Woodward
Manual MRM3-2 GB
24
TD_MRM3-2_09.08_GB_Rev.New
5.4.9 Tripping Time or Time Factor for the Phase Over current Element
(I>+t>)
Normally, after change of the trip characteristics, the tripping time or the time factor also has to be
changed accordingly. In order to avoid an unsuitable combination between trip characteristics and
tripping time or time factor, the following measures are initiated by the MRM3-2:
The LED for adjustment of the tripping time or time factor (I> + t>) starts to flash after the trip cha-
racteristics have changed. By this warning signal the operator is reminded to adjust the tripping
time or time factor to the changed operational mode or trip time characteristics. This warning signal
keeps flashing until the tripping time or time factor are re-adjusted. If re-adjustment has not been
done within 5 minutes (the time to enable parameter setting), then the processor sets the tripping
time or time factor to the highest sensible value (smallest possible tripping time ).
When adjusting to the “Definite Time” trip characteristic, the definite tripping time displayed is
shown as seconds (e.g. 0.35 = 0.35s). By pressing push-buttons <+><-> this time can be changed
step by step in the range 0.04s – 260s
When adjusting to the “Inverse Time” trip characteristics, the time factor (tI>) is displayed. This fac-
tor can be changed also by push buttons <+><-> step by step in the range 0.05 – 20.0.
If the tripping time or time factor are set to infinite long (on the display “EXIT” is shown) then trip of
the relay over current element is blocked. The WARNING/ALARM function is still active. During this
time LEDs I> and t> are red.
5.4.10 Reset Mode for the Trip Characteristics in the Phase Current Path
(I>+CHAR+t>)
In order to ensure that the trip function is reliable even with repeated error pulses, each of them
shorter than the set tripping time, the RESET mode for the trip characteristics can be changed
over. With a setting of “60s“ the elapsed tripping time is frozen and is only reset after 60s faultless
operation. Should another fault occur within these 60s, the tripping time counter re-mains in opera-
tion. With the setting “_ 0s“ the counter is immediately reset when the fault current is interrupted
and it is restarted when the fault current has returned again.
5.4.11 Phase Short-Circuit Trip (I>>) and (I>>+Start)
The short-circuit element has two threshold values and two time delays. The first threshold value
applies for the mode RUNNING and the second one for mode START.
Possible variations are:
•
During the start procedure the existing inrush current can be higher than the desired short-
circuit threshold during operation. e.g. direct start)
•
It is also possible that the slip rings are put at risk especially during the start procedure so
that for START a more sensitive adjustment is required than for RUNNING.
•
Both elements can be set to the same value so there is no differentiation.
If it is set to EXIT then the respective element is switched off.
Irrespectively of the selected trip characteristics for I>, the short-circuit high-speed trip element I>>
has a trip-ping time which does not depend on the current. This time applies for both elements
I>>START and I>>RUNNING.
