Toshiba GR200 Series Instruction Manual page 74

The SE can discriminate a faulted state from the steady state in the power system; it
blocks the WDE operation which otherwise may operate under the steady state.
CT secondary current waveforms
(ii)
Figure 2.4-2 shows three current waveforms: two currents at the incoming and outgoing
terminals on CT secondary circuits and a differential current at the outgoing terminal upon
occurrence of an external fault with the CT saturation.
Incoming
terminal current
Outgoing
terminal current
Differential
current
From the inception of a fault until the saturation at the outgoing terminal on the CT
secondary circuit, the differential current (Id) is zero; the change of the Id (ΔId) is obtained
from the equation (2.4-2) is zero. Conversely, the change in the restraining current (Ir) given
by the equation (2.4-3) is a sufficiently large positive value. That is, the equation (2.4-1) is
satisfied; the WDE can operate.
The SE detects current changes at the terminals and operates rapidly, when producing
an AND output with the WDE. After this, a certain time delay is inserted to block the operation
of the DIFL element. This is because there is a period during which the equation (2.4-1) is not
satisfied.
If, during an internal fault, there is a period during which the change in the
instantaneous value of the differential current (Id) is small due to CT saturation, the WDE
will not operate because the change in the restraining current (Ir) is also small during the
period. Thus, during the internal fault, the operation of the DIFL element is not blocked falsely.
In the case of an evolving fault from an external fault to an internal fault, the CTSC is
reset by the evolving fault detection element (EVD) if a change in the instantaneous value of
the differential current (Id) is more than a specified value. The DIFL function issues a trip
command with the absence of delay under such faults.
Figure 2.4-2
Waveforms for CT saturation
- 51 -
6F2S1914 (0.49)
No change period
GRL200 (Soft 033 & 037)