ABB Relion REG670 Technical Manual Download

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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. 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.

6.3.7.5

Open CT detection feature

Transformer differential protection has a built-in, advanced open CT detection
feature.

A sudden inadvertently opened CT circuit may cause an unexpected and unwanted
operation of the Transformer differential protection under normal load conditions.
Damage of secondary equipment may occur due to high voltage from open CT circuit
outputs. It is always an advantage, from the point of view of security and reliability,
to have the open CT detection function to block the transformer differential protection
function in case of an open CT condition, and produce an alarm signal to the
operational personnel to quickly correct the open CT condition.

The built-in open CT feature can be enabled or disabled by the setting parameter

OpenCTEnable

(

Off

). When enabled, this feature tries to prevent mal-operation

when a loaded main CT connected to Transformer differential protection is by mistake
open circuited on the secondary side. Note that this feature can only detect interruption
of one CT phase current at a time. If two or even all three-phase currents of one set of
CTs are accidentally interrupted at precisely the same time, this feature cannot
operate. Transformer differential protection generates a trip signal if the false
differential current is sufficiently high. An open CT circuit is typically detected in 12–
14 ms, and if the load in the protected circuit is relatively high, about the nominal load,

ABB Relion REG670, Technical Manual

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