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IEEE Rural Electric 2009 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 RGRG 46 Typical Distribution Transformer 46: Negative-Sequence Overcurrent Element (sees ground faults through bank) R G : Grounding Resistor (Industrial Load)

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Testing Numerical Transformer Differential Relays RGRG 46 E + ZTZT ZTZT ZTZT 3R G 46 I2I2 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 I2I2 I1I1 87 Y Y Y Y I 1 = Winding 1 per unit current (A, B or C-phase) I 2 = 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 I d = |I 1 – I 2 |, Differential Current I r = |I 1 | + |I 2 |, Restraint Current 2

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Testing Numerical Transformer Differential Relays Differential Characteristic IdId IrIr Minimum Pickup (per unit) Percent ( % ) Slope X Y % = Y X *100%

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Testing Numerical Transformer Differential Relays Differential Characteristic IdId IrIr Minimum Pickup Percent Slope Trip Region Block Region

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Testing Numerical Transformer Differential Relays Matrix IdIrIdIr [] = 1 -1 ½ [ ] * I1I2I1I2 []

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Testing Numerical Transformer Differential Relays Inverted Matrix I1I2I1I2 [] = ½ 1 -½ 1 [ ] * IdIrIdIr []

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Testing Numerical Transformer Differential Relays Differential Characteristic Test Current Equations I 1 = 0.5*I d + I r, Winding 1 Test Current (per unit) I 2 = -0.5*I d + I r, Winding 2 Test Current (per unit)

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Testing Numerical Transformer Differential Relays Differential Characteristic Test Points IdId IrIr Minimum Pickup Percent Slope Minimum Pickup = 0.2 per unit Slope = 28.6%

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Testing Numerical Transformer Differential Relays Differential Characteristic Test Points (Per Unit) IdIrI1I2IdIrI1I TABLE 1

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Testing Numerical Transformer Differential Relays Delta-Wye Differential Characteristic I2I2 I1I1 87 Y Y Y I 1 = Winding 1 per unit current (A, B or C-phase) I 2 = 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) I A1 relay = I A1 /TAP1 DAB WINDING I B1 relay = I B1 /TAP1 I C1 relay = I B1 /TAP1 I A2 relay = (I A2 - I B2 )/(TAP2*SQRT(3)) WYE WINDING I B2 relay = (I B2 - I C2 )/(TAP2*SQRT(3)) I C2 relay = (I C2 - I A2 )/(TAP2*SQRT(3)) TAP# = MVA# kV# LL *CTR#*SQRT(3)

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Testing Numerical Transformer Differential Relays Delta-Wye Differential Characteristic Single-Phase (Single-Phase Test for A-Phase Element) IdIrI1I2IdIrI1I From TABLE 1: I A1 = I 1 *TAP1 I A2 = I 2 *TAP2*SQRT(3) I A1 test = 0.8*TAP1 I A2 test = 0.6*TAP2*SQRT(3)

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Testing Numerical Transformer Differential Relays Ground Differential Element Sensitivity Test Directionality (I 0 vs. I G ) Ground Fault Location along Windings

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Testing Numerical Transformer Differential Relays Ground Differential Element Directional Element Directional Element +90 o -90 o IGIG I0I0 Disabled if |3I 0 | less than 140 mA (Improves Security for CT saturation during external faults) InternalExternal

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Testing Numerical Transformer Differential Relays Ground Differential Element Pickup Pickup Operate When: |3I 0 – I G | > Pickup

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Testing Numerical Transformer Differential Relays Ground Differential Element Sensitivity Test Power System Parameters: Source Impedance (Varies) X T = 10% R F (Varies) Ground Fault Location (5% from Transformer Neutral)

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Testing Numerical Transformer Differential Relays Ground Differential Element R F 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 > Unbalance Directional Element Disabled if 3I 0 Low IGIG R

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Testing Numerical Transformer Differential Relays I G2 = o Threshold = 0.14 Amps 3I 0 (2) = o (Threshold = 0.14 Amps) |CTCF*3I 0 (2) – I G2 | = 0.75 Amps Original Pickup = 0.3 Amps

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Testing Numerical Transformer Differential Relays I G2 CTCF*3I 0 (2) INTERNAL EXTERNAL I OP New Pickup = 1.0 Amps Directional Element

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

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Testing Numerical Transformer Differential Relays Even Harmonic Restraint during Inrush Auto-Transformer Model Auto-Transformer Model wye delta 13.2 kV 345 kV230 kV W1W1 W2W2 W3W3 600 MVA Auto-Transformer (Tertiary Winding DAC)

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Testing Numerical Transformer Differential Relays Auto-transformer Characteristics Z HM = per unit Z HL = per unit Z ML = per unit = per unit = per unit CTR W1 = 1200:5 (wye connected) CTR W2 = 2000:5 (wye connected) Even Harmonic Restraint during Inrush Auto-Transformer Model Auto-Transformer Model Z H = Z M = Z L = = per unit

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Testing Numerical Transformer Differential Relays Even Harmonic Restraint during Inrush Auto-Transformer Model Auto-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 unit *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 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 Energize Bank with Heavy A-Phase Residual Flux 2 nd Harmonic Phase Current

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Testing Numerical Transformer Differential Relays Even Harmonic Restraint I even = (I I 4 2 ) 1/2

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

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

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Testing Numerical Transformer Differential RelaysQUESTIONS

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