ABB RELION 670 Series Applications Manual page 118

Section 6
Differential protection
Oper Level
IEC06000062 V1 EN-US
Figure 51:
Note that the check zone minimum differential operational level
less than the corresponding operating level of the usual discriminating zones.
For substations where traditional "CT switching" is not required (that is, single busbar station or
breaker-and-a-half station) the check zone must not be used. For such applications the check zone
shall be disabled by setting check zone setting parameter
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.
For this reason, a so-called 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 can not 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 than 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.
There is no need for an external check zone 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.
• internal check zone feature is available
This means that a very cost effective solution can be achieved using REB670, producing extra
savings during scheme engineering, installation, commissioning, service and maintenance.
The pre-configured binary output contacts, are provided in the IED in order to alarm the open CT
circuit condition. At the same time, one of the LEDs on the local HMI can be programmed to light
up. It shall be noted that the Open CT Circuit alarm can only be manually reset by one of the three
following ways:
112
Operate
region
s=0.0-0.90 (settable)
I [Primary Amps]
out
Check zone operation characteristic
en06000062.vsd
OperLevel shall be set equal to or
Operation to Disabled .
1MRK 505 337-UUS A
M12115-3 v3
Application manual