Toshiba GRZ100 B Series, Инструкция по эксплуатации

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6 F 2 S 0 8 4 6
F3 F2 F1
C B A
Figure 2.4.5.2 Current Settings in Radial System
Time setting
Time setting is performed to provide selectivity in relation to the relays on adjacent lines. Consider
a minimum source impedance when the current flowing in the relay becomes a maximum. In
Figure 2.4.5.2, in the event of a fault at the near end, F2 of the adjacent line, the operating time is
set so that terminal A may operate by time grading Tc behind terminal B. The current flowing in
the relays may sometimes be greater when the remote end of the adjacent line is open. At this time,
time coordination must also be kept.
The reason why the operating time is set when the fault current reaches a maximum is that if time
coordination is obtained for a large fault current, then time coordination can also be obtained for
small fault current as long as relays with the same operating characteristic are used for each
terminal.
The grading margin Tc of terminal A and terminal B is given by the following expression for a
fault at point F2 in Figure 2.4.5.2.
Tc = T
1
+ T
2
+ T
m
where, T
1
: circuit breaker clearance time at B
T
2
: relay reset time at A
T
m
: time margin
When single-phase autoreclose is used, the minimum time of the earth fault overcurrent protection
must be set longer than the time from fault occurrence to reclosing of the circuit breaker. This is to
prevent three-phase final tripping from being executed by the overcurrent protection during a
single-phase autoreclose cycle.
2.4.5.2 Definite Time Overcurrent Backup Protection
In a system in which the fault current does not vary a great deal in relation to the position of the
fault, the advantages of the IDMT characteristics are not fully used. In this case, definite time
overcurrent protection is applied. The operating time can be set irrespective of the magnitude of
the fault current.
The definite time overcurrent protection consists of instantaneous overcurrent elements and
delayed pick-up timers started by them.
Identical current values can be set for terminals, but graded settings are better than identical
settings in order to provide a margin for current sensitivity. The farther from the power source the
terminal is located, the higher sensitivity (i.e. the lower setting) is required.
The operating time of the overcurrent element at each terminal is constant irrespective of the
magnitude of the fault current and selective protection is implemented by graded settings of the
delayed pick-up timer. As a result, the circuit breaker of the terminal most remote from the power
source is tripped in the shortest time.
When setting the delayed pick-up timers, time grading margin Tc is obtained in the same way as
explained in Section 2.4.5.1.