ABB Relion 670 Series Applications Manual page 102

Section 6
Differential protection
or three-phase faults, which can cause CT saturation. Comparison between these two characteristics is
shown in Figure 41.
DiffOperLevel
SensOperLevel
IEC06000142 V2 EN-US
Figure 41:
where
I
in
I
d
s = 0.53
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
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,
96
s=0.53
Differential protection operation characteristic
RMS value of the incoming current into the differential protection zone
RMS value of the differential current from the differential protection zone
operating slope for the differential function (fixed in the algorithm).
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Sensitive
differential
protection
Operate
region
Differential protection
operation characteristic
SensIinBlock
I [Primary Amps]
in
1MRK 505 370-UUS Rev. K
IEC06000142-2-en.vsdx
Busbar protection REB670
Application manual
M12474-3 v4