ABB Relion REG670, Technical Manual

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addition to this it is also possible to specify the cut-off time below which the IDMT
curve defined by the above equation will be used. Note that
tCutOff shall be always set
to the smaller value than tMax .
The operate time of the stator overload function is calculated by using the integration
principle (see Chapter “Inverse time characteristics” in the Technical Manual for
more information). As a service value from the stator overload function regarding this
integration process the Theta parameter, proportional to the stator heating content, is
provided. This parameter has value of 100% when function gives the trip signal. Thus,
in simple words, it represent a percentage of the function trip level. As soon as the
measured current is above the pick-up level I> and no block signal is present, the
output signal START is activated and the Theta parameter is incremented following
the inverse characteristic given in Figure
215 . When Theta has reached value 100%,
the output signal TRIP is set. The minimum trip signal duration is defined by the
parameter
tPulse . Note also that the BLKRECL signal will remain one as long as
Theta > ReclsLevTheta . Thus the parameter ReclsLevTheta represents the Theta value
below which is safe again to re-connect the tripped generator to the network.
The output signal START will reset if the measured current falls below reset level or
if BLOCK signal is set to one. However the Theta value is stored in the IED memory
and it starts to decay slowly following a linear reset characteristic defined by the
parameter
tReset. The parameter tReset actually defines the rest time for Theta from
value 100 to zero. Note that if an overload condition happens again, the last Theta
value is used as a starting point for the calculation. Thus the stator winding will be
properly protected against varying current levels.
Section 8
1MRK502052-UEN B
Current protection
460
Technical manual