GE Multilin L30 Line Current Differential System 9-7
9 APPLICATION OF SETTINGS 9.3 CHANNEL ASYMMETRY COMPENSATION USING GPS
9
9.3CHANNEL ASYMMETRY COMPENSATION USING GPS 9.3.1 DESCRIPTION
As indicated in the Settings chapter, the L30 provides three basic methods of applying channel asymmetry compensation
using GPS. Channel asymmetry can also be monitored with actual values and an indication signalled (FlexLogic operands
87L DIFF 1(2) MAX ASYM asserted) if channel asymmetry exceeds preset values. Depending on the implemented relaying
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 specified
time.
3. Continuously operate the 87L element but only enable GPS compensation when valid GPS signals are available. This
provides less sensitive protection on the loss of the GPS signal at any terminal and runs with higher pickup and
restraint settings.
9.3.2 COMPENSATION METHOD 1
Enable GPS compensation on the loss of the GPS signal at any terminal and continue to operate the 87L element until a
change 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 the
loss of the GPS signal, the L30 stores the last measured value of the channel asymmetry per channel and compensates for
the asymmetry until the GPS clock is available. However, if the channel was switched to another physical path during GPS
loss 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 menu
and 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 the
setting 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 channel
delay occurs during GPS loss conditions or on a startup before the GPS signal is valid. For three-terminal systems, the
87L DIFF 1 TIME CHNG operand must be ORed with the 87L DIFF 2 TIME CHNG FlexLogic™ operand. The Block
87L (VO1) output is reset if the GPS signal is restored and the 87L element is ready to operate.
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