Open Ct Detection - ABB RELION 650 Series Applications Manual

Section 6
Differential protection
IEC06000638 V2 EN-US
Figure 39: Example of LDCM signals as seen in the Signal matrix too (example showing
6.2.2.8

Open CT detection

Line differential protection has a built-in, advanced open CT detection feature. This feature
can block the unwanted operation created by the Line differential protection function in case
of an open CT secondary circuit under a normal load condition. However, there is no guarantee
that the Open CT algorithm prevents an unwanted disconnection of the protected circuit. The
Open CT can be detected in approximately 14 ± 2 ms, and the differential protection might by
that time in some cases have already issued a trip command. Nevertheless, the information on
an open CT as the reason for trip is still very important. An alarm signal can also be issued to
the substation operational personnel to make remedy action once the open CT condition is
detected. The open CT algorithm is active under normal load conditions and a little above, i.e.
in the range of load current from app. 10 % to app. 150 % rated load. Under normal conditions
load is considered to be well described by the bias current. The Open CT detection algorithm is
based on the principle that in the phase with an open CT, the current will suddenly drop to
zero, while in the other two phases the currents continue as before. The Open CT at each end
is enabled 60 seconds after the currents at that end are higher than 3 % rated current of the
protected line circuit. An open CT can only be declared if in the phase where an open CT is
suspected, exceeds 10 % of the rated current. The Open CT signal disappears when all currents
are again normal, or when all three currents become zero.
• Setting parameter
• Setting parameter
once an open CT is detected. When
issued.
• Setting parameter
given.
• Setting parameter
condition resets once the defective CT circuits have been repaired.
• Once the open CT condition has been detected, all the differential protection functions
are blocked except the unrestrained (instantaneous) differential protection. However,
there is no guarantee that an unwanted disconnection of the protected circuit can always
be prevented.
• For applications where the currents from two CTs are summated and sent over LDCM, the
output OPENCT must be connected to CTFAIL on LDLPSCH logic in order for the Open CT
detection to operate properly. Observe that summation of currents shall be avoided
where possible.
80
SMBI
670 device)l
OpenCTEnable enables and disables this feature.
OCTBlockEn enables and disables the blocking of differential function
tOCTAlarmDelay defines the time delay after which the alarm signal is
tOCTResetDelay defines the time delay after which the open CT
OCTBlockEn is set to OFF, only an alarm signal is
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Application manual