mistake open circuited on the secondary side. Note that this feature can only detect
interruption of one CT phase current at the time. If two or even all three-phase currents
of one set of CT are accidentally interrupted at the same time this feature cannot
operate and Line differential protection generates trip signal, if the false differential
current is sufficiently high. To ensure blocking of the differential protection for open
CT condition this algorithm must operate within 10 ms in order to be able to prevent
unwanted operation of Line differential protection under all loading conditions.
The principle applied to detect an open CT is a simple pattern recognition method,
similar to the waveform check which has been with advantage 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 (at least theoretically) 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. If the load currents are
very low or zero, the open CT condition cannot be detected. The open CT algorithm
only detects an open CT if the load on the power transformer is from 10% to 110% of
the rated load. Outside this range an open CT condition is not even looked for. The
search for an open CT starts after 60 seconds (50 seconds in 60 Hz systems) since the
bias current enters the 10–110% range. The Open CT detection feature can also be
explicitly deactivated by setting: OpenCTEnable = 0 (Disabled).
If an open CT is detected and the output OPENCT set to 1, then all the differential
functions are blocked, except of the unrestrained (instantaneous) differential. An alarm
signal is also produced after a settable delay (tOCTAlarmDelay) to report to
operational personal for quick remedy actions once the open CT is detected. When the
open CT condition is removed (that is, the previously open CT reconnected), the
functions remain blocked for a specified interval of time, which is also a setting
(tOCTResetDelay). The task of this measure is to prevent an eventual mal-operation
after the reconnection of the previously open CT secondary circuit.
The open CT feature works only during normal loading condition. Thus, the open CT
feature must be automatically disabled for all external faults, big overloads and inrush
conditions.
The open CT algorithm provides detailed information about the location of the
defective CT secondary circuit. The algorithm clearly indicates IED side, CT input and
phase in which open CT condition has been detected. These indications are provided
via the following outputs from Line differential protection function:
1MRK505222-UUS C Section 5
Differential protection
139
Technical reference manual
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