3.5.1 CHECK ZONE SUPERVISION
Just as the individual zones have a bias characteristic, so does the check zone. Therefore to confirm a trip
both the zone and checkzone thresholds must be exceeded. The checkzone thresholds are:
● The bias slope characteristic defined by the kCZ
setting in the GROUP X DIFF PROTECTION column
● The differential operating current threshold defined by the IDCZ>2 setting in the GROUP X DIFF
PROTECTION column.
The following diagram shows the bias characteristic for the check zone element, where:
Idiff is the vector sum of all the currents entering and leaving the check zone
Ibias is proportional to the scalar sum of all the currents entering and leaving the check zone.
Check zone supervision area
Idiff
Restrain area
Ibias
E00719
kCZ = 0% to 90%
IDCZ>2
Figure 32: Check zone supervision characteristic.
3.5.2 AUXILIARY CONTACTS DISCREPANCY
If the actual position of an isolator or circuit breaker in the substation is not the same as that shown in the
scheme, it can produce a differential current in one or more nodes. This is known as an auxiliary contacts
discrepancy and is one of the most common causes of maloperation of differential busbar protection
schemes. Therefore if a zone shows a differential current but the whole substation scheme does not, the
error may be in the zone’s assumption of the position of plant at that particular time.
3.6 CONTINUOUS SUPERVISION OF CURRENT CIRCUITS (CIRCUITRY FAIL)
Under normal operating conditions there should be no differential current, although there might be a small
amount due to CT mismatch. If a CT or its circuitry fails, a differential current may be seen. This product
monitors the scheme for such conditions but if you don’t want to use this circuitry fail feature you can disable
it.
MiCOM P747 Chapter 5 - Protection Functions
P747-TM-EN-1 107
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