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and if the load in the protected circuit is relatively high, about the nominal load, 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...120% 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 10% of IBase, the open CT condition cannot be detected. Therefore, the
Open CT algorithm only detects an open CT if the load on the power transformer
protected object is 10...120% of rated load and the differential current is higher
than 10% of IBase on that phase. The search for an open CT starts 60 seconds (50
seconds in 60 Hz systems) after the bias current has entered the 10...120% range.
The Open CT detection feature can also be explicitly deactivated by setting:
OpenCTEnable = 0 ( Off ).
The open CT function can be selected to either block the differential function or
issue the alarm signal via the setting
OCTBlockEn .
When the setting OCTBlockEn is set to ON and an open CT is detected, the output
OPENCT is set to 1 and 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.
Otherwise when the setting OCTBlockEn is set to OFF, only an alarm signal is
issued once an open CT is detected.
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 Line differential
protection function:
1MRK 505 394-UEN A
Section 6
Differential protection
Line differential protection RED650 2.2 IEC 105
Technical manual