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

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

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

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

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

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)

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

* 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)

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

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

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

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

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

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)

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

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

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)

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)

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 0.2 0.7 0.8 0.6 2 IA1test = 0.8*TAP1 IA2test = 0.6*TAP2*SQRT(3)

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

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)

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

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)

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

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

Testing Numerical Transformer Differential Relays IG2 = 0.3 Amps @ 177o 3I0(2) = 0.136 Amps @ 58o (Threshold = 0.14 Amps) |CTCF*3I0(2) – IG2| = 0.75 Amps Original Pickup = 0.3 Amps

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

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

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

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)

Testing Numerical Transformer Differential Relays Even Harmonic Restraint during Inrush Auto-Transformer Model Auto-transformer Characteristics ZHM = 0.01073 per unit ZHL = 0.04777 per unit ZML = 0.03123 per unit ZH = = 0.0140 per unit ZM = = -0.0029 per unit ZL = = 0.0340 per unit CTRW1 = 1200:5 (wye connected) CTRW2 = 2000:5 (wye connected)

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

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

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

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

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

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

Testing Numerical Transformer Differential Relays CONCLUSIONS

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

Testing Numerical Transformer Differential Relays QUESTIONS