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IEEE Rural Electric 2009
Testing Numerical Transformer Differential Relays
Testing Numerical Transformer Differential Relays
Steve Turner
Beckwith Electric Co., Inc.
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Testing Numerical Transformer Differential Relays
Commissioning versus Maintenance Testing
Types of Transformer Differential Protection: Restrained Phase Differential High Set Phase Differential Ground Differential
INTRODUCTION
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Testing Numerical Transformer Differential Relays
Topics: Transformer Differential Boundary Test
(Commissioning) Ground Differential Sensitivity Test
(Commissioning/Maintenance) Harmonic Restraint for Transformer Inrush
(Maintenance)
INTRODUCTION
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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
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Testing Numerical Transformer Differential Relays
Commissioning
Final Goal Ensure the transformer is properly
protected for the particular application
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Testing Numerical Transformer Differential Relays
RG46
Typical Distribution Transformer
46: Negative-Sequence Overcurrent Element (sees ground faults through bank)
RG: Grounding Resistor (Industrial Load)
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Testing Numerical Transformer Differential Relays
RG46
E
+
ZT
ZT
ZT
3RG
46
I2
Sensitive setting for ground fault - overreach for phase-to-phase fault
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Testing Numerical Transformer Differential Relays
Transformer Differential Characteristic Boundary Test
I2I1
87
Y Y
YY
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)
* Simulate Through Current
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Testing Numerical Transformer Differential Relays
Differential Characteristic Operating Equations
Id = |I1 – I2|, Differential Current
Ir = |I1| + |I2|, Restraint Current2
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Testing Numerical Transformer Differential Relays
Differential Characteristic
Id
Ir
MinimumPickup(per unit)
Percent (%)
Slope
XY
% = Y
X*100%
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Testing Numerical Transformer Differential Relays
Differential Characteristic
Id
Ir
MinimumPickup
Percent
Slope
Trip Region
Block Region
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Testing Numerical Transformer Differential Relays
Matrix
Id
Ir[ ] =
1 -1½ ½ [ ] * I1
I2[ ]
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Testing Numerical Transformer Differential Relays
Inverted Matrix
I1
I2[ ] =
½ 1-½ 1 [ ] * Id
Ir[ ]
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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)
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Testing Numerical Transformer Differential Relays
Differential Characteristic Test Points
Id
Ir
MinimumPickup
Percent
Slope
Minimum Pickup = 0.2 per unit
Slope = 28.6%
1 23
4
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Testing Numerical Transformer Differential Relays
Differential Characteristic Test Points (Per Unit)
Id Ir I1 I2
1 0.2 0.3 0.4 0.2
0.2 0.7 0.8 0.6
0.4 1.4 1.6 1.2
0.6 2.0 2.3 1.7
2
3
4
TABLE 1
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Testing Numerical Transformer Differential Relays
Delta-Wye Differential Characteristic
I2I1
87
Y
YY
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)
* Simulate Through Current
DAB
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Testing Numerical Transformer Differential Relays
Delta-Wye Differential Characteristic (Relay Internally Compensates Test Currents)
IA1relay = IA1/TAP1
DAB WINDING
IB1relay = IB1/TAP1
IC1relay = IB1/TAP1
IA2relay = (IA2- IB2)/(TAP2*SQRT(3))
WYE WINDING
IB2relay = (IB2 - IC2)/(TAP2*SQRT(3))
IC2relay = (IC2 - IA2)/(TAP2*SQRT(3))
TAP# = MVA#
kV#LL*CTR#*SQRT(3)
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Testing Numerical Transformer Differential Relays
Delta-Wye Differential Characteristic (Single-PhaseSingle-Phase Test for A-Phase Element)
Id Ir I1 I2
0.2 0.7 0.8 0.62
From TABLE 1:
IA1 = I1*TAP1
IA2 = I2*TAP2*SQRT(3)
IA1test = 0.8*TAP1
IA2test = 0.6*TAP2*SQRT(3)
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Testing Numerical Transformer Differential Relays
Ground Differential Element Sensitivity Test
Directionality
(I0 vs. IG)
Ground Fault Location along Windings
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Testing Numerical Transformer Differential Relays
Ground Differential Element Directional ElementDirectional Element
+90o
-90o
IGI0
Disabled if |3I0| less than 140 mA(Improves Security for CT saturation during external faults)
Internal External
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Testing Numerical Transformer Differential Relays
Ground Differential Element PickupPickup
Operate When:
|3I0 – IG| > Pickup
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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)
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Testing Numerical Transformer Differential Relays
87GD Sensitivity
0
20
40
60
80
100
120
140
160
180
0 20 40 60 80 100 120
Source Impedance
Fau
lt R
esis
tan
ce
IG = 200 mA IG = 500 mA IG = 1 Amp
Ground Differential Element RRFF Coverage vs. Source Strength Coverage vs. Source Strength
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Testing Numerical Transformer Differential Relays
Ground Differential Element Pickup Setting
Cold Load Pickup
Reclosing into Single-Phase Load
Pickup > UnbalanceDirectional Element Disabled if 3I0 Low
IG
R
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Testing Numerical Transformer Differential Relays
IG2 = 0.3 Amps @ 177o
3I0(2) = 0.136 Amps @ 58o (Threshold = 0.14 AmpsThreshold = 0.14 Amps)
|CTCF*3I0(2) – IG2| = 0.75 Amps
Original Pickup = 0.3 Amps
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Testing Numerical Transformer Differential Relays
IG2
CTCF*3I0(2)
INTERNAL EXTERNAL
IOP
New Pickup = 1.0 AmpsNew Pickup = 1.0 Amps
Directional ElementDirectional Element
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Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush COMTRADE PLAYBACKCOMTRADE PLAYBACK
Waveform Sources:•Events from Numerical Relays•Events from Digital Fault Recorders•Simulate using Transient Software
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Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush Traditional ApproachTraditional Approach
•2nd Harmonic Restraint•Cross Phase Blocking
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Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush Auto-Transformer ModelAuto-Transformer Model
wye wye
delta
13.2 kV
345 kV 230 kV
W1 W2
W3
600 MVA Auto-Transformer (Tertiary Winding DAC)
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Testing Numerical Transformer Differential Relays
Auto-transformer Characteristics ZHM = 0.01073 per unit ZHL = 0.04777 per unit ZML = 0.03123 per unit
2MLHLHM ZZZ
= 0.0140 per unit
2HLMLHM ZZZ
= -0.0029 per unit
2HMMLHL ZZZ
CTRW1 = 1200:5 (wye connected)
CTRW2 = 2000:5 (wye connected)
Even Harmonic Restraint during Inrush
Auto-Transformer ModelAuto-Transformer Model
ZH =
ZM =
ZL = = 0.0340 per unit
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Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush Auto-Transformer ModelAuto-Transformer Model
TAP1 =600 MVA
345 kV * 240 * SQRT(3)
TAP2 =600 MVA
230 kV * 400 * SQRT(3)
= 4.18
= 3.77
Minimum Pickup* = 0.5 per unit
Slope = 25%
*Original Pickup = 0.45 per unitOriginal Pickup = 0.45 per unit
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Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush Energize Bank with Heavy A-Phase Residual FluxEnergize Bank with Heavy A-Phase Residual Flux
Total Phase Current
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Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush Energize Bank with Heavy A-Phase Residual FluxEnergize Bank with Heavy A-Phase Residual Flux
2nd Harmonic Phase Current
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Testing Numerical Transformer Differential Relays
Even Harmonic RestraintEven Harmonic Restraint
Ieven = (I22 + I4
2)1/2
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Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush Energize Bank with Heavy A-Phase Residual FluxEnergize Bank with Heavy A-Phase Residual Flux
4th Harmonic Phase Current
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Testing Numerical Transformer Differential Relays
Even Harmonic Restraint during Inrush A-Phase Current (Winding 2)A-Phase Current (Winding 2)
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Testing Numerical Transformer Differential Relays
CONCLUSIONSCONCLUSIONS
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Testing Numerical Transformer Differential Relays
Transformer Differential Boundary Test(Commissioning)
Ground Differential Sensitivity Test(Commissioning/Maintenance)
Harmonic Restraint for Transformer Inrush(Maintenance)
CONCLUSIONSCONCLUSIONS
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Testing Numerical Transformer Differential Relays
QUESTIONSQUESTIONS