Toshiba GR200 Series Instruction Manual page 48

Segregated-phase current differential protection function (DIFL)
The principle of current differential protection (DIFL) function is to compare the currents
flowing into and out of the transmission line; the differential current is obtained by the
comparison between two currents in the DIFL function. In the DIFL function the value of the
differential current is almost zero when a fault occurs out of the protection zone (i.e., external
fault). Conversely the value of the differential current is nearly equal to the fault current when
the fault occurs in the protection zone (i.e., internal fault) The DIFL function can issue a trip
signal when the value of differential current is beyond to the DIFL settings.
The DIFL function can calculate the differential current when current data of two-
terminal line are obtained. That is, the relays at local-terminal and remote-terminal should be
connected by the communication. If a failure occurs on the communication, the DIFL function
may be affected by the error data due to the failure. Therefore, five check relays are provided
and are used to prevent the spurious operation in the DIFL function.
When a failure occurs on the CT circuit, the DIFL function cannot operate correctly.
Therefore, CT circuit failure-detection function is available in the DIFL function. Monitoring
function for differential current (DIFL-IDSV) is also provided so that the spuriousness of the
differential current can be detected during the CT circuit failure
Countermeasure features for through-fault-current and stub area are also available in
the DIFL function. These features are useful for the protection of the close-up external fault
and the stub-area fault.
The user can use the DIFL function for lines connected via a one-and-a-half breaker
busbar system; the DIFL function can sense stub faults and through fault currents.
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6F2S1914 (0.49)
GRL200 (Soft 033 & 037)