ABB REB670 Applications Manual page 128

Section 6
Differential protection
122
value or active binary input, while zone switching feature is active within the IED. This
setting is typically used for bus coupler bay in double busbar stations.
If for a particular CT input setting parameter ZoneSwitching is set to ForceIn, then this CT
input will be connected to both the differential zones, regardless of any other set value or
active binary input, while zone switching feature is active within the IED. This setting is
typically used for all feeders bay in a station with two single zone interconnected by a
sectionalizing disconnector.
If for a particular CT input setting parameter ZoneSwitching is set to Conditionally, then
this CT input will be connected to both the differential zones only if it was included to any
of the two zones for 2ms before the zone switching feature was activated. This setting is
typically used for all feeders bay in double busbar stations. With this setting all feeder
bays, which were not connected to any of the two zones before the zone interconnection
activation (that is, out for scheduled maintenance), will not either be included during zone
interconnection.
This practically means that for double busbar station, when zone switching feature is
active, all feeder bays will be connected to both differential zones, while bus coupler CT
will be disconnected from both zones. In this way simple but effective solution is formed.
It is as well important to notice that all necessary changes in the individual bay tripping
arrangements will be automatically performed within the internal logic.
A dedicated binary signal will be immediately activated in the internal logic when zone
interconnection feature is activated. If this feature is active longer than the pre-set time
separate alarm binary signal is activated, in order to alarm the station personnel about such
operating conditions. ZoneInterconnection feature can be disabled by a parameter setting
for substation arrangements where it is not required that is, single busbar stations, breaker-
and-a-half stations and so on.
Discriminating zones (that is, Zone A and Zone B) in the IED includes a sensitive
operational level. This sensitive operational level is designed to be able to detect busbar
ground faults in low impedance edgrounded power systems (that is, power systems where
the ground-fault current is limited to a certain level, typically between 300 A and 2000 A
by neutral point reactor or resistor) or for some other special applications where increased
sensitivity is required. Operation and operating characteristic of the sensitive differential
protection can be set independently from the operating characteristic of the main
differential protection. The sensitive differential level is blocked as soon as the total
incoming current exceeds the pre-set level. By appropriate setting then it can be insured
that this sensitive level is blocked for external phase-to-phase or three-phase faults, which
can cause CT saturation. Comparison between these two characteristics is shown in
figure 50.
1MRK 505 337-UUS -
Application manual