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P746/EN AP/G31 Application Notes
(AP) 6-
24
MiCOM P746
AP
6.1.2 Bias current in discriminitative zones
Magnitude of bias current in discriminative zones will be equal to scalar sum of all the
currents entering / leaving that particular zone ONLY.
Zone 1 Bias current: ibiasZ(t) = |i1| + | i3| + … + |ic| = Σ |in1|
Zone 2 Bias current: ibiasZ(t) = |ic| + |i2| + |im| = Σ |in2|
For Example:
Let’s consider we have total 8 feeders , and among that we have 5 feeders connected to
zone1 / bus1. All the currents involved in that zone1 will be summated as zone1 bias current.
If the remaining 3 feeders are connected to zone2 / bus2, all the currents in that zone2 will
be summated as zone2 bias current.
6.1.3 Bias current in check zone
Magnitude of bias current in check zone will be equal to scalar sum of all the currents
entering / leaving the WHOLE substation (the current flowing into coupling is not used)
CZ Bias current: ibiasCZ(t) = |iFeeder1| + … + |iFeederm| = Σ |iFeederm|
For Example:
Let’s consider we have a total of 8 feeders in the station and current through all the outgoing
/ incoming feeders will be summated and will be considered as check zone bias current.
Hence the differential current setting at any point of time could be different for discriminative
zones and check zone, based on the bias of discriminative zones and check zone.
6.2 Disavantages of busbar schemes with check zone AND high number of feeders
6.2.1 Effect on check zone sensitivity
Let’s consider double busbar system, where bus1 has least number of feeders (say 5
feeders) with weak source and busbar 2 has more number of feeders (say 30 feeders).
check zone (CZ) bias current will be extremely large since it sums up all the currents
entering / leaving the station. When there is a fault in bus1, discriminative zone 1 will operate
considering the zone1 bias current, whereas check zone will need higher differential current
for the same fault since the bias current for the same situation could be very huge.
This reduction in sensitivity will affect the operation of CZ element based on the level of
biasing which is based on the number of feeders, ID>CZ and slope kCZ. In certain cases it
can even lead to non-operation of the CZ element during an internal fault.
It’s generally not recommended to use check zone with large number of feeders (typically
more than 18) for a low impedance biased differential scheme
6.2.2 Effect of spill current in check zone
When high numbers of CTs (typically more than 18) are connected to check zone, more spill
current is expected during load conditions. Hence, CT circuitry fault setting needs to be set
with caution and should be more than the standing spill current in order to avoid spurious
blocking of busbar element. But at the same time, the setting should be lower than the
minimum loaded feeder current setting. In most cases it may be difficult to satisfy the
requirements.
Moreover spill current will increase when the busbar is extended with additional feeders,
where the characteristics of new CTs are different from the existing CTs , and it may
aggravate the situation.