- 151 -
by using only the reach settings, the user may be required to set the TZ3
(X)
longer than the
TZ2
(Y
).
(iv) Setting for backward lines (Zones4 and 5 serviced as backup relays)
If backward adjacent lines are required to protect by the relay
(X)
, the zone4
(X)
or the zone5
(X)
is
suitable to protect. For example, as shown in Figure 2.10-53, if setting the reach of the zone4
(X)
can be performed in the reverse to cover the busbar and the transformer, the zone4
(X)
will
operate as a back-up relays.
(v) Extra setting for the primary zone (Zone1X with autoreclosing)
The user should notice that disadvantages exist in the CTI schemes, because a fault occurred
a bit beyond the zone1 may be cleared in delay by the zone2. That is, the zone1 plus the zone2
cannot clear speedily for any fault on the line AB†. Thus, the extra zone1 (zone1X) is designed
to clear the fault occurred near the busbar B; its operation is taken together with the operation
of the autoreclose function (ARC‡).
Figure 2.10-56 CTIs for zone1 and zone1X
As shown in Figure 2.10-56, the reach of the zone1X should be set to overreach the
protected line AB. Once a fault occurs near busbar B, the ARC function must operate to follow
the TRIP COMMAND issued by the zone1X. The user must know that the zone1X is just
effective about the transient fault for which the ARC operation§ can recover the power
transmission.
If evolving faults occur, the zone1X will issue one-more TRIP COMMAND successively to
follow a CB reclose command issued by the ARC function. If the permanent fault occurs, the
zone1X will issue no TRIP COMMAND respond to a reclose-on-to-fault condition.
†Note: The command distance protection (DISCAR) is provided for the speedy protection
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