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Switch on to fault (SOTF-OC)
The switch-onto-fault function (SOTF-OC) is used to detect, and rapidly clear faults when
circuit breakers (CBs) is going to close onto a pre-existing fault in the protected zone. If a circuit
breaker has closed onto a permanent fault, the memory circuit in the IED will be unable to
memorize the voltage data required to retain the pre-fault values. This is because the voltage
input to distance measuring elements is absent continuously prior to and after the fault
particularly with voltage transformers installed on the line side; as a result, the fault cannot
be detected by conventional standard protection functions. Therefore, in order to avoid such
failures, SOTF-OC is applied so that the fault can be detected for a specific period following
the closure of the CB.
Scheme logic
2.7.1
The SOTF-OC function provides signals to the TRC function: SOTFOC-OPT-A,-B,-C, SOTFOC-
TRIP, and SOTFOC-ARC-BLOCK. The SOTF-OC function issues a trip signal upon the
operation of the OCSOTF element if CBs are closed onto a fault when energizing a
transmission line following a period of non-operation. The criterion for the transmission line
being in the non-operation state are when (1) the CBs are open for more than 0.5 seconds and
(2) the undervoltage elements in the PROT COMMON function remain operated for more than
0.5 seconds. The TRIP and PROT COMMON functions are discussed separately (See Chapters
2.23 and 2.24).
Setting [OCSOTF] is provided to determine the SOTF threshold. Setting [OCSOTF-2fBlk]
is used to override the SOTF function when the SOTF-OC detects a second harmonic. If Block-
AllP is set for the setting [OCSOTF-2fBlk], all of the OCSOTF elements are overridden when
second harmonic is present. Alternatively, if Block-PerP is set for the setting [OCSOTF-2fBlk],
each individual OCSOTF element of the SOTF function is overridden for second harmonic
currents. The detection of the second harmonic is determined by the function of the inrush
current detector (ICD). The ICD function is discussed separately. (See Chapter 2.10.)
The SOTF-OC function is active when On is set for the setting [SOTFOC-EN]. Setting
[SOTF-test] is used for the mandatory condition of cold load when testing. When wishing to
add another condition into the logic for the SOTF operation, the user can inject the signal using
PLC connection point "ADD.SOTF.EN".
- 113 -
6F2T0207 (0.01)
GRE200 (5,6)
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