GE Multilin L90 Line Current Differential System 5-815 SETTINGS 5.4 SYSTEM SETUP5• Channel asymmetry prior to losing the GPS time reference. This value is measured by the L90 and a user-pro-grammable threshold is applied to it. The corresponding FlexLogic operands are produced if the asymmetry isabove the threshold ( 87L DIFF MAX 1 ASYM and 87L DIFF 2 MAX ASYM ). These operands can be latched in Flex-Logic and combined with other factors to decide, upon GPS loss, if the relays continue to compensate using thememorized correction. Typically, one may decide to keep compensating if the pre-existing asymmetry was low.• Change in the round trip travel time. This value is measured by the L90 and a user-programmable thresholdapplied to it. The corresponding FlexLogic operands are produced if the delta change is above the threshold ( 87LDIFF 1 TIME CHNG and 87L DIFF 2 TIME CHNG ). These operands can be latched in FlexLogic and combined withother factors to decide, upon GPS loss, if the relays continue to compensate using the memorized correction. Typ-ically, one may decide to disable compensation if the round trip time changes.• BLOCK GPS TIME REF: This setting signals to the L90 that the time reference is not valid. The time reference may benot accurate due to problems with the GPS receiver. The user must to be aware of the case when a GPS satellitereceiver loses its satellite signal and reverts to its own calibrated crystal oscillator. In this case, accuracy degrades intime and may eventually cause relay misoperation. Verification from the manufacturer of receiver accuracy not worsethan 250 s and the presence of an alarm contact indicating loss of the satellite signal is strongly recommended. If thetime reference accuracy cannot be guaranteed, it should be relayed to the L90 via contact inputs and GPS compensa-tion effectively blocked using the contact position in conjunction with the BLOCK GPS TIME REF setting. This setting istypically a signal from the GPS receiver signaling problems or time inaccuracy.Some GPS receivers can supply erroneous IRIG-B signals during power-up and before locking to satellites. If thereceiver’s failsafe contact opens during power-up (allowing for an erroneous IRIG-B signal), then set a dropout delayup to 15 minutes (depending on GPS receiver specifications) to the failsafe contact via FlexLogic to prevent incorrectrelay response.• MAX CHNL ASYMMETRY: This setting detects excessive channel asymmetry. The same threshold is applied to boththe channels, while the following per-channel FlexLogic operands are generated: 87L DIFF 1 MAX ASYM and 87L DIFF 2MAX ASYM . These operands can be used to alarm on problems with communication equipment and/or to decidewhether channel asymmetry compensation remains in operation should the GPS-based time reference be lost. Chan-nel asymmetry is measured if both terminals of a given channel have valid time reference.If the memorized asymmetry value is much greater than expected (indicating a significant problem with GPS clock tim-ing), then this operand can be also used to block GPS compensation, forcing the relay to use the memorized asymme-try value.• ROUND TRIP TIME CHANGE: This setting detects changes in round trip time. This threshold is applied to both chan-nels, while the 87L DIFF 1 TIME CHNG and 87L DIFF 2 TIME CHNG ASYM per-channel FlexLogic operands are generated.These operands can be used to alarm on problems with communication equipment and/or to decide whether channelasymmetry compensation remains in operation should the GPS-based time reference be lost.• LOCAL GPS TROUBLE: This signal is On when any of the following conditions are present:• The BCS selection is none or SNTP• The BCS selection is IRIG-B and the IRIG-B SIGNAL TYPE setting is Amplitude Modulated• The RTC and/or the synchrophasor clock are not synchronized to the selected synchronizing source. This canpossibly occur on power up, when transferring between sources, and when the selected source's holdover timertimes out.• The quality bits in the messages from the a PTP source used for synchronizing indicate worst-case error greaterthan 250 μs, or accuracy less than 250 μs, or unknown accuracy/error (that is, not locked to an international timestandard). Apply 2 security counts (2 seconds) to both set and reset of this operand when change is based onaccuracy. There is no corresponding quality test for IRIG-B sources here.
