Overexcitation inverse definite minimum time curve (IDMT)
In the inverse time modes, the trip time depends on the momentary value of the excitation:
the higher the excitation level, the shorter the trip time. The trip time calculation or
integration starts immediately when the excitation level exceeds the set
Pickup value
and
the
PICKUP
output is activated.
The
TRIP
output is activated when the cumulative sum of the integrator calculating the
overexcitation situation exceeds the value set by the inverse time mode. The set value
depends on the selected curve type and the setting values used.
The
Minimum trip time
and
Maximum trip time
settings define the minimum trip time and
maximum trip time possible for the IDMT mode. For setting these parameters, a careful
study of the particular IDMT curves is recommended.
The tripping time of the function block can vary much between different
operating curve types even if other setting parameters for the curves were
not changed.
Once activated, the timer activates the
PICKUP
output for the IDMT curves. If the
excitation level drops below the
Pickup value
setting before the function trips, the reset
timer is activated. If the fault reoccurs during the reset time, the tripping calculation is
made based on the effects of the period when
PICKUP
was previously active. This is
intended to allow a tripping condition to occur in less time to account for the heating
effects from the previous active pickup period.
When the fault disappears, the reset time can be calculated:
reset time
PICKUP
DUR
Cooling time
=
⋅
_
100
GUID-14537CFF-A8C8-49DF-A0D3-02FF269FE516 V2 EN
(Equation 4)
For the IDMT curves, when the fault disappears, the integral value calculated during
PICKUP
is continuously decremented by a constant that causes its value to become zero
when the reset time elapses during the reset period. If a fault reoccurs, the integration
continues from the current integral value and the pickup time is adjusted, as shown in
. The pickup time becomes the value at the time when the fault dropped off
minus the amount of reset time that occurred. If the reset period elapses without a fault
being detected, the saved values of the pickup time and integration are cleared.
1MRS240050-IB C
Section 4
Protection functions
REF615R
251
Technical Manual
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