Where
：
"n” means the number of sides shared by transformers.
The left part of above equation is restraint current, while the right part is differential current. The
equation is not fulfilled in case of either external fault or external fault under the condition of
saturated CT. As a matter of fact, since CT saturation induced differential current always comes
into being after a certain period of time of CT saturation, the device determines if saturation has
occurred by taking advantage of the temporal consistency between restraint current and
differential current. If saturation has occurred, the percentage differential restraint coefficient would
be automatically driven up so as to assure the reliability of differential protection and the quick
operation in case of conversion of external fault saturation into internal fault.
3.42.2.8 CT Circuit Failure
This is a differential protection CT circuit failure criterion.
First of all, the possibility of concurrence of multi-side CT line-break and fault is not taken into
consideration. Under this premise, it’s possible to distinguish between CT circuit failure and fault
based on the following characteristic. In case of CT circuit failure, it’s necessary to specifically
identify circuit failure phase.
Single-phase or two-phase CT circuit failure:
In case the abrupt current variation is greater than 5~10%IN, abrupt current variation would be
deemed to have occurred.
Alarm signal will be issued when CT circuit failure lasts for 10s (or 40ms if the differential
protection trips), and in such a case, whether or not to instantaneously block relevant differential
protection is determined by setting. The condition for reset of CT circuit failure is that there's no
negative-sequence current at this side. The negative-sequence current at this side is lower than
threshold of CT line-break negative-sequence current (fixed value).
Blocking of biased differential protection by CT circuit failure follows the following principle:
When “CTS_Blk_Ena” is set to “1”, REF protection would be blocked in case of CT circuit
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