The setting NegSeqROA represents the so-called Relay Operate Angle, which
determines the boundary between the internal and external fault regions. It can be
selected in the range from 30 degrees to 90 degrees, with a step of 1 degree. The
default value is 60 degrees. The default setting 60 degrees somewhat favors security in
comparison to dependability. If the user has no well-justified reason for another value,
60 degrees shall be applied.
If the above conditions concerning magnitudes are fulfilled, the internal/external fault
discriminator compares the relative phase angle between the negative sequence current
contributions from the HV side and LV side of the power transformer using the
following two rules :
• If the negative sequence currents contributions from HV and LV sides are in phase
or at least in the internal fault region, the fault is internal.
• If the negative sequence currents contributions from HV and LV sides are 180
degrees out of phase or at least in the external fault region, the fault is external.
Under external fault condition and with no current transformer saturation, the relative
angle is theoretically equal to 180 degrees. During internal fault and with no current
transformer saturation, the angle shall ideally be 0 degrees, but due to possible
different negative sequence source impedance angles on HV and LV side of power
transformer, it may differ somewhat from the ideal zero value.
The internal/external fault discriminator has proved to be very reliable. It has been
given a great power. If a fault is detected,that is, PICKUP signals set by ordinary
differential protection, and at the same time the internal/external fault discriminator
characterizes this fault as an internal, any eventual blocking signals produced by either
the harmonic or the waveform restraints are ignored.
If the bias current is more than 110% of IBase, the negative sequence threshold
(IMinNegSeq) is increased internally.. This assures response times of the new and
advanced of the differential protection below one power system cycle (below 16.66mS
for 60 Hz system) for all more severe internal faults. Even for heavy internal faults
with severely saturated current transformers this new differential protection operate
well below one cycle, since the harmonic distortions in the differential currents do not
slow down the differential protection operation. Practically, an unrestrained operation
is achieved for all internal faults.
External faults happen ten to hundred times more often than internal ones as far as the
power transformers are concerned. If a disturbance is detected and the internal/external
fault discriminator characterizes this fault as an external fault, the conventional
additional criteria are posed on the differential algorithm before its trip is allowed. This
assures high algorithmstability during external faults. However, at the same time the
differential function is still capable of trip ping quickly for evolving faults.
1MRK504116-UUS C Section 3
IED application
105
Application manual
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