Toshiba GR200 Series Instruction Manual page 72

CT saturation countermeasure in DIFL (CTsat in DIFL)
Counter measure for CT saturation is required when a large current is induced by the
occurrence of through-fault current† and so on. If any CT saturation occurs, the DIFL function
may see a differential current erroneously; thus, the counter measure for the CT saturation is
requisite in the DIFL operation. Consequently, the DIFL function can operate stably even if
the CT saturation occurs. The operation availability for the counter measure is dependent upon
the communication capacity‡ and the number of CTs§ in the IED.
†Note: We shall see the countermeasure of through-fault current in the DIFL application
of 1.5CB arrangement in section 2.6.1.
‡Note: Communication capacity is determined by the transmission control function (TC).
Notation "N" is referred to the bandwidth of communication channel. When N=1,
the bandwidth is referred normal (scheme switch [CH*_COM_WIDTH] = Normal]).
When N=3, the bandwidth is referred wide (scheme switch [CH*_COM_WIDTH] =
Wide]). Extended data frame at N=3 is ready for exclusive analog input
quantities, which are only used in WDE element. Data frame at N=1 is ready for
the other analog input quantities, which are not used in the WDE (say,
conventional DIF calculations are only performed with the data transferred at
N=1 frame). For more information about Wide, see Chapter
Transmission control function
§Note: The number of CTs is decided by switch [APPL-CTSELECT]. For more
information, see Chapter
polarity, and busbar arrangements
Stabilization for CT saturation owing to through-fault current
2.4.1
Principle of operation
(i)
Even when CTs begins to be 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 2.4-1 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
, separately.
Technical description: Settings for residual voltage, CT
.
- 49 -
6F2S1914 (0.49)
Relay application:
GRL200 (Soft 033 & 037)