ABB Relion REG670, Technical Manual, Page: 166 / 1298

the unwanted trip cannot always be prevented. Still, the information about what was
the cause of the open CT secondary circuit, is vital.
The principle applied to detect an open CT is a simple pattern recognition method,
similar to the waveform check used by the Power transformer differential protection
in order to detect the magnetizing inrush condition. The open CT detection principle
is based on the fact that for an open CT, the current in the phase with the open CT
suddenly drops to zero (that is, as seen by the protection), while the currents of the
other two phases continue as before.
The open CT function is supposed to detect an open CT under normal conditions, that
is, with the protected multi-terminal circuit under normal load (10...110% of the rated
load). If the load currents are very low or zero, the open CT condition cannot be
detected. In addition to load condition requirement, Open CT function also checks the
differential current on faulty phase. If the differential current is lower than 50% of
IdMin, the open CT condition cannot be detected. Therefore, the Open CT algorithm
only detects an open CT if the load on the power transformer is 10...110% of rated load
and the differential current is higher than 50% of IdMin on that phase. The search for
an open CT starts 60 seconds (50 seconds in 60 Hz systems) after the transformer is
energized. The Open CT detection feature can also be explicitly deactivated by
setting:
OpenCTEnable = 0 ( Off ).
If an open CT is detected and the output OPENCT set to 1, then all the differential
functions are blocked, except the unrestrained (instantaneous) differential. An alarm
signal is also produced after a settable delay (
tOCTAlarmDelay ) to report to
operational personnel for quick remedy actions once the open CT is detected. When
the open CT condition is removed (that is, the previously open CT is reconnected), the
functions remain blocked for a specified interval of time, which is also defined by a
setting (
tOCTResetDelay ). This is to prevent an eventual mal-operation after the
reconnection of the previously open CT secondary circuit.
The open CT algorithm provides detailed information about the location of the
defective CT secondary circuit. The algorithm clearly indicates the IED side, CT input
and phase in which an open CT condition has been detected. These indications are
provided via the following outputs from the Transformer differential protection
function:
1. Output OPENCT provides instant information to indicate that an open CT circuit
has been detected.
2. Output OPENCTAL provides a time-delayed alarm that the open CT circuit has
been detected. Time delay is defined by the parameter tOCTAlarmDelay .
3. Integer output OPENCTIN provides information on the local HMI regarding
which open CT circuit has been detected (1=CT input No 1; 2=CT input No 2).
4. Integer output OPENCTPH provides information on the local HMI regarding in
which phase an open CT circuit has been detected (1=Phase L1; 2= Phase L2; 3=
Phase L3).
Once the open CT condition is declared, the algorithm stops to search for further open
CT circuits. It waits until the first open CT circuit has been corrected. Note that once
Section 6
1MRK502052-UEN B
Differential protection
160
Technical manual