9-12 D60 Line Distance Protection System GE Multilin9.4 SERIES COMPENSATED LINES 9 APPLICATION OF SETTINGS9This setting is a sum of the overvoltage protection levels for all the series capacitors located between the relay potentialsource and the far-end busbar, or location for which the zone must not overreach. The setting is entered in pu of the phaseVT nominal voltage (RMS, not peak value).If a minimum fault current level (phase current) is causing a voltage drop across a given capacitor that prompts its air gap toflash over or its MOV to carry practically all the current, then the series capacitor shall be excluded from the calculations(the capacitor is immediately by-passed by its overvoltage protection system and does not cause any overreach problems).If a minimum fault current does not guarantee an immediate capacitor by-pass, then the capacitor must be included in thecalculation: its overvoltage protection level, either air gap flash-over voltage or MOV knee-point voltage, shall be used(RMS, not peak value).Assuming none of the series capacitors in the sample system is guaranteed to get by-passed, the following calculationsapply:For the Sending Bus: 0.5 + 0.7 = 1.2 pu if the line-side (B) VTs are used0.6 + 0.5 + 0.7 = 1.8 pu if the bus-side (A) VTs are usedFor the Receiving Bus: 0.6 + 0.5 = 1.1 pu if the line-side (B) VTs are used0.6 + 0.5 + 0.5 = 1.6 pu if the bus-side (A) VTs are used9.4.3 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 > 0; there is no need for offset.• Net inductive reactance from relay through far-end busbar = –4 + 10 – 3 = 3 ; the offset cannot be higher than 3 .• It is recommended to use 1.5 offset impedance.For the Sending Bus relay, line-side VTs:• Net inductive reactance from relay into local system = –2 + 3 – 4 = –3 < 0; an offset impedance 3 must be used.• Net inductive reactance from relay through far-end busbar = 10 – 3 = 7 ; the offset cannot be higher than 7 .• It is recommended to use 5 offset impedance.For the Receiving Bus relay, bus-side VTs:• Net inductive reactance from relay into local system = –5 + 7 = 2 > 0; there is no need for offset.• Net inductive reactance from relay through far-end busbar = –3 + 10 – 4 = 3 ; the offset cannot be higher than 3 .• It is recommended to use 1.5 offset impedance.For the Receiving Bus relay, line-side VTs:• Net inductive reactance from relay into local system = –3 – 5 + 7 = –1 < 0; an offset impedance 1 must be used.• Net inductive reactance from relay through far-end busbar = 10 – 4 = 6 ; the offset cannot be higher than 6 .• It is recommended to use 3.5 offset impedance.9.4.4 HIGH-SET PHASE OVERCURRENTThe setting rules for high-set overcurrent protection are explained in the High-Set Overcurrent Elements section.
