Current restraint feature
The overcurrent protection step operation can be made dependent of a restraining
current quantity (see table 334). Practically then the pickup level of the overcurrent
step is not constant but instead increases with the increase in the magnitude of the
restraining current.
IsetHigh
IsetLow
I
Measured
Restraint
atan(RestrCoeff)
en05000255.vsd
Operate area
I>RestrCoeff*Irestrain
IEC05000255 V1 EN
Figure 276: Current pickup variation with restraint current magnitude
This feature will simply prevent overcurrent step to start if the magnitude of the
measured current quantity is smaller than the set percentage of the restrain current
magnitude. However this feature will not affect the pickup current value for
calculation of operate times for IDMT curves. This means that the IDMT curve
operate time will not be influenced by the restrain current magnitude.
When set, the start signal will start definite time delay or inverse (IDMT) time delay
in accordance with the end user setting. If the start signal has value one for longer time
than the set time delay, the overcurrent step will set its trip signal to one. Reset of the
start and trip signal can be instantaneous or time delay in accordance with the end user
setting.
11.1.7.4 Built-in undercurrent protection steps
Two undercurrent protection steps are available. They are absolutely identical and
therefore only one will be explained here. Undercurrent step simply compares the
magnitude of the measured current quantity (see table
332) with the set pickup level.
The undercurrent step will pickup and set its start signal to one if the magnitude of the
measured current quantity is smaller than this set level. The start signal will start
definite time delay with set time delay. If the start signal has value one for longer time
than the set time delay the undercurrent step will set its trip signal to one. Reset of the
start and trip signal can be instantaneous or time delay in accordance with the setting.
1MRK502052-UEN B Section 11
Multipurpose protection
583
Technical manual
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