BDD Transformer Differential Relay With …engineering.richmondcc.edu/Courses/EUS 230/Notes/BDD...

Module 13

General Electric TypeBDD Transformer

Differential Relay WithPercentage and

Harmonic Restraint

PROTECTIVE RELAYMAINTENANCE – BASIC

Introduction ........................................................ 2Objectives............................................................ 2Applications ........................................................ 3Components ........................................................ 4Operating Principles .......................................... 5Protection Scheme .............................................. 9Testing ................................................................ 10

Types of Tests ..................................................... 11Adjustments........................................................ 28Summary ............................................................ 30Lab

General Electric Type BDD TransformerDifferential Relay with Percentage andHarmonic Restraint

Module 13

PROTECTIVE RELAY MAINTENANCE – BASIC

2

INTRODUCTION

The General Electric Type BDD Transformer Differential relay withpercentage and harmonic restraint is designed specifically for trans-former protection. The BDD is available for application on powertransformers for protection of two-winding transformers, three-winding transformers, or three-winding transformers and a sectionof bus.

The BDD relay has currents applied to it from instrument currenttransformers located on each side of the transformer. These two cur-rents are compared, and if they are of the same magnitude there isno protection problem; however, if the two currents are not the same,a protection problem exists. The relay operates at a percentage differ-ence between the two currents.

NOTE: From this point forward all General Electric TypeBDD transformer differential relays will be refer-red to as type BDD relays.

OBJECTIVES

Upon completion of this module and lab exercise, the participantwill be able to perform the following tasks related to the type BDDTransformer differential relay with percentage and harmonicrestraint:

1. Describe its application.

2. Describe its components.

3. Describe its operating principles.

General Electric Type BDD TransformerDifferential Relay with Percentage and

Harmonic Restraint

Module 13

3

4. Interpret an external wiring connection diagram.

5. Interpret an internal schematic diagram for it.

6. Describe and perform the minimum pickup test.

7. Describe and perform the through fault test.

8. Describe and perform the slope test.

9. Describe and perform the harmonic restraint unitpickup test.

10. Describe and perform the instantaneous unit pickup test.

11. Describe and make calibration adjustments.

APPLICATIONS

The BDD relay is applied to power transformers to detect internalfaults. The current entering one end of a winding must be within aspecified percentage of the current at the other end. The differencein this current is called slope. The slope incorporated is to allow forsmall instrument current transformer (CT) errors, due possibly toCT ratio mismatch and/or CT saturation. Available percentageslopes include 15%, 25%, and 40%.

On internal faults, percentage differential relays are sensitive andfast operating. For external faults, they are reasonably safe fromfalse operation.


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4

COMPONENTS

The BDD relay is a single-unit transformer differential relay. Princi-pal parts of the relay and their locations are shown in Figure 1. TheBDD uses a sensitive, polarized, plug-in, electromechanical relay asthe operating element.

FIGURE 1General Electric Type BDD Transformer Differential

with Harmonic Restraint Relay

AuxiliaryUnit

PercentSlope

CalibratingResistor

(R3)

RatioMatching

Taps

InstantaneousOvercurrentUnit (DOC)

Series TuningCapacitor(C-1)

DC ControlVoltage TapPlate

DifferentialCurrentTransformer(DCT)

RectifierTerminalBoard

ThroughCurrentRestraintTransformer(TCT)

96-0104-1

Percent SlopeTap

Plate


Harmonic Restraint

Module 13

5

There are two tap blocks found on the type BDD relay, as shown.These ratio matching taps are to match current input from currenttransformers on both sides of the protected transformer. There isalso an instantaneous unit and auxiliary trip unit. The auxiliarytrip unit is not a seal-in unit.

OPERATING PRINCIPLES

The differential unit is supplied with currents, which are modifiedby current restraint taps so “normal” mismatch is at a minimum.In addition, they have a slope characteristic, to compensate for mis-match up to a percent of difference current. This gives the relaygreater selectivity to restrain from operation on through faults, andstill be sensitive for operation on internal faults. In addition, trans-former differential relays must be able to discriminate betweeninrush current on transformer energization and actual internal faultcurrent. Therefore, they are designed with harmonic restraint cir-cuitry. The instantaneous unit is provided to ensure high-speedoperation, with no restraint on heavy internal faults.

To better understand the operation of the BDD, the internal sche-matic (Figure 2) is used to explain the relay’s operation under differ-ent system conditions.

RANGE TAPS

2.9 – 8.7 2.9 3.2 3.5 3.8 4.2 4.6 5 8.7

2.9 – 8.7 2.9 3.2 3.5 3.8 4.2 4.6 5 8.7


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6

FIGURE 2Internal Schematic for General Electric Type BDD

Transformer Differential with Harmonic Restraint Relay

Aux. Aux.Wdg. #1

ThroughCurrent

RestraintTransformer

Rectifier Lead No.to Terminal Board

LO D.C.V. Tap

R7

R8

HI D.C.V. Tap

Restr.1

8 2

7

3456

OperateR1

R2

7

3

4

HarmonicFrequencyRestraintFilter

L2C1

L1

5

910

HarmonicRestr.Adj.

Thyrite

I(Instan.)

DifferentialCurrentTransformer

Am

ps.

2.9

3.2

3.5

3.8

4.2

4.6 5 8.7

Wdg.12

Wdg. #2

Percent SlopeAdjustment

R31

6

8

2

Short Brushes

2 =

C2

97-0368-2

Inst.Inst.

AuxilaryRelay

Main Unit(Polar relay)

Fundamental FrequencyFilter

10

9

8

7

6

5

42

1


Harmonic Restraint

Module 13

7

NORMAL CONDITIONS

When the system is under normal conditions, current flows throughthe differential transformer and the restraint transformer. Thedifferential transformer output is the vector sum (difference) of thetwo relay input currents. This output feeds the instantaneous unit,then the input terminals of the fundamental filter and the har-monic filter. Since the waveform should be 60 Hz, only the funda-mental filter should have an output. This output is fed to the oper-ating coil of the main unit (polar relay).

The two relay input currents also flow to the restraint transformer.The output of the through restraint transformer is the vector sum(additive) of the two relay inputs. This output is fed to the restraintcoil of the main unit, after it passes through the percent slope adjust-ment resistor.

Under normal system conditions, within the slope characteristic,the restraint coil is stronger than the operating coil of the main unit;therefore, the main unit does not close its contacts.

EXTERNAL FAULT

When the system is under external fault condition, current flowsthrough the restraint transformer and the differential transformer.Everything, as previously stated under normal conditions, occursonce again. The difference is in the magnitude of current input,however the results will be the same (that is, there is no contactclosure of the main unit).


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8

INTERNAL LOSS/HIGH IMPEDANCE INTERNAL FAULT

When the transformer has an internal loss or a high impedanceinternal fault, the output of the differential transformer increasesand the output of the restraint transformer decreases. This causesthe operating coil of the main unit to increase in strength. If thisincrease is above the slope characteristic of the relay, the operatingcoil overcomes the restraint coil and the main unit closes its contacts.

When the main unit contacts close, the path for the auxiliary relayunit is complete. The auxiliary relay unit operates and closes itscontacts. The closure of the auxiliary relay unit contacts either causea direct trip of the breaker, or more typically, energize an 86 relay.

INTERNAL FAULT

When the transformer has an internal fault, the output of the differ-ential transformer increases and the output of the restraint trans-former decreases. This causes the operating coil of the main unitto increase in strength. If this increase is above the slope charac-teristic of the relay, the operating coil overcomes the restraint coil,and the main unit closes its contacts.

If the internal fault is a low impedance fault (bolted) and the faultis above the instantaneous unit pickup setting, the instantaneousunit picks up and closes its contacts.


Harmonic Restraint

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9

INRUSH CONDITION

When the transformer is being energized, a current surge takesplace. The current surge is the current required to establish themagnetic field of the transformer core. This magnetizing current iscalled inrush current. The inrush current could cause the differ-ential relay to operate and the transformer to trip off line.

Since the inrush current has not only a large fundamental current(60 Hz) content, but also a large harmonic content (120 Hz plus),the waveform output from the differential transformer also has aharmonic content. This waveform is fed to both the fundamentalfilter and the harmonic filter. The fundamental filter blocks the har-monic current, but allows the fundamental current to pass.

The harmonic filter passes the harmonic current and blocks thefundamental current. The output of the harmonic filter feeds therestraint coil of the main unit relay, thereby increasing the restraintcoil’s strength. Therefore, even though the output from the funda-mental filter has increased due to the imbalance the transformeris under during this inrush, the main unit relay does not close itscontacts, because the restraint coil overcomes the operating coil.

PROTECTION SCHEME

Typical external wiring diagrams for the type BDD relay is shownin Figure 3.


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10

TESTING

The following is a guide for the tests that should be performed on theBDD relay. Figure 2 is a typical internal schematic for a BDD15Brelay.

When performing tests and adjustments, always refer to the manu-facturer’s instruction leaflet for that particular relay.

FIGURE 3Protective Scheme for a Two-Winding Transformer

1 2 3 1 2 3

H2

H1H3

X2

X1

X3

H1

H2

H3

X1

X2

X3

5

6WDG.

187-1TCT

87-1DCT

WDG.2

87-1TCT

4

87-2 87-3

Same asPhase 1

87-1DHR

87-1DHR

87-1AUX

1

87-1AUX

2

7

86

86

125V.DC

87-1I

87-2 87-3

Similar toPhase 1

86

52-Ha

52-HTC

(+) DC Trip Bus

1 2 3 1 2 3

250V.DC

AlternateConnectionsfor ∆-Y Bank

aTCDCTTCTIDHR865287

- Aux. switch, open when breaker is open- Trip coil- Differential current transformer- Through current transformer- Instantaneous overcurrent- Differential unit with harmonic restraint- Hand reset relay type HEA- Power circuit breaker-Type BDD differential relay

97-0369-1

86

52-La

52-LTC

NUMBER DEVICE CHART

52-H52-H

52-L 52-L


Harmonic Restraint

Module 13

11

* The procedures for performing insulation resistance testing can be foundin the Appendix. Check the manufacturer’s instruction literature for thespecific way to perform this test.

TYPES OF TESTS

1. Insulation Resistance*

2. Minimum Pickup

3. Through Fault

4. Slope

5. Harmonic Restraint

6. Instantaneous Unit Pickup

7. Main Unit Dropout

PICKUP TEST

This test is performed to determine the minimum operating currentof the relay, that is, the minimum current needed to close the relaycontacts for a particular TAP setting. A test circuit connection dia-gram for performing this test is shown in Figures 4A and 4B. Thetype BDD15B transformer differential relay is designed for a pickupequal to 30% of tap value ±10%.


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12

FIGURE 4AWinding 1 Pickup Test Connection Diagram

Aux. Aux.Wdg. #1

ThroughCurrent



LO D.C.V. Tap

R7

R8

HI D.C.V. Tap

Restr.1

8 2

7

3456

OperateR1

R2

7

3

4

HarmonicFreq.Restr.

L2FundFreq.Oper.

C1

L1

5

910

HarmonicRestr.Adj.

Thyrite

I(Instan.)


Am

ps.

2.9

3.2

3.5

3.8

4.2

4.6 5 8.7

Wdg.12

Wdg. #2Percent SlopeAdjustment

R31

6

8

2

2 =

C2

97-0370-1

Inst.Inst.


AC CurrentSource

DC VoltageSource

MonitorCircuit

+

10

9

8

7

6

5

42

1


Harmonic Restraint

Module 13

13

FIGURE 4BWinding 2 Pickup Test Connection Diagram

Aux. Aux.Wdg. #1

ThroughCurrent



LO D.C.V. Tap

R7

R8

HI D.C.V. Tap

Restr.1

8 2

7

3456

OperateR1

R2

7

3

4

HarmonicFreq.Restr.

L2FundFreq.Oper.

C1

L1

5

910

HarmonicRestr.Adj.

Thyrite

I(Instan.)


Am

ps.

2.9

3.2

3.5

3.8

4.2

4.6 5 8.7

Wdg.12


R31

6

8

2

2 =

C2

97-0370-1

Inst.Inst.


AC CurrentSource

DC VoltageSource

MonitorCircuit

+

10

9

8

7

6

5

2

1

4


Module 13


14

The following is an outline of the winding 1 pickup test:

1. Connect the relay as per Figure 4A.

2. Apply the proper DC voltage to relay studs 1(+) and 7(–).

3. Connect an AC current across relay studs 5 and 6 withcurrent polarity on stud 5.

4. Increase the AC current until the relay operates. Thisshould be 30% of the winding 1 tap value.

5. Record this current value as the winding 1 pickup.

The following is an outline of the winding 2 pickup test:

1. Connect the relay as per Figure 4B.



4. Increase the AC current until the relay operates. Thisshould be 30% of the winding 2 tap value.

5. Record this current value as the winding 2 pickup.


Harmonic Restraint

Module 13

15

TESTING THROUGH FAULT RESTRAINT

The through fault restraint test is performed to check that the re-lay does not operate upon an external fault condition. A test circuitconnection diagram for performing this test is shown in Figure 5.

1. Connect the relay as per Figure 5.



4. Apply 10 amps to studs 4 and 6.

5. Ensure that the auxiliary unit contacts do not close.

6. Record the fact that the relay did not operate on throughfault current.

TESTING SLOPE CHARACTERISTIC

The slope test is conducted to check the relay’s percentage slope, inaccordance with the manufacturer’s curves. The BDD relay slopecharacteristic curve is shown in Figure 6.

The slope characteristic tolerance is plus 10% of the curve value.In addition, care should be taken not to overheat the relay. Allowsufficient cooling time between tests.


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16

FIGURE 5Through Fault Test Connection Diagram

Aux. Aux.Wdg. #1

ThroughCurrent



LO D.C.V. Tap

R7

R8

HI D.C.V. Tap

Restr.1

8 2

7

3456

OperateR1

R2

7

3

4

HarmonicFreq.Restr.

L2FundFreq.Oper.

C1

L1

5

910

HarmonicRestr.Adj.

Thyrite

I(Instan.)


Am

ps.

2.9

3.2

3.5

3.8

4.2

4.6 5 8.7

Wdg.12


R31

6

8

2

5

2 =

C2

97-0371-1

Inst.Inst.


AC CurrentSource

DC VoltageSource

MonitorCircuit

+

7 9

4 6 8 102

1


Harmonic Restraint

Module 13

17

FIGURE 6Slope Characteristic

0 2 4 6 8 10 12 14 16

Note : For two-winding transformer relays “throughcurrent” is taken as the smaller of the twocurrents. For three-winding transformers, it istaken as the sum of the incoming or outgoingcurrents, whichever is smaller. (Each currentto be expressed as a multiple of tap.)

0

10

20

30

40

50

60

70

80

Through Current In Multiples of Tap

Min

imum

Per

cent

Slo

pe

97-0373-1

40 %

25 %

15 %


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18

Use the values in Figure 8 to determine the amount of current tobe applied to the relay.

The following is an outline of the slope test:

NOTE: Two current source with no phase shift betweenthem is necessary to perform this test.


2. Apply appropriate DC voltage to studs 1(+) and 7(–).

3. Connect a current source to relay studs 4 and 6 withpolarity on 4. (This will be the restraining current of30 amps.)

4. Connect a current source to relay studs 5 and 6 withpolarity on stud 5. (This is the operate current).

5. Turn on the 30 restraint amps.

6. Turn on and increase the operating current until therelay operates. (Approximately 7.5 – 8.3 amps)

7. Turn off both currents and calculate % slope using the% slope formula.

8. Record the percent slope.


Harmonic Restraint

Module 13

19

FIGURE 7Percent Slope Test Connection Diagram

Aux. Aux.Wdg. #1

ThroughCurrent



LO D.C.V. Tap

R7

R8

HI D.C.V. Tap

Restr.1

8 2

7

3456

OperateR1

R2

7

3

4

HarmonicFreq.Restr.

L2FundFreq.Oper.

C1

L1

5

910

HarmonicRestr.Adj.

Thyrite

I(Instan.)


Am

ps.

2.9

3.2

3.5

3.8

4.2

4.6 5 8.7

Wdg.12


R31

6

8

2

2 =

C2

97-0374-1

Inst.Inst.


AC CurrentSource #1 DC Voltage

Source

MonitorCircuit

AC CurrentSource #2

+ ++

6

5

4

7

8

9

102

1


Module 13


20

IN-SERVICE RELAY TAPS

When the type BDD15B transformer differential relay is requiredto be tested as found (in-service), the manufacturer’s literature mustbe followed for the specific test connection diagrams and charac-teristic curves. Figure 8 shows the manufacturer’s recommendedcurrent values and formula for performing an in-service tap test.

FIGURE 8Slope Test Operating Values

96-0110-1

40

11.6

37.4

38.9

12.8

36.2

37.7

14.0

35.0

36.5

15.2

33.8

35.3

16.8

32.2

33.7

18.4

30.6

32.1

20.0

29.0

30.5

34.8

14.2

15.7

TAPS

T1

2.9

3.2

3.5

3.8

4.2

4.6

5.0

8.7

25

11.6

32.2

33.1

12.8

31.0

31.9

14.0

29.8

30.7

15.2

28.6

29.5

16.8

27.0

27.9

18.4

25.4

26.3

20.0

23.6

24.7

34.8

9.0

9.9

15

11.6

28.7

29.3

12.8

27.5

28.1

14.0

26.3

26.9

15.2

25.1

25.7

16.8

23.5

24.1

18.4

21.9

22.5

20.0

20.3

20.9

34.8

5.5

6.1

40

17.4

24.6

25.8

19.2

22.8

24.0

21.0

21.0

22.2

22.8

19.2

20.4

25.2

16.8

18.0

27.6

14.4

15.6

30.0

12.0

13.2

25

17.4

20.1

20.9

19.2

18.3

19.1

21.0

16.5

17.3

22.8

14.7

15.5

25.2

12.3

13.1

27.6

9.9

10.7

30.0

7.5

8.3

15

17.4

17.1

17.6

19.2

15.3

15.8

21.0

13.5

14.0

22.8

11.7

12.2

25.2

9.3

9.8

27.6

6.9

7.4

30.0

4.5

5.0

8.75.0

40

17.4

21.2

22.4

19.2

19.4

20.6

21.0

17.6

18.8

22.8

15.8

17.0

25.2

13.4

14.6

27.6

11.0

12.2

25

17.4

17.1

17.8

19.2

15.3

16.0

21.0

13.5

14.2

22.8

11.7

12.4

25.2

9.3

10.0

27.6

6.9

7.6

15

17.4

14.3

14.8

19.2

12.5

13.0

21.0

10.7

11.2

22.8

8.9

9.4

25.2

6.5

7.0

27.6

4.1

4.6

40

17.4

17.9

18.9

19.2

16.1

17.1

21.0

14.3

15.3

22.8

12.5

13.5

25.2

10.1

11.1

25

17.4

14.1

14.8

19.2

12.3

13.0

21.0

10.5

11.2

22.8

8.7

9.4

25.2

6.3

7.0

15

17.4

11.6

12.0

19.2

9.8

10.2

21.0

8.0

8.4

22.8

6.2

6.6

25.2

3.8

4.2

40

17.4

14.5

15.4

19.2

12.7

13.6

21.0

10.9

11.8

22.8

9.1

10.0

25

17.4

11.1

11.7

19.2

9.3

9.9

21.0

7.5

8.1

22.8

5.7

6.3

15

17.4

8.8

9.2

19.2

7.0

7.4

21.0

5.2

5.6

22.8

3.4

3.8

40

17.4

12.0

12.8

19.2

10.2

11.0

21.0

8.4

9.3

25

17.4

8.8

9.4

19.2

7.0

7.6

21.0

5.2

5.8

15

17.4

6.7

7.1

19.2

4.9

5.3

21.0

3.1

3.5

40

17.4

9.5

10.3

19.2

7.7

8.5

25

17.4

6.6

7.1

19.2

4.8

5.3

15

17.4

4.6

5.0

19.2

2.9

3.2

40

17.4

7.0

7.7

25

17.4

4.3

4.8

15

17.4

2.6

2.9

I3I1(min)

I1(max)

I3I1(min)

I1(max)

I3I1(min)

I1(max)

I3I1(min)

I1(max)

I3I1(min)

I1(max)

I3I1(min)

I1(max)

I3I1(min)

I1(max)

I3I1(min)

I1(max)

T2SLOPES

CURRENTS

4.64.23.83.53.22.9


Harmonic Restraint

Module 13

21

PERCENT HARMONIC RESTRAINT

The manufacturer’s harmonic restraint characteristic curve is givenin Figure 9. The following is a brief explanation of the manufac-turer’s suggested test. Two currents are passed into the operatingcoil and one restraint coil in series. The first current (I1) is a pure60 Hertz sine wave. The second current I2 has a known quantity of2nd harmonic content. I2 is either a 1/2 wave rectified (by a seriesdiode) sine wave or a 120 Hertz sine wave generated by an electronictest set. I2 is preset, and I1 is adjusted until the relay operates.

For a half wave rectified I2 of 4 amperes DC, the relay should oper-ate for I1 between 4.5 and 5.5 amperes. These values correspond to21% and 19% 2nd harmonic respectively. If I2 is set to 2 amperes at120 Hertz, the relay should operate for I1 values between 9.3 (21%)and 10.3 amperes (19%). A typical test circuit connection diagramfor performing this test is shown in Figure 10.

The % 2nd harmonic restraint can be calculated for the BDD relaybased on whether the test current is 2nd harmonic based or half-wave based.


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22

FIGURE 9Percent Harmonic Restraint

Characteristic Curve

By-

Pas

s C

urre

nt (

I) In

Am

pere

s

Percent Second Harmonic1517 161819202122232425

3

4

5

6

7

8

I (DC) = 4.0 Amperes

97-0376-1


Harmonic Restraint

Module 13

23

FIGURE 10Percent Harmonic RestraintTest Connection Diagram

Aux. Aux.Wdg. #1

ThroughCurrent



LO D.C.V. Tap

R7

R8

HI D.C.V. Tap

Restr.1

8 2

7

3456

OperateR1

R2

7

3

4

HarmonicFreq.Restr.

L2FundFreq.Oper.

C1

L1

5

910

HarmonicRestr.Adj.

Thyrite

I(Instan.)


Am

ps.

2.9

3.2

3.5

3.8

4.2

4.6 5 8.7

Wdg.12

Wdg. #2

Percent SlopeAdjustment

R31

6

8

2

4

2 =

C2

97-0375-1

Inst.Inst.


AC CurrentSource

DC VoltageSource

MonitorCircuit +

1/2 Wave DCSource or

120 Hz CurrentSource

2

++

5

6 8 10

7 91


Module 13


24

The following formula is for a 2nd harmonic current produced bythe AVO relay test sets.

% 2nd Harmonic Restraint =

whereI2nd

= 2nd Harmonic Current

IF

= Fundamental Current

The following formula is for a half-wave current source.

% 2nd Harmonic Restraint =

0.212 × IDC

0.45 × IF + 0.5 × IDC× 100

whereIF

= Fundamental Test Current

IDC

= DC Current

2nd

F 2nd

I

I + I 100

2 2( ) ( )×


Harmonic Restraint

Module 13

25

The following is an outline of the percent harmonic restraint testusing a diode:



3. Apply a half wave rectified current of 4 amps across relaystuds 5 and 6 with current polarity on stud 5.


5. Increase the AC current until the relay operates.

6. Calculate the percent harmonic based on the formula onthe bottom of page 23.

7. Calibrate the relay using resistor R2 if necessary.

The following is an outline of the percent harmonic test using atrue 120 Hz current source:



3. Apply a 120 Hz current of 1 amp across relay studs 5 and6 with current polarity on stud 5.

4. Apply 60 Hz current across relay studs 5 and 6 withcurrent polarity on stud 5.


Module 13


26

5. Increase the 60 Hz current until the relay operates.

6. Calculate the percent harmonic based on the formula onthe bottom of page 24.

7. Calibrate the relay using resistor R2 if necessary.

INSTANTANEOUS

The instantaneous unit is tested by pulsing or jogging currentthrough the instantaneous unit of the relay until the instantaneousunit contacts pick up. The BDD relay’s instantaneous is typicallyset to pick up at 8 times the winding 1 or winding 2 tap value. Atypical test circuit connection diagram for performing this test isshown in Figure 11.

The following is an outline of the instantaneous unit pickup test:

NOTE: Do not connect the DC control voltage to therelay.

1. Connect a current source capable of 8 times tap valuecurrent to relay studs 5 and 6.

2. Jog the current up until the instantaneous unit contactsclose.

3. Record this value of current as the instantaneous unitpickup.


Harmonic Restraint

Module 13

27

FIGURE 11Instantaneous Test Connection Diagram

Aux. Aux.Wdg. #1

ThroughCurrent



LO D.C.V. Tap

R7

R8

HI D.C.V. Tap

Restr.1

8 2

7

3456

OperateR1

R2

7

3

4

HarmonicFreq.Restr.

L2FundFreq.Oper.

C1

L1

5

910

HarmonicRestr.Adj.

Thyrite

I(Instan.)


Am

ps.

2.9

3.2

3.5

3.8

4.2

4.6 5 8.7

Wdg.12


R31

6

8

2

4

2 =

C2

97-0377-1


AC CurrentSource

MonitorCircuit

6 8 10

5

2

1 7 9


Module 13


28

DROPOUT OF MAIN UNIT

This test is performed to check that the polar unit relay is operatingcorrectly. Put all windings in the 2.9 tap and set the relay for 25%slope. Rapidly apply 20 amperes to terminals 5 and 6. Decrease thecurrent quickly at first, and then more slowly. The polar unit shoulddrop out at 0.1 amperes or more. If the dropout current is less than0.1 amperes, the polar unit is defective.

A typical test circuit connection diagram for performing this test isshown in Figure 12.

ADJUSTMENTS

The relay’s calibration is performed by adjusting resistors R1, R2,and R3. Changes made in any one of these resistors affect the othertwo settings. Adjustments have to be made, until the relay’s threesettings are within manufacturer’s tolerances.

MINIMUM PICKUP

You can calibrate the minimum pickup value by adjusting theresistor R1. Resistor R1 is located at the top of the relay, left-handside.

HARMONIC RESTRAINT

You can calibrate the harmonic restraint value by adjusting resis-tor R2. Resistor R2 is located at the top of the relay, next to R1.


Harmonic Restraint

Module 13

29

FIGURE 12Typical Test Connection Diagram

Aux. Aux.Wdg. #1

ThroughCurrent



LO D.C.V. Tap

R7

R8

HI D.C.V. Tap

Restr.1

8 2

7

3456

OperateR1

R2

7

3

4

HarmonicFreq.Restr.

L2FundFreq.Oper.

C1

L1

5

910

HarmonicRestr.Adj.

Thyrite

I(Instan.)


Am

ps.

2.9

3.2

3.5

3.8

4.2

4.6 5 8.7

Wdg.12


R31

6

8

2

4

2 =

C2

97-0378-1


AC CurrentSource DC Voltage

Source

MonitorCircuit

+

2 6

51 7

8

9

10


Module 13


30

SLOPE

You can calibrate the slope characteristic value by adjusting the re-sistor R3 based on the selected slope tap. Resistor R3 is locatedbehind the nameplate. There are three adjustable resistor bands.From left to right, there are the 15%, 25% and 40% adjustments.

INSTANTANEOUS UNIT

Adjust the instantaneous unit’s pickup by means of the core screw.Remember to release the lock nut before making any adjustments,then secure it after the desired pickup is achieved.

SUMMARY

The transformer differential relay has special features that distin-guish it from other differential relays: (1) percentage restraint per-mits accurate (adjustable) determination between internal andexternal faults at high current; (2) harmonic restraint enables therelay to distinguish, by the differences in waveform, between the dif-ferential current caused by internal fault and that caused by trans-former magnetizing inrush; and (3) the instantaneous is a backupoperating on very high currents.

To ensure proper operation, the relay is tested to verify minimumpickup, through fault, slope, instantaneous pickup, and main unitdropout. This module also discusses proper calibration procedures,if the relay is found to be out of the allowable tolerances.

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