Supplementary Criteria - ABB Relion Technical Manual

Section 6
Differential protection
6.4.6.3
142

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
nd
components, the 2 , and the 5
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 sequence system, faults can
be distinguished as internal or external, even for three-phase faults.
The internal or external fault discriminator responds to the relative phase angles of the
negative sequence fault currents at both ends of the stator winding. Observe that the source
of the negative sequence currents at unsymmetrical faults is at the fault point.
th
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1MRK 502 048-UUS A
Technical manual