The differential element is completely dependent on receiving data from the relay at the remote end of the line, therefore,
upon startup, the differential element is disabled until the time synchronization system has aligned both relays to a common
time base. After synchronization is achieved, the differential is enabled. Should the communications channel delay time
increase, such as caused by path switching in a SONET system or failure of the communications power supply, the relay
will act as outlined in the next section.
The L90 incorporates an adaptive differential algorithm based on the traditional percent differential principle. In the tradi-
tional percent differential scheme, the operating parameter is based on the phasor sum of currents in the zone and the
restraint parameter is based on the scalar (or average scalar) sum of the currents in the protected zone - when the operat-
ing parameter divided by the restraint parameter is above the slope setting, the relay will operate. During an external fault,
the operating parameter is relatively small compared to the restraint parameter, whereas for an internal fault, the operating
parameter is relatively large compared to the restraint parameter. Because the traditional scheme is not adaptive, the ele-
ment settings must allow for the maximum amount of error anticipated during an out-of-zone fault, when CT errors may be
high and/or CT saturation may be experienced.
The major difference between the L90 differential scheme and a percent differential scheme is the use of an estimate of
errors in the input currents to increase the restraint parameter during faults, permitting the use of more sensitive settings
than those used in the traditional scheme. The inclusion of the adaptive feature in the scheme produces element character-
istic equations that appear to be different from the traditional scheme, but the differences are minimal during system
steady-state conditions. The element equations are shown in the OPERATING CONDITION CALCULATIONS section.
On startup of the relays, the channel status will be checked first. If channel status is OK, and BER and CRC values are
below their limits, all relays will send a special "startup" message and the synchronization process will be initiated. It will
take about 2 minutes 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 deterioration in performance. If communications are interrupted for more than 4
cycles, the following applies:
In 2-terminal mode:
With second redundant channel, relays will not lose functionality at all if second channel is live.
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:
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 2-3 power cycles of valid PFLL calculations, the PFLL will be declared as
OK and the configuration will revert back to Master-Master.
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 2 minutes) 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 87L BLOCKED, reflecting whether or not the local current differential func-
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.
8.1.20 DIFFERENTIAL ELEMENT CHARACTERISTICS
L90 Line Differential Relay
8 THEORY OF OPERATION
8.1.21 RELAY SYNCHRONIZATION
GE Power Management