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
TRIP
output of the function is
activated when the time calculation exceeds the set
Trip 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 operation 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 operation timer.
1MAC059074-MB A
Section 11
General function block features
615 series ANSI
1073
Technical Manual
Summary of Contents for Relion 615 series
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