GE Multilin L90 Line Current Differential System 9-339 THEORY OF OPERATION 9.4 PHASE DISTANCE APPLIED TO POWER TRANSFORMERS9If installed at the location X, the relay would use 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 – I B = 2.576 kA –27.6° primary or 42.93 A –27.6° secondaryAnd consequently it would see an apparent impedance of:Zapp = V / I = 30.05 85° primary or 0.687 85° secondaryIf applied at location H, the relay sees the following input signals:The 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 9.37)(EQ 9.38)This results in the following apparent impedance:(EQ 9.39)The above value is a correct measure of the distance from the VT location to the fault. For relay location 2, this certainlyincludes the positive-sequence impedance of the transformer:(EQ 9.40)Thus, 0.127 90° + 0.05779 85° = 0.1847 88.4° primary side or 2.569 88.4° on the secondary side.The above example illustrates how the relay maintains correct reach for fault behind power transformers. When installed atX, the relay shall be set to 0.687 85° secondary in order to reach to the fault shown in the figure. When installed at H,the relay shall be set to 2.569 88.4° to ensure exactly same coverage.See Chapter 9: Application of Settings for more information on setting calculations.INPUT PRIMARY SECONDARYVA 100.4 kV –7.32° 38.25 V –7.32°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°V 13-------V AB V BC 10.861 kV 59.9 primary or 94.45 V 59.9 secondaryI 3I B 58.860 kA 27.6 primary or 36.75 A 27.6 secondaryZ appVI---- 94.45 kV 59.936.75 kA 27.6---------------------------------------------- 2.570 87.5 secondaryZ T at 13.8 kV 10100---------- 13.8 kV 2150 MVA---------------------------- 0.127 90Z L at 13.8 kV 30.11 13.8315----------- 2 0.05779 85
