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and also the differential current at the time of an external fault with outgoing terminal CT
saturation.
Incoming terminal
current
Outgoing terminal
current
Differential
current
No change period
Figure 2.2.2.2 CT Secondary Current Waveforms and Differential Current for an External
Fault with CT Saturation
From the inception of the fault until the CT secondary current at the outgoing terminal saturates,
the differential current Id is zero and the change in the differential current ΔId obtained from
equation (2) is also zero. However, the change in the restraining current given by equation (3) is a
sufficiently large positive value, so equation (1) is met and WDE operates.
SE detects changes in the terminal currents and rapidly operates, producing an AND output with
WDE. After this, since there is a period during which equation (1) is not satisfied, a certain time
delay is inserted to reliably block the operation of the DIFT_DIF differential element.
If, during an internal fault, there is a period during which the change in the instantaneous value of
the differential current is small due to CT saturation, WDE will not operate because the change in
the restraining current is also small during that period. Thus, during an internal fault, operation of
the differential element is not blocked falsely.
The CTS function can be disabled by the scheme switch [CTSEN].