ABB REB670 Applications Manual page 130

Section 6
Differential protection
124
Oper Level
IEC06000062 V1 EN
Figure 51: Check zone operation characteristic
Note that the check zone minimum differential operational level OperLevel shall be set
equal to or less than the corresponding operating level of the usual discriminating zones.
For substations where traditional "CT switching" is not required (that is, single busbar
station or breaker-and-a-half station) the check zone must not be used. For such
applications the check zone shall be disabled by setting check zone setting parameter
Operation to Disabled.
When CT-circuits are switched depending on the position of the busbar disconnectors
there is a possibility that some of the CT secondary circuits can be open circuited by a
mistake. At the same time this can cause unwanted operation of the differential protection
scheme.
For this reason, a so-called check zone is often required for a traditional high-impedance
busbar protection scheme when switching in CT-circuit is done. The check zone is fixed
and has no switching of CTs in any of the outgoing circuits and is not connected to busbar
section and busbar coupler bays. The check zone, will detect faults anywhere in the
substation but can not distinguish in which part of the station the fault is located. When the
check zone detects a fault it gives a release signal to the busbar protection relays in all
individual, discriminating zones. The busbar protection discriminating zones will than
trip the part of the substation that is faulty. However, this principle creates not only a high
cost as separate CT cores are required, but also a need for extra cabling and a separate
check zone differential relay.
There is no need for an external check zone due to the following facts:
Operate
region
s=0.0-0.90 (settable)
I [Primary Amps]
out
en06000062.vsd
1MRK 505 337-UUS -
Application manual