change in circuit breaker position or from circuit breaker close command pulse. The
setting parameter SOTFSel can be set for activation of CB position open change, CB
position closed change or CB close command. In case of a residual current pickup from
step 2 or 3 (dependent on setting) the function will give a trip after a set delay tSOTF.
This delay is normally set to a short time (default 200 ms).
The Under-Time logic always uses the pickup signal from the step 4. The Under-Time
logic will normally be set to operate for a lower current level than the SOTF function.
The Under-Time logic can also be blocked by the 2
nd
harmonic restraint feature. This
enables high sensitivity even if power transformer inrush currents can occur at breaker
closing. This logic is typically used to detect asymmetry of CB poles immediately after
switching of the circuit breaker. The Under-Time logic is activated either from change
in circuit breaker position or from circuit breaker close and open command pulses. This
selection is done by setting parameter ActUnderTime. In case of a pickup from step 4
this logic will give a trip after a set delay tUnderTime. This delay is normally set to a
relatively short time (default 300 ms). Practically the Under-Time logic acts as circuit
breaker pole-discrepancy protection, but it is only active immediately after breaker
switching. The Under-Time logic can only be used in solidly or low impedance
grounded systems.
1MRK505222-UUS C Section 7
Current protection
411
Technical reference manual
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