GE Multilin L30 Line Current Differential System 10-710 APPLICATION OF SETTINGS 10.3 CHANNEL ASYMMETRY COMPENSATION USING GPS1010.3CHANNEL ASYMMETRY COMPENSATION USING GPS 10.3.1 DESCRIPTIONAs 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™ oper-ands 87L DIFF 1(2) MAX ASYM asserted) if channel asymmetry exceeds preset values. Depending on the implementedrelaying philosophy, the relay can be programmed to perform the following on the loss of the GPS signal:1. 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.2. Enable GPS compensation on the loss of the GPS signal at any terminal and block the 87L element after a specifiedtime.3. 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.10.3.2 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.If GPS is enabled at all terminals and the GPS signal is present, the L30 compensates for the channel asymmetry. On theloss of the GPS signal, the L30 stores the last measured value of the channel asymmetry per channel and compensates forthe asymmetry until the GPS clock is available. However, if the channel was switched to another physical path during GPSloss conditions, the 87L element must be blocked, since the channel asymmetry cannot be measured and system is no lon-ger accurately synchronized. The value of the step change in the channel is preset in L30 POWER SYSTEM settings menuand signaled by the 87L DIFF 1(2) TIME CHNG FlexLogic™ operand. To implement this method, follow the steps below:1. Enable Channel Asymmetry compensation by setting it to ON. Assign the GPS receiver failsafe alarm contact with thesetting Block GPS Time Ref.2. Create FlexLogic™ similar to that shown below to block the 87L element on GPS loss if step change in the channeldelay occurs during GPS loss conditions or on a startup before the GPS signal is valid. For three-terminal systems, the87L DIFF 1 TIME CHNG operand must be ORed with the 87L DIFF 2 TIME CHNG FlexLogic™ operand. The Block87L (VO1) output is reset if the GPS signal is restored and the 87L element is ready to operate.
