1. Testing Numerical Transformer Differential Relays
IEEE Rural Electric 2009
Testing Numerical Transformer Differential Relays
Steve Turner
Beckwith Electric Co., Inc.

2. Testing Numerical Transformer Differential Relays
INTRODUCTION
Commissioning versus Maintenance Testing
Types of Transformer Differential Protection:
Restrained Phase Differential
High Set Phase Differential
Ground Differential

3. Testing Numerical Transformer Differential Relays
INTRODUCTION
Topics:
Transformer Differential Boundary Test (Commissioning)
Ground Differential Sensitivity Test (Commissioning/Maintenance)
Harmonic Restraint for Transformer Inrush (Maintenance)

4. Testing Numerical Transformer Differential Relays
Commissioning
Common Practice:
Test all numerical relay settings –
verify settings properly entered
Easily facilitated using computer –
automate test set & store results
Hundreds of tests are possible –
numerical relays have many settings

5. Testing Numerical Transformer Differential Relays
Commissioning
Final Goal
Ensure the transformer is properly protected for the particular application

6. Typical Distribution Transformer
Testing Numerical Transformer Differential Relays
Typical Distribution Transformer 46 RG
46: Negative-Sequence Overcurrent Element (sees ground faults through bank)
RG: Grounding Resistor
(Industrial Load)

7. Testing Numerical Transformer Differential Relays+ 46 RG ZT I2
3RG
Sensitive setting for ground fault -
overreach for phase-to-phase fault ZT 46 ZT

8. * Simulate Through Current
Testing Numerical Transformer Differential Relays
Transformer Differential Characteristic
Boundary Test I1 I2 Y Y Y Y 87
* Simulate Through Current
I1 = Winding 1 per unit current (A, B or C-phase)
I2 = Winding 2 per unit current (A, B or C-phase)
3 elements per function (A, B & C-phase)

9. Testing Numerical Transformer Differential Relays
Differential Characteristic
Operating Equations
Id = |I1 – I2|, Differential Current
Ir = |I1| + |I2|, Restraint Current 2

10. Testing Numerical Transformer Differential Relays
Differential Characteristic Id Y
% =
*100% X
Percent (%) Slope Y X
Minimum
Pickup
(per unit) Ir

11. Testing Numerical Transformer Differential Relays
Differential Characteristic Id
Percent Slope
Trip Region
Block Region
Minimum
Pickup Ir

12. Testing Numerical Transformer Differential Relays
Matrix [ ] [ ] [ ] -1
½ ½ Id Ir I1 I2 * =

13. Testing Numerical Transformer Differential Relays
Inverted Matrix [ ] [ ] [ ]
½ 1
-½ 1 I1 I2 Id Ir * =

14. Testing Numerical Transformer Differential Relays
Differential Characteristic
Test Current Equations
I1 = 0.5*Id + Ir, Winding 1 Test Current (per unit)
I2 = -0.5*Id + Ir, Winding 2 Test Current (per unit)

15. Testing Numerical Transformer Differential Relays
Differential Characteristic
Test Points
Minimum Pickup = 0.2 per unit Id
Slope = 28.6%
Percent Slope 4 3 1 2
Minimum
Pickup Ir

16. Testing Numerical Transformer Differential Relays
Differential Characteristic
Test Points (Per Unit)
Id Ir I1 I2 1 2 3 4
TABLE 1

17. Testing Numerical Transformer Differential Relays
Delta-Wye Differential Characteristic
DAB I1 I2 Y Y Y 87
* Simulate Through Current
I1 = Winding 1 per unit current (A, B or C-phase)
I2 = Winding 2 per unit current (A, B or C-phase)
3 elements per relay (A, B & C-phase)

18. Testing Numerical Transformer Differential Relays
Delta-Wye Differential Characteristic
(Relay Internally Compensates Test Currents)
DAB WINDING
WYE WINDING
IA1relay = IA1/TAP1
IA2relay = (IA2- IB2)/(TAP2*SQRT(3))
IB1relay = IB1/TAP1
IB2relay = (IB2 - IC2)/(TAP2*SQRT(3))
IC1relay = IB1/TAP1
IC2relay = (IC2 - IA2)/(TAP2*SQRT(3))
MVA#
TAP# =
kV#LL*CTR#*SQRT(3)

19. Testing Numerical Transformer Differential Relays
Delta-Wye Differential Characteristic
(Single-Phase Test for A-Phase Element)
IA1 = I1*TAP1
IA2 = I2*TAP2*SQRT(3)
From TABLE 1:
Id Ir I1 I2 2
IA1test = 0.8*TAP1
IA2test = 0.6*TAP2*SQRT(3)

20. Testing Numerical Transformer Differential Relays
Ground Differential Element
Sensitivity Test
Directionality
(I0 vs. IG)
Ground Fault Location along Windings

21. Testing Numerical Transformer Differential Relays
Ground Differential Element
Directional Element
+90o
Internal
External IG I0
-90o
Disabled if |3I0| less than 140 mA
(Improves Security for CT saturation during external faults)

22. Testing Numerical Transformer Differential Relays
Ground Differential Element
Pickup
Operate When:
|3I0 – IG| > Pickup

23. Testing Numerical Transformer Differential Relays
Ground Differential Element
Sensitivity Test
Power System Parameters:
Source Impedance (Varies)
XT = 10%
RF (Varies)
Ground Fault Location (5% from Transformer Neutral)

24. Testing Numerical Transformer Differential Relays
Ground Differential Element
RF Coverage vs. Source Strength

25. Testing Numerical Transformer Differential Relays
Ground Differential Element
Pickup Setting R IG
Cold Load Pickup
Reclosing into Single-Phase Load
Pickup > Unbalance
Directional Element Disabled if 3I0 Low

26. Testing Numerical Transformer Differential Relays
IG2 = o
3I0(2) = o (Threshold = 0.14 Amps)
|CTCF*3I0(2) – IG2| = 0.75 Amps
Original Pickup = 0.3 Amps

27. Testing Numerical Transformer Differential Relays
Directional Element
INTERNAL
EXTERNAL
IG2
IOP
CTCF*3I0(2)
New Pickup = 1.0 Amps

28. Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush
COMTRADE PLAYBACK
Waveform Sources:
Events from Numerical Relays
Events from Digital Fault Recorders
Simulate using Transient Software

29. Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush
Traditional Approach
2nd Harmonic Restraint
Cross Phase Blocking

30. Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush
Auto-Transformer Model
wye
delta
13.2 kV
345 kV
230 kV W1 W2 W3
600 MVA Auto-Transformer (Tertiary Winding DAC)

31. Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush
Auto-Transformer Model
Auto-transformer Characteristics
ZHM = per unit
ZHL = per unit
ZML = per unit
ZH =
= per unit
ZM =
= per unit
ZL =
= per unit
CTRW1 = 1200:5 (wye connected)
CTRW2 = 2000:5 (wye connected)

32. Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush
Auto-Transformer Model
600 MVA
TAP1 =
= 4.18
345 kV * 240 * SQRT(3)
600 MVA
= 3.77
TAP2 =
230 kV * 400 * SQRT(3)
Minimum Pickup* = 0.5 per unit
Slope = 25%
*Original Pickup = 0.45 per unit

33. Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush
Energize Bank with Heavy A-Phase Residual Flux
Total Phase Current

34. Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush
Energize Bank with Heavy A-Phase Residual Flux
2nd Harmonic Phase Current

35. Testing Numerical Transformer Differential Relays
Even Harmonic Restraint
Ieven = (I22 + I42)1/2

36. Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush
Energize Bank with Heavy A-Phase Residual Flux
4th Harmonic Phase Current

37. Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush
A-Phase Current (Winding 2)

38. Testing Numerical Transformer Differential Relays
CONCLUSIONS

39. Testing Numerical Transformer Differential Relays
CONCLUSIONS
Transformer Differential Boundary Test (Commissioning)
Ground Differential Sensitivity Test (Commissioning/Maintenance)
Harmonic Restraint for Transformer Inrush (Maintenance)

40. Testing Numerical Transformer Differential Relays
QUESTIONS