Section 11 General function block features
11.1 Definite time characteristics
11.1.1 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.
1YHT530004D05 D Section 11
General function block features
615 series 791
Technical Manual
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