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**Testing Numerical Transformer Differential Relays**

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

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**Testing Numerical Transformer Differential Relays**

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

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**Testing Numerical Transformer Differential Relays**

INTRODUCTION Topics: Transformer Differential Boundary Test (Commissioning) Ground Differential Sensitivity Test (Commissioning/Maintenance) Harmonic Restraint for Transformer Inrush (Maintenance)

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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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**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)

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**Testing Numerical Transformer Differential Relays**

+ 46 RG ZT I2 3RG Sensitive setting for ground fault - overreach for phase-to-phase fault ZT 46 ZT

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*** 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)

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**Testing Numerical Transformer Differential Relays**

Differential Characteristic Operating Equations Id = |I1 – I2|, Differential Current Ir = |I1| + |I2|, Restraint Current 2

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**Testing Numerical Transformer Differential Relays**

Differential Characteristic Id Y % = *100% X Percent (%) Slope Y X Minimum Pickup (per unit) Ir

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**Testing Numerical Transformer Differential Relays**

Differential Characteristic Id Percent Slope Trip Region Block Region Minimum Pickup Ir

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**Testing Numerical Transformer Differential Relays**

Matrix [ ] [ ] [ ] -1 ½ ½ Id Ir I1 I2 * =

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**Testing Numerical Transformer Differential Relays**

Inverted Matrix [ ] [ ] [ ] ½ 1 -½ 1 I1 I2 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 Minimum Pickup = 0.2 per unit Id Slope = 28.6% Percent Slope 4 3 1 2 Minimum Pickup Ir

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**Testing Numerical Transformer Differential Relays**

Differential Characteristic Test Points (Per Unit) Id Ir I1 I2 1 2 3 4 TABLE 1

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**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)

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**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)

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**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)

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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 Element +90o Internal External IG I0 -90o Disabled if |3I0| less than 140 mA (Improves Security for CT saturation during external faults)

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**Testing Numerical Transformer Differential Relays**

Ground Differential Element Pickup 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**

Ground Differential Element RF Coverage vs. Source Strength

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**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

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**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

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**Testing Numerical Transformer Differential Relays**

Directional Element INTERNAL EXTERNAL IG2 IOP CTCF*3I0(2) New Pickup = 1.0 Amps

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**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

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**Testing Numerical Transformer Differential Relays**

Even Harmonic Restraint during Inrush Traditional Approach 2nd Harmonic Restraint Cross Phase Blocking

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**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)

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**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)

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**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

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**Testing Numerical Transformer Differential Relays**

Even Harmonic Restraint during Inrush Energize 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 Flux 2nd Harmonic Phase Current

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**Testing Numerical Transformer Differential Relays**

Even Harmonic Restraint Ieven = (I22 + I42)1/2

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**Testing Numerical Transformer Differential Relays**

Even Harmonic Restraint during Inrush Energize 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)

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**Testing Numerical Transformer Differential Relays**

CONCLUSIONS

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**Testing Numerical Transformer Differential Relays**

CONCLUSIONS Transformer Differential Boundary Test (Commissioning) Ground Differential Sensitivity Test (Commissioning/Maintenance) Harmonic Restraint for Transformer Inrush (Maintenance)

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**Testing Numerical Transformer Differential Relays**

QUESTIONS

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