ABB RELION REX640, Техническое руководство, Страница: 1858 / 1960

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.
General function block features 1MRS759142 F
1858 REX640
Technical Manual