Harmonic Restrain; Cross-Block Logic Scheme; Simplified Block Diagrams - ABB REG650 Technical Manual

1MRK 502 034-UEN -
5.3.6.4
5.3.6.5
5.3.6.6
Technical Manual

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. The harmonic analysis
is only executed in those phases, where start 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 start signals have been set, the harmonic analysis is
initiated in the phases with start 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 start signal set (start = 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.

Simplified block diagrams

The principle design of the generator differential protection is shown in figure 53.
Section 5
Differential protection
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