Harmonic Restrain - ABB RELION 670 Series Technical Manual

Section 6
Differential protection
Sensitive negative sequence differential protection
The difference from the unrestrained negative sequence differential protection, described above,
is that the sensitive one only operates without any pickup signal or harmonic blocking signal to be
set. It is enough that both of the negative sequence currents, contributions to the total negative
sequence differential current, which should be compared, are above the setting
this protection can be made very sensitive. Further, an intentional delay of two cycles are added
not to inadvertently operate for some eventual transients. Further, the sensitive negative
sequence differential protection is automatically disabled when the bias current exceeds 1.5 times
the rated current of the protected generator. Operation of this protection is signaled on the
output of the function as TRNSENS.
Desensitization of operate-bias characteristic under external fault condition
The operate-bias characteristic can be temporarily desensitized by applying the adaptive DC
biasing method. When an external fault is detected, the adaptive DC biasing method will
temporarily shift the operate-bias characteristic by adding DC components to the operate level
IdMin . The DC component is extracted online from the instantaneous differential currents and the
highest DC in all three phases is selected to be added to
of a differential function against the CT errors during external faults. The adaptive DC biasing will
be reset if any of the following conditions is fulfilled:
• External fault signal disappears
• Internal fault is issued or differential currents become higher than the bias current
6.3.7.4

Harmonic restrain

Harmonic restrain is the classical restrain method traditionally used with power transformer
differential protections. The goal there was to prevent an unwanted trip command due to
magnetizing inrush currents at switching operations, due to magnetizing currents at over-
voltages, or external faults. Harmonic restrain is just as useful with Generator differential
protection GENPDIF (87G). The harmonic analysis is only executed in those phases, where pickup
signals have been set.
There is no magnetizing inrush to a generator, but there may be some in case of shunt reactors.
The false initial differential currents of a shunt reactor have an appreciable amount of higher
harmonic currents.
At external faults dangerous false differential currents can arise for different reasons, mainly due
to saturation of one or more current transformers. The false differential currents display in this
case a considerable amount of higher harmonics, which can, therefore, be used to prevent an
unwanted trip of a healthy generator or shunt reactor.
If a fault is recognized as external by the internal/external fault discriminator, but nevertheless
one or more pickup signals have been set, the harmonic analysis is initiated in the phases with
pickup signal, as previously described. If all of the instantaneous differential currents, where trip
signals have been set, are free of higher harmonics (that is the cross-block principle is imposed
temporarily), a (minor) internal fault is assumed to have happened simultaneously with a
predominant external one. A trip command is then allowed.
160
The desensitization of operate-bias characteristic by applying the adaptive DC
biasing is disabled when
IdMin . This feature improves the security
NegSeqDiffEn is set to Off .
1MRK 502 066-UUS B
SEMOD155649-80 v2.1.1
IMinNegSeq . Thus,
GUID-2D3A6212-9A5B-4571-9A43-AE2D0EC1533B v1
SEMOD155649-83 v3
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