8-6 L30 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUALCHANNEL ASYMMETRY COMPENSATION USING GPS CHAPTER 8: APPLICATION OF SETTINGS8• For relay 3, channel 1 communicates to relay 1 and channel 2 communicates to relay 2Consequently, to achieve the maximum sensitivity of 0.2 pu at the terminal with a CT ratio of 2000/5 (400 amps lineprimary differential current), the following pickup values are chosen:Eq. 8-11Choosing relay as a reference with a breakpoint value of 5.0 pu, breakpoints for relays 2 and 3 are calculated as follows:Eq. 8-12Eq. 8-13To verify the calculated values, we have:Eq. 8-14This satisfies the equality condition indicated earlier.During on-load tests, the differential current at all terminals should be the same and generally equal to the chargingcurrent if the tap and CT ratio settings are chosen correctly.8.3 Channel asymmetry compensation using GPS8.3.1 IntroductionAs indicated in the Settings chapter, the L30 provides three basic methods of applying channel asymmetry compensationusing GPS. Channel asymmetry can also be monitored with actual values and an indication signalled (FlexLogic operands87L DIFF 1(2) MAX ASYM asserted) if channel asymmetry exceeds preset values. Depending on the implemented relayingphilosophy, the relay can be programmed to perform the following on the loss of the GPS signal:• Enable GPS compensation on the loss of the GPS signal at any terminal and continue to operate the 87L element(using the memorized value of the last asymmetry) until a change in the channel round-trip delay is detected.• Enable GPS compensation on the loss of the GPS signal at any terminal and block the 87L element after a specifiedtime.• Continuously operate the 87L element but only enable GPS compensation when valid GPS signals are available. Thisprovides less sensitive protection on the loss of the GPS signal at any terminal and runs with higher pickup andrestraint settings.8.3.2 Compensation method 1Enable GPS compensation on the loss of the GPS signal at any terminal and continue to operate the 87L element until achange in the channel round-trip delay is detected.
