, and I2 represents the equivalent current on the protectedtransformer LV s ide. The Algorithm calculates the functions I1, I2and these twosignals are processed to form the operate and restraint signals respectively.Both signals are digitally filtered to remove any unwanted DC and the filteredsignals are converted to RMS value calculated over the previous cycle beforecomparison. Second harmonic quantities are calculated from the signal I1 I2to

provide an inhibit signal to prevent the protection operating for magnetizinginrush conditions. The high set protection operates when the RMS value of thesignal I1 I2is greater than a Pre set value which can be varied. All settings onthe characteristic are under software control and characteristic has been madeflexible to cover the wide range of application encountered in transformerprotection.

Typical Connections Diagram :


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9.4 COMPARISION OF OLD AND NEW RELAYS

S.No. Old Version New Version

1. Static circuiting is employedMicroprocessors based circuiting is

employed.

2.5

thharmonic by pass circuit is

employed.

5thharmonic restraint circuit is

employed.

3. Fixed pickup only Variable pick up is possible.

4. Bias range is limited Wide bias range is possible.

5.Only one trip indication for all

phases

Individual phase has separate

indication for trip.

6.No separate indication for High Set

trip.

There is separate indication for

High Set.

7. High set is fixed High set is selectable

8. Limited Auxiliary supply range Wide range is available.

9. Operating time is large Operating time is lesser

10. Burden is high Burden is minimum

11.

Mechanical checking is necessary to

flag indicator and output armature

pickup relay for free from dust.

There is no need for mechanical

checking because of separate

enclosure for dust proof.

12. No Data Storage Facility

Data Storage facility is available

for pre-fault and post fault

information.

13.Self Monitoring Facility is not

availableSelf Monitoring Facility is available

14. ICT maybe required ICT is not required

15.

External auxiliary relays are required

for other protection like Buchholz, oil

and winding temperature etc.,

Auxiliary relays function is also

provided in a single unit of

Differential relay.


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9.5 TYPICAL TEST RESULTS - I

Tamilnadu Electricity BoardGobi Electricity Distribution Circle, GobiSub Division: MRT

Date: 19-01-05Name of the SS : Chennampatty 110/22KVSSRelay Make : ABBType : RADSBArt No. : IN7454 3344 CESerial No. : 807062Current Rating : 1AVoltage Rating : 110V

(Isr) Bias Setting Available : 0.2, 0.25, 0.35 & 0.5 In Adopted: 0.35 In(Isu) High Set Available : 8, 13 & 20 In Adopted: 13 InFlag Operation : LED

Connections

Phase Injection Terminals RADSB Aux-supply terminal

A H.V. 3

L.V. 6

Spill12RXTUG2H

B H.V. 4 T.B. TerminalL.V. 7 +ve116

Spill13 -ve 118

C H.V. 5L.V. 8

Spill14

IR Values:-

i) D.C. Circuit to Earth : 40 M ohm

ii) C.T. Circuit to Earth : 50 M ohm

iii) C.T. Circuit to D.C. Circuit : 40 M ohm

iv) Main C.T. to ICT Circuit : 70 M ohm


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Pick Up Test

Actual Relay Pick-up (Amps)Phase

RelayPick-upset Isr HV LV

ManufacturerLimit

A Phase 0.35 0.33 0.33B Phase 0.35 0.33 0.33C Phase 0.35 0.33 0.33

+ 10% of setCurrent

Bias Characteristic:

Setting PhaseOperating Current

Measured Id(Amps)

ManufacturerIdLim its(Amps)

A 0.78B 0.75Ib: 1.5A

C 0.74

0.65 to 0.95A

A 1.78B 1.78

Isr set at0.35 In

Ib: 3.0A

C 1.73

1.6 to 2.2A

2nd

Harmonics Test :

0.472 I2% Restraint = ---------------------- x 100

I1+ 1.11 I2

Keep I2 = 0.8A DC Constant for 1 A relay

PhaseIDCSet I2

(Amps)

IACmeasured

I1 (Amps)

% Restraint

calculated

Manufacturer

Limits of I1

HV 0.8 1.52 15.6A Phase

LV 0.8 1.59 15.2

HV 0.8 1.67 14.7B Phase

LV 0.8 1.68 14.6

HV 0.8 1.69 14.6C Phase

LV 0.8 1.68 14.6

14% to 18%

(1.2 to 1.8A)


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High Set Test :


RelayPick-upset Isu HV LV

Manufacturer

Limits of setcurrent

A Phase 13A 12.7 12.7B Phase 13A 12.3 12.3

C Phase 13A 12.6 12.6

+ 10%

Operating Time Measurement

Manufacturer Data : 3 Isr = approx 30 m sec.Isr = 0.35A

3Isr = 0.35 X 3 = 1.05 A

Operating time measured for 3Isr= 27 m sec.

High Set Operating Data : 2 Isu = 10 to 20 m sec.Is u = 13 Amps

2 Isu = 13x2 = 26 Amps

Operating time measured for 2 Isu = 12 m sec.

BIAS SLOPE CURVE


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9.6 TYPICAL TEST RESULTS - II

Tamilnadu Electricity BoardGobi Electricity Distribution Circle, GobiSub Division: MRT

Date : 26.05.05Name of the SS : Nambiyur 110 / 11 KVSSRelay Make : EEType : DTH31Art No. : DTH31FF8011A (m)Serial No. : m432840Current Rating : 1AVoltage Rating : 30V

Pickup Setting Available : 15% Fixed% Bias setting Available : 15, 30 and 45 Adopted : 30%High set Available : 10 Amps Fixed Adopted : -Flag Operation : Flag

Connections

Phase Injection Terminals DTH31 Aux-supply terminal

A H.V. 11

L.V. 14

Spill 12

B H.V. 15 T.B. TerminalL.V. 18 +ve19

Spill 16 -ve 20

C H.V. 7L.V. 10

Spill 8

IR Values:-

i) D.C. Circuit to Earth : 30 M ohm

ii) C.T. Circuit to Earth : 40 M ohm

iii) C.T. Circuit to D.C. Circuit : 30 M ohm

iv) Main C.T. to ICT Circuit : -


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Pick Up Test

Actual Relay Pick-up (Amps)Phase RelayPick-upFixed HV LV

ManufacturerLimit

A Phase 0.15 0.15 0.15B Phase 0.15 0.15 0.15C Phase 0.15 0.17 0.17

12% to 18%of ratedcurrent

Bias Characteristic:Id

% Bias = --------------- x 100

Ib+ (0.5 Id)

Setting Phase

OperatingCurrent

Measured Id(Amps)

Calculated %Bias

ManufacturerLimits

A 0.34 29%

B 0.34 29%Ib:1.0A

C 0.36 30.5%A 0.68 29%

B 0.68 29%

% Bias :30%

Ib:2.0A

C 0.69 29.4%

+ 10% ofBias Setting

(%)

2ndHarmonics Test:

0.212 I2% Restraint = ---------------------- x 100

0.45 I1+ 0.5 I2

Keep I2 = 0.467A DC Constant for 1 A relay

PhaseIDCSet I2

(Amps)IACmeasured

I1 (Amps)% Restraintcalculated

ManufacturerLimits

HV 0.467 0.99 14.5%A Phase

LV 0.467 0.97 14.7%

HV 0.467 1.11 13.5%B Phase

LV 0.467 1.11 13.5%

HV 0.467 1.07 13.8%C Phase

LV 0.467 1.07 13.8%

12% to 20%


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High Set Test :


RelayPick-upFixed HV LV

Manufacturer

Limits of setcurrent

A Phase 10A 9.2 9.2B Phase 10A 9.4 9.4

C Phase 10A 9.5 9.5

+ 10%

Operating Time Measurement

Manufacturer Data : 45 m sec at 2 times Setting CurrentSet Current Id= 0.15 Amps

Operating time measured for 2 Id= 46 m sec.

Bias Slope Curve :


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10. ANALYSIS OF DIFFERENTIAL PICKUP FEATURE

A series of tests have been done with the object of confirming that the

relay is capable of differentiating between the harmonic content of a magnetizingsurge and harmonics present in the output current of a heavily saturated C.T.under internal fault conditions.

The relay is extremely stable during through faults and provides highspeed operation on internal faults.

In addition, fifth harmonic restraint to avoid mal operation under over-excited condition.

While conducting the test, the following problems are associated with the

application of differential relay.

10.1 Pickup Test:

An object of confirming that, the relay required minimum current inoperating coil to cause operation.

a) What is the setting of bias at pickup test?

During the test, the current is passed through one half of the bias windingand the operating winding.

The pickup value of operating coil is independent of the bias setting.Therefore, keep the bias setting at the lowest value.

b) How to select pickup setting?

In old relays, the biased differential element has a fixed setting of the ratedcurrent. It indicates the minimum current in terms of percentage of the ratedcurrent of the relay at which it would operate.

In practice there will always be some differential current flowing due toC.T. errors, ratio mismatch and tap changing. Therefore, the differential current isrequired to be a certain percentage of the mean through current before the relaywill operate. To prevent the relay operating on the steady state magnetizingcurrent of the transformer, a fixed setting is incorporated of 15% (or) 20% of therelay rated current. Initial pickup is continued with bias slope characteristic.


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The following conditions should be considered in addition, while selectinga proper pickup setting.

a) The interconnecting leads of the main CTs are of the same length(or) different length on each side of the relay.

b) Interpose C.T.s are connected on one side of the differential relay.

c) The prior loading of the current transformer meant for the differentialrelay is in series with breaker over current relays.

d) Whether main current transformers are of different rating, their over-current factors and their accuracy class.

Conclusion:

Normal setting of pickup for Power Transformer Differential relayis 20% of rated current.

At any time, the pickup setting should not exceed 50% of rated current.

11. ANALYSIS OF BIAS CHARACTERISTIC FEATURE

An object of confirming that the relay is capable of differentiating betweenthe faults is external or internal. The relay is extremely stable during throughfault and high speed operation on internal fault and yet maintains high s ensitivityfor internal faults when the differential current may be relatively small.

SettingRange( 0.1-0.5 )


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a) What is the setting of pickup at Bias characteristic test?

Selected pickup as per previous discussion may be adopted. If the relay

is tested for the first time, conduct bias slope characteristic with minimum pickupand then conduct with selected pick up as per our requirement. The two slopecharacteristics may be plotted and recorded for Bench Marking.

b) What is the effect of bias slope characteristic at higher pickup?

If higher pickup is adopted inadvertently, the bias slope characteristic ofthe relay not followed set bias characteristic initially. The through fault currentexceeds the 200% of rated current then it followed set bias characteristic.

Test Example: 1

Name of the SS : Thalavaipettai 110/22KV SS

Relay Make : Ashida, 1 Amp, 110V

Type : ACDF31HA

S.No. : 2KBDF005

Date : 25-08-2005

Setting Ib PhaseOperating CurrentMeasured Id

(Amps)

Calculated% Bias

0.5 A 0.20 33.331.0 A 0.34 29.061.5 A 0.52 29.542.0 A 0.69 30.532.5 A 0.87 29.64

% Bias : 30 %Pickup : 20 %

3.0 A 1.06 30.03

0.5 A 0.59 74.211.0 A 0.59 45.55

1.5 A 0.60 33.332.0 A 0.67 28.692.5 A 0.86 29.35

% Bias : 30 %Pickup : 60 %

3.0 A 1.04 29.54


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The fig. has shows that, the Relay operating bias characteristic if selectedhigher pickup of 0.6 x In instead of 0.2 x In, it is clear that, the relay is sluggish atlow incipient fault because of non-operating region increases at low through fault.

c) What is the exact bias slope?

The differential current (Id) Vs through fault current (Ib) gives the bias slopecharacteristic. The relationship between % bias, differential current and thethrough fault current is,

Id% Bias = ------------------------- x 100

Ib+ (0.5 Id)

The operating current Idat various through fault current Ibis given below.

BiasCurrent

0.5 1 1.5 2 2.5 3

20% 0.11 0.22 0.33 0.44 0.55 0.6625% 0.14 0.29 0.43 0.57 0.71 0.8630% 0.18 0.35 0.53 0.71 0.89 1.135% 0.21 0.42 0.64 0.85 1.06 1.2740% 0.25 0.5 0.75 1.0 1.25 1.545% 0.29 0.58 0.87 1.16 1.45 1.74

Id limit is + 10% Therefore % bias limit is + 2%


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Bias Slope Curve

For Example,

Test Example : 2

Name of the SS : Kolappalur 110/11KV SSRelay Make : ABB, 1 Amp, 110VType : RADSBS.No. : 407093Date : 27-08-2005

Setting Ib PhaseOperating Current

Measured Id(Amps)

Calculated% Bias

0.5 A 0.34 50.741.0 A 0.53 41.89

1.5 A 0.69 37.392.0 A 0.94 38.052.5 A 1.39 43.50

3.0 A 1.84 46.933.5 A 2.32 49.78

0.35 In

4.0 A 2.75 51.16

It is clear that, the various bias at various through current (Ib). The % biasis high at First and Last. The % bias is low at Middle as compared with other.

ie. 0.35 x Ingives 50% bias at First and last.0.35 x Ingives 40% bias at middle.

Hence the Protection Engineer has to select particular bias feature forparticular power transformer carefully.

% Bias Vs Ib


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4) How to decide the healthiness of relay if slightly beyond the limits oftest results?

If the test results are beyond the limit as per manufacture prescribedsettings and instruction, the relay may not be declared as defective immediately.The Protection Engineer has to analyze what are the options available in therelay for re-use effectively and to avoid procurement of new relay at a heavy costand revenue loss due to interruption.

Hence the following steps should be carried out and desired.

1) Conduct the bias test at various bias current.Ie. Ib= 0.5 ,1, 1.5, 2, 2.5, 3.0, 3.5 and 4.0 Amps.

2) Bias curve plotted from the test reading.

3) The drawn curve is to be compared with the slope curve mentioned incompany manufacturer.

4) In case of major deviation during comparison, the particular relay shouldnot be allowed for commissioning.

5) If there is a small deviation from manufacturer bias slope, the deviationregion is to be considered as,

1stregion, Ib= 0 to 1 A, Idrequired to operate relay is 0.5 In.

2nd

region, Ib= 1 to 2.5A, Idrequired to operate relay is 1 In.3rdregion, Ib= 2.5 A and above, Idrequired to operate relay is above 1.5

Permissible Limits

Ist

Region :

The relay is sensitive up to differential current 0.5 Inat 100% through faultcurrent. Hence, deviation may be allowed differential current up to 0.5 In.

2nd

Region :

This region is very im portant due to presence of incipient fault. The

deviation in that region may be allowed + 2% only.

3rd

Region :

The through fault stability is desired in that region at 250% and abovethrough fault current. The slope of bias may be deviated up to 80% ispermissible.

(or)

If you select either lower (or) higher bias settings to achieve the requiredbias s lope for particular power transformer.


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12. ANALYSIS OF 2ND

HARMONICS RESTRAINT FEATURES :

Test has been made to verify that the relay will remain inoperative under

magnetizing in rush current up to 30 times the C.T. rating when the transformeris energized. In addition, confirming that the relay is capable of differentiatingbetween the inrush current and fault currents.

1) What is inrush? How will affect the differential relay?

During power transformer charging, an instantaneous change in fluxlinkage in a power transformer will cause abnormally large magnetizing(2

ndharmonics) current to flow and has no counterpart on the secondary winding

side. Consequently there is a spill current in the differential circuit which cancause the differential relay to mal operate.

2) What is the purpose of diode in the 2ndharmonics restraint test circuit?

The second harmonic restraint is effective to block the relay operationwhen the second harmonic current content of the differential current exceedsabout 20% of the fundamental. For testing this circuit at site, wheneverarrangements to generate second harmonic current is not available, an alternatecircuit with test rectifier can be made use of in this circuit the relay is connectedto receive both AC and half-wave rectified D.C. current.


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The percentage of second harmonic current component generated in thiscircuitry is given in the formula.

0.2 1 2 I2------------------------------ x 100 < 20%

0.45 I1+ 0.5 I2

I1 = A.C. Current I2 = D.C. Current at constant value of 0.467 A

Another method to derive the % 2nd

harmonics restraint particularly ABBrelay recommends the following setting and the formula is given below.

Set I2 D.C. Current at constant value of 0.8ATherefore, 2

ndHarmonic restraint calculated from the formula,

0.472 x I2------------------ x 100 = 14 to 18%I1+ (1.11 x I2)

The comparison of I2settings (0.467 and 0.8 A) :

Test Example : 3

Name of the SS : Kolappalur 110/11KV SSRelay Make : ABB, 1 Amp, 110VType : RADSBS.No. : 407093Date : 27-08-2005

Setting I2 PhaseOperating Current

Measured I1(Amps)

Calculated% Restraint

0.8 A 1.53 15.610.8 B 1.54 15.55

IsrSetting0.35 In%

0.8 C 1.59 15.230.467 A 1.0 14.48

0.467 B 1.0 14.48

Isr

Setting0.35 In%

0.467 C 0.98 14.47

The variation of the percentage restraint of the above two DC constantsare very small for the same relay at the same settings.

3) What is the Bias setting at harmonic restraint test?

The harmonic restraint values are independent of bias and pickup setting.Hence the bias and pickup settings are kept at selected rating.


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Test Example:

Name of the SS : Thalavaipettai 110/22KV SS

Relay Make : Ashida, 1 Amp, 110VType : ACDF31HAS.No. : 2KBDF005Date : 25-08-2005

BiasSetting

PickupSetting I2 Phase

Operating CurrentMeasured I1

(Amps)

Calculated% Restraint

20% 20% 0.467 A 0.49 21.845% 20% 0.467 A 0.49 21.8

20% 60% 0.467 A 0.5 21.5945% 60% 0.467 A 0.5 21.59

4) How to decide the healthiness of relay, if the 2nd

harmonics % restraintexceeds slightly more than 20 %?

Before we decide the healthiness of relay, we should understand theinrush phenomena when the transformer is energized.

If the transformer is re-energized at the instant the voltage wave formcorresponds to the residual magnetic density within the core, there would be asmooth density within the core, there would be smooth continuation of theprevious operation with no magnetic transient.

1


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In practice, however, the instant when switching takes place in rush currentcannot be controlled and a magnetizing transient is practically unavoidable. Ifhowever, it is assumed that the circuit is re-energized at the instant when the flux

would normally be at its negative maximum value (-max) the residual fluxwould have a positive value. Since magnetic flux can neither be created nor

destroyed instantly, the flux wa ve, will start with the residual (R) and trace thecurve (1).

Since power transformers operate near the knee of the saturation curve, a flux

demand of 2mdrives the transformer core deep into saturation, causing it todraw a very large magnetizing current with a peaky non-sinusoidal waveform.Such non-sinusoidal or distorted waveform is known as harmonics. Themagnetizing current is, therefore, very high, of the order of 8 to 30 times the full-lode current. This current is known as inrush current. The inrush currentdecays rapidly for the first few cycles and then very slowly. Sometimes they take4 to 6 seconds to subside.

The inrush wave form is predominantly 2nd

Harmonic content, whereas,the internal fault current consists of only of the fundamental. Thus, we candevelop additional restraint based on harmonic content of the inrush current.


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The Percentage second harmonic restraint value should be less than20%. Even if the Percentage restraint exceeds slightly more than 20%, the relayis considered to be healthy if the differential relay does not actuate for inrush

current for ten operations of power transformer without load.

In recent years improvem ents in core steel and design has resulted in lessinrush current harmonics with possibilities of the second harmonics being as lowas 7%. Now a days power quality analyzerinstruments are available to studyactual harmonic present in the current wave while transformer energized withoutload. Record the values for benchmark with future comparison.

5) What is the working principle of zero crossing detection methodand how it will effectively utilize 2

ndharmonic & 5

th harmonic restraint in

new differential relays?

This method, does not involve harmonic filter circuit and their associateddelay. A different approach, working on the direction of zero crossing in thecurrent wave form, was therefore evolved to distinguish between genuine faultconditions and conditions of inrush and over fluxing.

An examination of the wave form reveals there is a substantial period ofthe half cycle when the current remains zero (or) near zero, unlike in a faultcurrent wave form passes through zero current quickly. It can be seen that the

gap detecting method as shown can distinguish between fault condition andother by looking at the duration of the gap produced. This basically comprisestwo timers. Their operation in conjunction with the differential comparator (theheart of the relay).

In simple terms, the arrangement is such that timer 2 is set to trip after acertain time while timer 1 is set to inhibit timer 2 after its operating time, unlessit itself is prevented from doing so by the comparator. The latter does not inhibittim er 1 unless the current magnitude is above the set threshold. The thresholdlevel is varied by the current setting on the front of the relay.

DifferentialComparator

Timer 1 Timer 2Trip

Bias

Differential

Threshold

Inhibit Inhibit


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Circuit Diagram

D=Diode (IN4007 / 5 Amps)

Relay OperationPhases S1

S2 Closed S2 Open

HV ClosedA Phase

LV Closed

HV ClosedB Phase

LV Closed

HV Closed

C PhaseLV Closed

Testing Procedure:

1) Connect tes t setup as shown in circuit diagram.2) Switch on Auxiliary supply.3) Close Switch S1and S2and set the current equal to rated current of relay,

check that the relay operates.4) Open Switch S2, Close Switch S1and check that relay does not operate.

In this test, % 2nd

harmonics restraint could not be measured. Themanufacturer has recommended energizing transformer 10 times without loadand checking that the relay does not mal - operating. If the % 2nd harmonicsrestraint value is required, the routine 2

nd harmonic bias test will be conducted

and record the % value for future comparison.


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13. ANALYSIS OF HIGH SET FEATURE

The differential relays are provided with a set instantaneous element for

high speed protection against internal faults in the transformers. Such faultinvolves very high fault currents and this causes the C.Ts to go into partialsaturation and the resulting harmonics slow down relay operation. To take careof such a contingency, the differential relay is provided with an unrestrained highset element. A setting of at least ten times rated current is usual to preventoperation of this element during transformer inrush.

High set is determined by the m agnitude of the inrush current to the powertransformer and thus affected by the rating and the connection of the powertransformer. High set should be set as low as possible but not less than themaximum 3 phases through current and not less than maximum magnetizing

current. If the power transformer connection is Dy and the rating between 10 to100 MVA, the recommend high set value is around 13 In. If below 10 MVA, the

high set value will be around 20 In. The operating time of high set will be alwaysless than 20 milli seconds. When the differential relay is also to provide busprotection, the setting (20 In) should be chosen to ensure stability for through busbar faults . To ensure still better through fault stabil ity this element has also anauto ranging setting feature whereby it is caused to increase in the throughcurrent. Hence optimum performance is ensured at all times.


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Voltage drop e = 2 x R x I

(or)

e = 2 x L x I/ (K x q)

Where,

e = Voltage drop in voltsR = Resistance in ohm (Lead One Way)

I = Line current in AmpsL = Length in meter (Lead One Way)q = Conductor cross section in sq.mm.K = Conductivity (for Cu = 56, Al = 35)

14.3 Secondary Current Injection:

Open intentionally two pilot wire terminals at the C.Ts secondary terminalsand inject current just above the pickup value and ensure D relay picks up andtripping command is sent from the D relay to the master relay. This can bedone on either sides H.V. as well as L.V. and on all three phases. After theabove test, ensure that all the pilot wires are properly connected to the terminalswith adequate tightness, and test the connections by primary current injection.Do not open circuit the secondary circuit of a current transformer under load

conditions since the high voltage produced may be lethal and could damageinsulation.

14.4 Testing the Protective Scheme by Injecting Three Phase PrimaryCurrent:

This method is mainly applied to check the differential protections toensure that the current transformer connections have been made according tothe vector group. Possible errors on the diagram or in the actual secondarywiring are,

a) C.T. s starred on wrong side.

b) Wrong delta connection used on C.T.

c) Pilots between C.T.s crossed over.

d) C.T. polarities are interchanged.


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Figure shows the test circuit set up for checking a differential protectionscheme of a power transformer. The transformer is solidly shorted on the lowvoltage side by means of a set of temporary test short circuit conductors andthree phase supply from 400V tes t main is applied to high voltage winding.

The through currents measured at relay terminals both H.V. and L.V. s idewill be in milli amps. The operating current measured should be negligible if therelay is connected correctly to the current transformers and if the C.T. polaritiesare correct.

14.5. The possibilities of secondary wrong connections :

The possibilities of secondary wrong connections of powertransformer bushing C.Ts and the corresponding spill currents at

differential relay end were taken at

THALAVAIPETTAI 110/22 KV Sub-Station

Power transformer ANDREW YULE make 10 MVA

Tested date : 10-06-2004


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1. Correct Connection:

2. Wrong Connection:

B and C phase pilot wire transposed on H.V. Side

Current measured at the Di fferentialRelay Terminals:-

HVA : 42.6 ma (I)

HVB : 42.2 ma (I)HVc : 42.7 ma (I)

LVa : 42.3 ma (I)LVb : 41.8 ma (I)

LVc : 41.7 ma (I)Spill A : 0.3 ma

Spill B : 0.3 maSpill C : 0.5 ma

HVN : 0.1 ma

Current measured at the Di fferential

Relay Terminals:-

HVA : 42.8 ma (I)HVB : 41.9 ma (I)

HVc : 43.0 ma (I)LVa : 42.5 ma (I)

LVb : 41.9 ma (I)LVc : 41.8 ma (I)

Spill A : 0.3 ma

Spill B : 72.7 ma (3I)Spill C : 70.4 ma (3I)HVN : 0.1 ma


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C phase C.T. reversed on H.V. Side

4. Wrong Connection :

B & C Phase C.T. Reversed on H.V. Side


HVA : 42.8 ma (I)

HVB : 42.3 ma (I)

HVc : 42.6 ma (I)

LVa : 42.6 ma (I)LVb : 42.6 ma (I)

LVc : 42.0 ma (I)

Spi ll A : 0.4 maSpi ll B : 0.6 ma

Spill C : 84.1 ma (2I)

HVN : 84.7 ma (2I)


HVA : 42.7 ma (I)

HVB : 42.3 ma (I)HVc : 42.3 ma (I)

LVa : 42.5 ma (I)LVb : 41.9 ma (I)

LVc : 41.5 ma (I)Spi ll A : 0.5 ma

Spill B : 84.5 ma (2I)Spill C : 83.4 ma (2I)

HVN : 85.5 ma (2I)


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S1 Star on H.V. Side


B & C Phase transposed and A Phase C.T. Reversed on H.V. Side


HVA : 42.3 ma (I)HVB : 42.0 ma (I)HVc : 42.0 ma (I)

LVa : 42.6 ma (I)LVb : 41.5 ma (I)

LVc : 41.6 ma (I)

Spill A : 84.4 ma (2I)

Spill B : 84.0 ma (2I)Spill C : 83.4 ma (2I)

HVN : 0.6 ma


HVA : 42.3 ma (I)

HVB : 39.3 ma (I)HVc : 43.0 ma (I)LVa : 42.0 ma (I)

LVb : 41.5 ma (I)

LVc : 41.7 ma (I)Spill A : 84.2 ma (2I)

Spill B : 72.5 ma (3I)Spill C : 72.4 ma (3I)HVN : 84.5 ma (2I)


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S1 - out on LV Side


Reverse Delta, S2 - out on LV Side


Relay Terminals:-

HVA : 43.0 ma (I)

HVB : 42.5 ma (I)HVc : 42.9 ma (I)

LVa : 41.6 ma (I)LVb : 41.3 ma (I)

LVc : 42.6 ma (I)

Spill A : 72.6 ma (3I)Spill B : 72.6 ma (3I)Spill C : 73.0 ma (3I)HVN : 0.4 ma

Current measured at theDifferential Relay Terminals:-

HVA : 42.1 ma (I)

HVB : 41.8 ma (I)HVc : 42.8 ma (I)

LVa : 42.0 ma (I)LVb : 41.8 ma (I)LVc : 43.2 ma (I)

Spill A : 40.8 ma (I)Spill B : 42.5 ma (I)Spill C : 41.6 ma (I)

HVN : 0.4 ma


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Reverse Delta, S1 - out on LV Side


a & c PhasePilot Wire transposed on LV Side



LVa : 42.2 ma (I)LVb : 41.6 ma (I)

LVc : 41.6 ma (I)

Sp ill A : 83.8 ma (2I)

Sp ill B : 84.6 ma (2I)Sp ill C : 83.6 ma (2I)

HVN : 0.5 ma

Current measured at theDifferential Relay Terminals:-


LVa : 41.2 ma (I)LVb : 41.7 ma (I)

LVc : 42.4 ma (I)Spill A : 71.3 ma (3I)Spill B : 0.6 ma

Spill C : 72.5 ma (3I)HVN : 0.4 ma


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c Phase CT Reversed on LV Side


b &c phase CT reversed on LV Side


HVA : 41.5 ma (I)

HVB : 42.4 ma (I)

HVc : 42.3 ma (I)

LVa : 23.8 ma (I/3)LVb : 41.5 ma (I)

LVc : 24.0 ma (I/3)

Sp ill A : 47.7 ma (2I/3)Spill B : 0.4 ma

Sp ill C : 48.0 ma (2I/3)HVN : 0.2 ma


HVA : 42.0 ma (I)

HVB : 42.3 ma (I)HVc : 42.4 ma (I)

LVa : 24.0 ma (I/3)LVb : 23.7 ma (I/3)

LVc : 41.5 ma (I)Spill A : 47.7 ma (2I/3)Spill B : 47.8 ma (2I/3)Spill C : 83.2 ma (2I)

HVN : 0.7 ma


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a & c phase Pilot Wire Transposed and c phase CT reversed on LV Side


a & c phase Pilot wire Transposed and b & c phase CT reversed on LV side


HVA : 42.2 ma (I)

HVB : 42.4 ma (I)HVc : 42.6 ma (I)

LVa : 24.6 ma (I/3)LVb : 41.9 ma (I)

LVc : 24.2 ma (I/3)Sp ill A : 23.7 ma (I/3)

Spill B : 0.5 maSp ill C : 23.8 ma (I/3)HVN : 0.1 ma


HVA : 42.9 ma (I)

HVB : 42.9 ma (I)HVc : 42.7 ma (I)

LVa : 41.3 ma (I)

LVb : 23.7 ma (I/3)LVc : 24.1 ma (I/3)Sp ill A : 40.5 ma (I)

Sp ill B : 47.9 ma (2I/3)Sp ill C : 24.0 ma (I/3I)HVN : 0.2 ma

LV


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C phase CT reversed on HV Side and a phase CT reversed on LV side


C phase CT reversed on HV side, a & b phase CT reversed on LV side


Relay Terminals:-

HVA : 42.1 ma (I)

HVB : 41.7 ma (I)

HVc : 42.4 ma (I)

LVa : 23.4 ma (I/3)LVb : 23.9 ma (I/3)

LVc : 41.6 ma (I)Spill A : 47.7 ma (2I/3)Spill B : 48.8 ma (2I/3)Spill C : 83.2 ma (2I)HVN : 85.2 ma (2I)


Relay Terminals:-

HVA : 42.4 ma (I)

HVB : 42.8 ma (I)HVc : 42.6 ma (I)

LVa : 23.5 ma (I/3)

LVb : 41.3 ma (I)LVc : 24.2 ma (I/3)Spill A : 47.9 ma (2I/3)Spill B : 84.0 ma (2I)

Spill C : 48.3 ma (2I/3)HVN : 84.6 ma (2I)


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CONCLUSION:

H.V. Side C.T. reverse reflects in common neutral (H.V.N) and Spill. H.V. Side Pilot transposed reflects in spill. L.V. Pilot transposed reflects in spill. L.V. Side Pilot taping at positive junction (S1out) reflects in spill. L.V. Side C.T. DELTA B connection (reverse delta) reflects in spill. L.V. Side C.T. reverse reflects in spill and L.V. pilots . H.V. and L.V. side C.T. reverse reflects in common neutral (H.V.N), L.V.

Pilots and spill.

NOTE: Any transposition in pilot wires of HV & LV side of differential relays is

found out by injecting current at C.T. terminals of power transformer.

14.6. Tripping and Inter Tripping Tests :

Connect the auxiliary supplies to relay. Operate relay by hand (testprovision) and see that circuit breakers operate only when the appropriate linksare inserted and that the indicators operate correctly.

14.7. Before Putting In To Service :

a) See that trip supply is connected.

b) See that all relays are reset.c) Restore alarm link and trip link normal position.

14.8. Magnetising S urge Tests :

Perform ten switching operations of the power transformer with out load,the relay should not operate.

14.9. Tests When Power Transformer Is Energized and In Load Service :

a) Current measurement should be made at relay terminals with the help of

digital clip on millimeter of suitable range.b) This spill current when expressed as a percentage of the load current

used in the test, indicates the minimum amount of bias necessary on therelay to maintain stability on through faults. It is advisable to measure thespill current through the relay operating coil during load test with tapchanger set on its maximum and minimum tap.

The transient condition cause unequal C.T. saturation and spill which canplay with the differential relay stability. C.T. requirement has been made possibleto improve stability.


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Under through fault conditions, the protection is stable with fault currentequivalent to 25 times rated current with up to 30% mismatch of the line currenttransformer and the content of the third harmonics can be up to approximately

60% of the fundamental.

Max. Through fault current beyond which scheme (mal) operatesStability ratio = ------------------------------------------------------------------------------------------------

Min. Internal fault current required for tripping

The higher the stability ratio, the better is the ability of the system todiscriminate between external and internal faults. Ensure whether the adopted %bias setting is adequate. (This can be ensured during through fault condition, ifnecessary raise the % bias).

14.10. Maintenance :

The following test should be performed yearly and recorded.

a) General check of connectionsb) Relay calibrationc) IR value measurementd) Tests using load current

14.11. Relay requirement :

The biased (%) differential relay meet requirement as per IEC 255-13.

15. TROUBLE SHOOTING AND REMEDIES

It is not very uncommon to have certain unexplainable operation of relaysin spite of very careful selection of relay settings. In many such cases definitefaults were found to exist outside the sphere normally scrutinized by theprotection engineer. It is therefore, necessary that engineer should proceed withan open mind to inves tigate such apparent mal operations. The followingoccurrence is an example.

15.1 Bushing C.T. terminal loose connection :

At one sub station, the transformer was tripped by differential relay. Thetes t on transformer and relay were found normal. Repeated tests on transformerand relays were in vain. Finally the wiring and connections were checked fromrelay to power transformer and found that bushing C.T. terminal connection wasloose. After tightness, the power transformer was put back into service andfound normal.


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C.Ts and relay as when ICTs are introduced the circuit connection to the primaryand secondary sides of the ICTs will be considered as different domains forgrounding the neutral circuit. The intent is to emphasize that all neutral circuits in

the protection have an earth reference but one and only a s ingle earth reference.

The reason why the ground should be made at only one point is to avoidimproper relay operation and cause damage to the C.T. interconnections. Ifgrounds are made at two or more locations circulating current may be caused toflow in the differential circuit because of difference of potential between thegrounding points owing to the flow of large ground fault current.

A ground in the yard and another in the switch house put the secondarywiring in parallel with the ground mat so that part of the heavy fault current canflow in the secondary winding directly to either damage or cause mal-operation.

Only one ground in the circuit is sufficient to minimize any electrostatic potentialref. 9(b).

15.7. Effect of Ambient Temperature:

The relay is suitable for indoor m ounting and should be installed in a placefree from damp, dust, corrosive vapors and the like. There must also be a freecirculation of air around the relay. The operation of relay may be nuisance ifambient temperature limits is exceeded.

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3.15& 5 MVA Power Transformer Testing