ABB RELION REB670, Applications Manual

Page: 120 / 554

Additionally the sensitive differential protection can be time delayed and it must be
externally enabled by a binary signal (that is, from external open delta VT overvoltage
relay or power transformer neutral point overcurrent relay).
Please refer to the technical reference manual for more details about the working
principles of the differential function algorithm.
6.1.3.3 Check zone protection
M12474-3 v4
An integrated overall differential zone, so-called check zone, is available for multiple
busbar stations to secure stability of the busbar differential protection, in case of
problem with disconnector auxiliary contacts (stuck contacts or wiring problems).
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.
Therefore, an external 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 cannot 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 then 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.
In REB670, an internal built-in check zone in the IED is provided, therefore there is no
need for such costly external check zone. This is possible, mainly due to the following
facts:
• the CT switching is made only in software, and CT secondary current circuits do
not include any auxiliary contacts, as shown in Figure
63 .
• the IED is always supplied with a special zone and phase selective “Open CT
Detection” algorithm, which can instantly block the differential function in case of
an open CT secondary circuits caused by accidents or mistakes.
Consequently a very cost effective solution can be achieved producing extra savings
during scheme engineering, installation, commissioning, service and maintenance.
Operating characteristics for the check zone can be set independently from the two
discriminating zones. However, it is to be observed that the check zone has slightly
different operating characteristic from the usual discriminating zones. For the check
Section 6
1MRK 505 370-UUS A
Differential protection
114 Busbar protection REB670 2.2 ANSI
Application manual