Toshiba GR200 Series Instruction Manual page 27

Reliability
(iii)
The relay is in a quiescent state normally, but the relay shall respond to a fault instantly once
it occurs on the power system. Hence, the operation availability shall be checked whenever the
line is energized in unfaulted state. Additionally, to improve the operation reliability, the
protection scheme is checked externally by fail-safe function (FS). That is, the relay operations
are checked not to issue a trip command mistakenly by the FS relay; consequently, erroneous
tripping can be prevented dramatically if the prime (main) relay operates mistakenly. The
check results of the FS relay are combined together with the operation results of the prime
relay at the last stage (i.e., AND logics runs on the trip circuit).
Behavior of backup protections
1.1.3
Faults shall be securely cleared and clearing the faults shall be made as soon as possible, or
the power system may result in the outage. As to protect the power system, as a rule, a main
(primary) relay operates along with a number of backup relays. A protection function
represents the main (primary) relay, whereas the other protection functions operate as the
backup relays. Accordingly, faults could be clear if the main (primary) relay were in the failure
to operate.
Protection for the line
(i)
The element† in the current differential protection function (DIF) is designed to operate for
the main relay. However, we shall note that the main protection cannot operate perfectly at
any time. For example, the DIF protection function cannot operate correctly once the
telecommunication incident or others occur. Thus, we shall design that clearing the faults could
be performed by the operation in backup relays in any case. Note that we should figure the
capability for clearing the faults being dependent on the system integrity. The conventional
relays (such as distance relay and others) are serviced as the backup protections, which we
discuss successively.
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6F2S1914 (0.49)
GRL200 (Soft 033 & 037)