Supplementary Criteria - ABB REG650 Technical Manual

Section 6
Differential protection
6.3.6.3
124
This DC desensitization is not active, if a disturbance has been detected
and characterized as internal fault.

Supplementary criteria

To relieve the burden of constructing an exact optimal operate-restrain characteristic,
two special features supplement the basic stabilized differential protection function,
making Generator differential protection GENPDIF (87G) a very reliable one.
The supplementary criteria are:
• Internal/external fault discriminator (enhances, or blocks, the trip command)
• Harmonic restrain (blocks only)
The internal/external fault discriminator is a very reliable supplementary criterion. It
discriminates with a high speed between internal and external faults. The discriminator
is the main part of what is here called the negative-sequence-current-based differential
protections. It is recommended that this feature is always used (that is, enabled,
OpNegSeqDiff = On).
If a fault is classified as internal, then any eventual block signals by the harmonic
criterion are ignored, and the differential protection can operate very quickly without
any further delay.
If a fault (disturbance) is classified as external, then generally, but not unconditionally,
a trip command is prevented. If a fault is classified as external, harmonic analysis of
the fault conditions is initiated.
If all the differential currents which caused their respective pickup signals to be set, are
free of harmonic pollution, that is, if no harmonic block signal has been set, then a
(minor) internal fault, simultaneous with a predominant external fault, can be
suspected. This conclusion can be drawn because at external faults, major false
differential currents can only exist when one or more current transformers saturate
transiently. In this case, the false instantaneous differential currents are highly polluted
by higher harmonic components, the 2
The existence of relatively high negative-sequence currents is in itself an indication of
a disturbance, as the negative-sequence currents are superimposed, pure-fault
quantities. The negative-sequence currents are measurable indications of abnormal
conditions. The negative sequence currents are particularly suitable for directional
tests. The negative sequence internal or external fault discriminator works satisfactorily
even in case of three-phase faults. Because of the fundamental frequency components
(50/60 Hz) of the decaying DC offset of the fault currents, the system is not fully
symmetrical immediately after the fault. Due to the transient existence of the negative
nd th
, and the 5 .
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Technical Manual