6.3.7.3 Supplementary criteria
SEMOD155649-53 v2.1.1
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. Meanwhile, the operate-bias characteristic can also be temporarily desensitized by
applying the adaptive DC biasing method.
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
nd
, and the 5
th
.
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.
• If the two negative sequence currents, as seen by the differential relay, flow in the same
direction (that is with the CTs oriented as in figure
51), the fault is internal. If the two negative
sequence currents flow in opposite directions, the fault is external.
• Under external fault condition, the relative angle is theoretically equal to 180°. Under internal
fault condition, the angle is ideally 0°, but due to possible different negative-sequence
impedance angles on both sides of the internal fault, it may differ somewhat from 0°.
The setting
NegSeqROA
, as shown in figure 56, represents the so called Relay Operate Angle,
which determines the boundary between the internal and external fault regions. It can be selected
Section 6 1MRK 502 066-UUS B
Differential protection
158
Technical manual
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