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Section 6
Differential protection
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
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 62.
•
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 zone the outgoing
current is used as stabilizing current instead of total incoming current in order to guarantee
the check zone operation for all possible operating conditions in the station. The check zone
operating characteristic is shown in Figure 41. Note that the check zone minimum differential
operational level
of the usual discriminating zones.
96
OperLevel shall be set equal to or less than the corresponding operating level
1MRK 505 370-UEN D
M12474-3 v4
Busbar protection REB670
Application manual
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