Harmonic Restrain - ABB Relion Technical Manual

1MRK 502 048-UUS A
6.4.6.4
6.4.6.5
Technical manual
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.

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.
Cross-block logic scheme
The cross-block logic says that in order to issue a common trip command, the harmonic
contents in all phases with a pickup signal set (pickup = TRUE) must be below the limit
defined with the setting HarmDistLimit. In the opposite case, no trip command will be
issued.
The cross-block logic is active if the setting OpCrossBlock = Yes. By always using the
cross-block logic, the false trips can be prevented for external faults in cases where the
internal or external fault discriminator should for some reason fail to declare an external
fault. For internal faults, the higher frequency components of an instantaneous differential
current are most often relatively low, compared to the fundamental frequency component.
While for an external (heavy) fault, they can be relatively high. For external faults with
moderate fault currents, there can be little or no current transformer saturation and only
small false differential currents.
Section 6
Differential protection
145