ABB REG650 ANSI, Техническое руководство

Страница: 140 / 728

6.4.6.4 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. 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.
6.4.6.5 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.
6.4.6.6 Simplified block diagrams
The principle design of the generator differential protection is shown in figure 56 .
Section 6
1MRK 502 048-UEN A
Differential protection
134
Technical manual