10-18 L90 Line Current Differential System GE Multilin10.6 SERIES COMPENSATED LINES 10 APPLICATION OF SETTINGS1010.6.2 GROUND DIRECTIONAL OVERCURRENTGround directional overcurrent function (negative-sequence or neutral) uses an offset impedance to guarantee correct faultdirection discrimination. The following setting rules apply.1. If the net impedance between the potential source and the local equivalent system is inductive, then there is no needfor an offset. Otherwise, the offset impedance shall be at least the net capacitive reactance.2. The offset cannot be higher than the net inductive reactance between the potential source and the remote equivalentsystem. For simplicity and extra security, the far-end busbar may be used rather than the remote equivalent system.As the ground directional functions are meant to provide maximum fault resistance coverage, it is justified to assume thatthe fault current is very low and none of the series capacitors is guaranteed to get by-passed. Consider settings of the neg-ative-sequence directional overcurrent protection element for the Sample Series Compensated System.For the Sending Bus relay, bus-side VTs:• Net inductive reactance from the relay into the local system = –2 + 3 = 1 Ω > 0; there is no need for offset.• Net inductive reactance from relay through far-end busbar = –4 + 10 – 3 = 3 Ω; the offset cannot be higher than 3 Ω.• It is recommended to use 1.5 Ω offset impedance.For the Sending Bus relay, line-side VTs:• Net inductive reactance from relay into local system = –2 + 3 – 4 = –3 Ω < 0; an offset impedance ≥3 Ω must be used.• Net inductive reactance from relay through far-end busbar = 10 – 3 = 7 Ω; the offset cannot be higher than 7 Ω.• It is recommended to use 5 Ω offset impedance.For the Receiving Bus relay, bus-side VTs:• Net inductive reactance from relay into local system = –5 + 7 = 2 Ω > 0; there is no need for offset.• Net inductive reactance from relay through far-end busbar = –3 + 10 – 4 = 3 Ω; the offset cannot be higher than 3 Ω.• It is recommended to use 1.5 Ω offset impedance.For the Receiving Bus relay, line-side VTs:• Net inductive reactance from relay into local system = –3 – 5 + 7 = –1 Ω < 0; an offset impedance ≥1 Ω must be used.• Net inductive reactance from relay through far-end busbar = 10 – 4 = 6 Ω; the offset cannot be higher than 6 Ω.• It is recommended to use 3.5 Ω offset impedance.
