ABB Relion 670 series, Technical Reference Manual

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Current restraint feature
The overcurrent protection step operation can be made dependent of a restraining
current quantity (see table
299 ). 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
Op
er
ate
ar
ea
I>R
es
trC
oe
ff*I
res
tra
in
IEC05000255 V1 EN
Figure 292: Current pickup variation with restraint current magnitude
This feature will simply prevent overcurrent step to pickup 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 pickup signal will start definite time delay or inverse (IDMT) time delay
in accordance with the end user setting. If the pickup 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 pickup and trip signal can be instantaneous or time delay in accordance with the
end user setting.
10.1.2.4 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
297 ) with the set pickup level.
The undercurrent step will pickup and set its pickup signal to one if the magnitude of
the measured current quantity is smaller than this set level. The pickup signal will start
definite time delay with set time delay. If the pickup signal has value one for longer
Section 10
1MRK505222-UUS C
Multipurpose protection
576
Technical reference manual