Toshiba GR200 Series Instruction Manual page 39

Stabilization for CT saturation with through-fault current
2.1.3
For current the differential protection of bus-bars, a counter measure against CT saturation is
essential. If any CTs saturate due to a large through-fault current, an apparent differential
current is generated in the differential circuit and can cause false operation of the differential
protection.
Principle of Operation
(i)
Even when CTs become saturated due to very large primary currents, the waveform of the
saturated CT secondary current has two periods in each cycle: a non-saturation period and a
saturation period. This phenomenon is utilized to provide very secure operation for external
faults with a large through-fault current.
Figure 2.1-6 shows the block diagram for the CT saturation countermeasure (CTSC).
CTSC has a waveform discriminating element (WDE) and a starting element (SE). The WDE
will operate if a change in the instantaneous value of the differential current is less than a
specified percentage corresponding change in the instantaneous value of the restraining
current. For through-fault currents, during the CT non-saturation period, the differential
current is theoretically zero and hence, this element will operate during this period.
An evolving fault detection element (EVD) is included to monitor the no-change period of
differential current, and will reset the CTSC function in the case of an evolving fault i.e. in
this case a fault that evolves from an external fault to an internal fault.
Current
Differential Element
Input
(DIFCH, DIFDZ)
Waveform Discriminating
Element (WDE)
Starting Element (SE)
Evolving fault detection
element (EVD)
Figure 2.1-6 Differential element with CT saturation countermeasure
Note: The no-change period is generally motioned with the CTSC function (i.e., [CTSC*-
EN]=On), but blocking the CTSC function is possible when Off is set.
The algorithm used in this element is given by Equations (2.1-1) to (2.1-3):
&
Starter
t
1 cycle
CTSC*-EN On
∆Id < 0.15 × ( ∆Ip + ∆In )
- 25 -
0
t
&
3 cycles
CTSC
0
EVD
6F2S1931 (0.20)
Tripping
&
Output
&
(2.1-1)
Centralized GRB200
(Soft: 030, 031, 032, 033, 034)