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As the example application in Figure
showed, for an internal fault in the blind spot,
the differential zone ZA will unnecessarily operate and open the bus section breaker
and all other feeder breakers associated with it. Nevertheless the fault will still exists
on other busbar section, but it is outside the current transformer in the bus section bay
and hence outside the zone ZB ( that is, it is external fault for zone ZB). Similar
problem will also exist if section/coupler circuit breaker was open before the internal
fault in the blind zone. Therefore, the busbar protection scheme does not protect the
complete busbar.
In order to improve the busbar protection scheme with this type of station layout, it is
often required to disconnect the bus-section or bus-coupler CT from all affected
differential zones as soon as the bus-section or bus-coupler circuit breaker is opened.
This arrangement can be easily achieved within the IED. In such applications, the bus
section or bus coupler current transformer shall be wired just to one current input of the
IED. Meanwhile this current should be given to both related differential zones. To
ensure this, in two-zone busbar protections, it requires to set the corresponding bay the
parameter
ZoneSel
to
FIXEDtoZA&-ZB
in PST. In six-zone busbar protections, this
single CT current should be connected to both CT inputs on the bus section or bus
coupler function block, and then invert one of them in software by using the parameter
setting
OperMode
in PST. This shall be done taking into account the actual location of
the CT star point in this bay.
When the bus coupler/section breaker is open, to disconnect this current from both
zones, additional configuration logic is required by using the binary input, for example
ZEROCUR
in two-zone differential protections . Figure
provides an example of
such logic. Moreover, the following two binary inputs are at least necessary in order to
guaranty proper operation of such logic:
•
Normally closed contact of the bus section or bus coupler circuit breaker
•
Signal from the bus section or bus coupler circuit breaker closing circuit that
somebody wants to close the breaker.
This solution does not depend on contact timing between the main contacts and
auxiliary contact of the circuit breaker. It directly follows the philosophy used for
RADSS/REB 103 schemes used for similar applications before. Principle connection
between the bus-coupler CB normally closed auxiliary contact (“b” contact), REB670
and internal configuration logic, as shown in Figure
Section 6
1MRK 505 370-UUS A
Differential protection
124
Busbar protection REB670 2.2 ANSI
Application manual
Содержание RELION REB670
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