10-36 L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUALPHASE DISTANCE APPLIED TO POWER TRANSFORMERS CHAPTER 10: THEORY OF OPERATION10If installed at the location X, the relay uses the following input signals for its phase AB distance element:V = VAB = 77.402 kV ∠57.5° primary or 29.49 V ∠57.5° secondaryI = I A – IB = 2.576 kA ∠–27.6° primary or 42.93 A ∠–27.6° secondaryAnd consequently it sees an apparent impedance ofZapp = V / I = 30.05 Ω ∠85° primary or 0.687 Ω ∠85° secondaryIf applied at location H, the relay sees the following input signals:Table 10-11: Relay input signals at location HThe relay is set as follows:XFMR VOL CONNECTION = “Dy11”XFMR CUR CONNECTION = “Dy11”Consequently, the following signals are applied to the phase AB distance element:Eq. 10-40Eq. 10-41This results in the following apparent impedance:Eq. 10-42This value is a correct measure of the distance from the VT location to the fault. For relay location 2, this certainly includesthe positive-sequence impedance of the transformer:Eq. 10-43Thus, 0.127 Ω ∠90° + 0.05779 Ω ∠85° = 0.1847 Ω ∠88.4° primary side or 2.569 Ω ∠88.4° on the secondary side.This example illustrates how the relay maintains correct reach for fault behind power transformers. When installed at X,the relay needs to be set to 0.687 Ω ∠85° secondary in order to reach to the fault shown in the figure. When installed at H,the relay needs to be set to 2.569 Ω ∠88.4° to ensure exactly same coverage.VB 97.23 kV ∠–53.4° 37.04 V ∠–53.4°VC 181.8 kV ∠–150.0° 69.26 V ∠–150.0°IA 1.288 kA ∠–27.6° 21.47 A ∠–27.6°IB 1.288 kA ∠152.4° 21.47 A ∠152.4°IC 0 0Input Primary SecondaryVA 7.584 kV ∠–5.59° 69.95 V ∠–5.59°VB 6.269 kV ∠–120.1° 54.52 V ∠–120.1°VC 7.751 kV ∠125.5° 65.84 V ∠125.5°IA 16.976 kA ∠–27.6° 10.61 A ∠–27.6°IB 33.952 kA ∠152.4° 21.22 A ∠152.4°IC 16.976 kA ∠–27.6° 10.61 A ∠–27.6°Input Primary Secondary
