ABB Relion 611 Series, Technical Manual

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Section 9 General function block features
9.1 Definite time characteristics
9.1.1 Definite time operation
The DT mode is enabled when the Operating curve type setting is selected either as
"ANSI Def. Time" or "IEC Def. Time". In the DT mode, the
OPERATE
output of the
function is activated when the time calculation exceeds the set Operate delay time .
The user can determine the reset in the DT mode with the Reset delay time setting,
which provides the delayed reset property when needed.
The Type of reset curve setting has no effect on the reset method when
the DT mode is selected, but the reset is determined solely with the
Reset delay time setting.
The purpose of the delayed reset is to enable fast clearance of intermittent faults, for
example self-sealing insulation faults, and severe faults which may produce high
asymmetrical fault currents that partially saturate the current transformers. It is typical
for an intermittent fault that the fault current contains so called drop-off periods,
during which the fault current falls below the set start current, including hysteresis.
Without the delayed reset function, the operate timer would reset when the current
drops off. In the same way, an apparent drop-off period of the secondary current of the
saturated current transformer can also reset the operate timer.
1MRS757454 D
Section 9
General function block features
611 series 439
Technical Manual