GE Multilin L90 Line Current Differential System 10-1710 APPLICATION OF SETTINGS 10.6 SERIES COMPENSATED LINES1010.6SERIES COMPENSATED LINES 10.6.1 DISTANCE SETTINGS ON SERIES COMPENSATED LINESTraditionally, the reach setting of an underreaching distance function shall be set based on the net inductive impedancebetween the potential source of the relay and the far-end busbar, or location for which the zone must not overreach. Faultsbehind series capacitors on the protected and adjacent lines need to be considered for this purpose. For further illustrationa sample system shown in the figure below is considered.Figure 10–4: SAMPLE SERIES COMPENSATED SYSTEMAssuming 20% security margin, the underreaching zone shall be set as follows.At the Sending Bus, one must consider an external fault at F1 as the 5 : capacitor would contribute to the overreachingeffect. Any fault behind F1 is less severe as extra inductive line impedance increases the apparent impedance:Reach Setting: 0.8 x (10 – 3 – 5) = 1.6 : if the line-side (B) VTs are usedReach Setting: 0.8 x (10 – 4 – 3 – 5) = –1.6 : if the bus-side (A) VTs are usedThe negative value means that an underreaching zone cannot be used as the circuit between the potential source of therelay and an external fault for which the relay must not pick-up, is overcompensated, i.e. capacitive.At the Receiving Bus, one must consider a fault at F2:Reach Setting: 0.8 x (10 – 4 – 2) = 3.2 : if the line-side (B) VTs are usedReach Setting: 0.8 x (10 – 4 – 3 – 2) = 0.8 : if the bus-side (A) VTs are usedPractically, however, to cope with the effect of sub-synchronous oscillations, one may need to reduce the reach even more.As the characteristics of sub-synchronous oscillations are in complex relations with fault and system parameters, no solidsetting recommendations are given with respect to extra security margin for sub-synchronous oscillations. It is strongly rec-ommended to use a power system simulator to verify the reach settings or to use an adaptive L90 feature for dynamicreach control.If the adaptive reach control feature is used, the PHS DIST Z1 VOLT LEVEL setting shall be set accordingly.This 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 used10 Ω-4 Ω -3 Ω -5 Ω7 Ω-2 Ω3 ΩSENDINGBUSRECEIVINGBUSProtected LineA B B AINFINITEBUSINFINITEBUSF1F20.5 pu 0.6 pu 0.5 pu 0.7 pureactancevoltageprotectionlevel
