Relay Synchronization - GE L90 Instruction Manual

8 THEORY OF OPERATION
On startup of the relays, the channel status will be checked first. If channel status is OK, all relays will send a special "star-
tup" message and the synchronization process will be initiated. It will take about 5 to 7 seconds to declare PFLL status as
OK and to start performing current differential calculations. If one of the relays was powered off during the operation, the
synchronization process will restart from the beginning. Relays tolerate channel delay (resulting sometimes in step change
in communication paths) or interruptions up to 4 power cycles round trip time (about 66 ms at 60 Hz) without any deteriora-
tion in performance. If communications are interrupted for more than 4 cycles, the following applies:
In 2-terminal mode:
1. With second redundant channel, relays will not lose functionality at all if second channel is live.
2. With one channel only, relays have a 5 second time window. If the channel is restored within this time, it takes about 2-
3 power cycles of valid PFLL calculations (and if estimated error is still within margin) to declare that PFLL is OK. If the
channel is restored later than 5 seconds, PFLL at both relays will be declared as failed and the re-synch process will
be initiated (about 2 minutes) after channel status becomes OK.
In 3-terminal mode:
1. If one of the channels fails, the configuration reverts from Master-Master to Master-Slave where the Master relay has
both channels live. The Master relay PFLL keeps the 2 Slave relays in synchronization, and therefore there is no time
limit for functionality. The PFLL of the Slave relays will be "suspended" (87L function will not be performed at these
relays but they can still trip via DTT from the Master relay) until the channel is restored. If the estimated error is within
margin upon channel restoration and after 2 to 3 power cycles of valid PFLL calculations, the PFLL will be declared as
OK and the configuration will revert back to Master-Master.
2. If 2 channels fail, PFLL at all relays will be declared as failed and when the channels are back into service, the re-
synch process will be initiated (about 5 to 7 seconds) after channel status becomes OK.
Depending on the system configuration (number of terminals and channels), the 87L function operability depends on the
status of channel(s), status of synchronization, and status of channel(s) ID validation. All these states are available as Flex-
Logic™ operands, for viewing in actual values, logged in the event recorder (if events are enabled in 87L menu), and also
trigger Targets (if targets are enabled in 87L menu). These FlexLogic™ operands are readily to be used to trigger alarm, lit
LED and to be captured in oscillography.
There is, however, a single FlexLogic™ operand
tion is blocked due to communications or settings problems. The state of this operand is based on the combination of con-
ditions outlined above and it is recommended that it be used to enable backup protection if 87L is not available.
The FlexLogic™ operand
87L BLOCKED
apply:
1. Channel fail as indicated below:
At least one channel failed either at 3 Terminal or 2 Terminal-1 Channel systems, or
Both channels failed at 2 Terminal-2 Channels
2. PFFL fail or suspended,
3. Channel ID failure detected on at least one channel at either system.
GE Multilin
, reflecting whether or not the local current differential func-
87L BLOCKED
is set when the 87L function is enabled and any of the following three conditions
L90 Line Differential Relay
8.1 OVERVIEW

8.1.20 RELAY SYNCHRONIZATION

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