ABB RELION REX640 Technical Manual page 791

1MRS759142 F
DC restraint
On detection of a DC component, the function temporarily desensitizes the
differential protection. The functioning of this module depends on the
Enable setting. The DC components are continuously extracted from the three
instantaneous differential currents. The highest DC component of all three is taken
as a kind of DC restraint in a sense that the highest effective, temporary sensitivity
of the protection is temporarily decreased as a function of this highest DC offset.
The calculated DC restraint current is not allowed to decay (from its highest ever
measured value) faster than with a time constant of one second. The value of the
temporarily effective sensitivity limit is limited upwards to the rated current of the
machine or 3.3 times that of
extra limit decays exponentially from its maximum value with a time constant of one
second.
This feature should be used in case of networks where very long time constants
are expected. The temporary sensitivity limit is higher to the set operating
characteristics. In other words, the temporary limit has superposed the unchanged
operating characteristics and temporarily determines the highest sensitivity of the
protection. The temporary sensitivity is less than the sensitivity in section 1 of the
operating characteristic and is supposed to prevent an unwanted trip during the
external faults with lower currents.
Biased low stage
The current differential protection needs to be biased because of the possible
appearance of a differential current which can be due to something else than an
actual fault in the machine. In case of differential protection, a false differential
current can be caused by:
• CT errors
• CT saturation at high currents passing through the machine
The differential current caused by CT errors increases at the same percent ratio as
the load current.
The high currents passing through the protected object can be caused by the
through fault. Therefore, the operation of the differential protection is biased with
respect to the load current. In the biased differential protection, the higher the
differential current required for the protection of operation, the higher the load
current.
Based on the conditions checked from the through-fault module, the DC
(component) detection module and the CT saturation-based blocking modules,
the biased low-stage module decides whether the differential current is due to
the internal faults or some false reason. In case of detection of the TF, DC or CT
saturation, the internal differential blocking signal is generated, which in turn blocks
the operating signal. In case of internal faults, the operation of the differential
protection is affected by the bias current.
The
determined with the equation:
Low operate value
The
REX640
Technical Manual
Low operate value setting for the stabilized stage of the function block is
I
=
d 1
Slope section 2 and Slope section 3 settings are determined correspondingly:
Low operate value , whichever is smaller. The temporary
Protection functions
DC restrain
(Equation 203)
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