External/Internal Fault Discriminator - ABB RELION 650 Series Applications Manual

Section 6
Differential protection
6.1.2.6

External/Internal fault discriminator

The external/internal fault discriminator operation is based on the relative position of the two
phasors (in case of a two-winding transformer) representing the W1 and W2 negative
sequence current contributions, defined by matrix expression see the technical reference
manual. It practically performs a directional comparison between these two phasors.
In order to perform a directional comparison of the two phasors their magnitudes must be
high enough so that one can be sure that they are due to a fault. On the other hand, in order to
guarantee a good sensitivity of the internal/external fault discriminator, the value of this
minimum limit must not be too high. Therefore this limit value (
range from 1% to 20% of the differential protections IBasecurrent, which is in our case the
power transformer HV side rated current. The default value is 4%. Only if the magnitude of
both negative sequence current contributions are above the set limit, the relative position
between these two phasors is checked. If either of the negative sequence current
contributions, which should be compared, is too small (less than the set value for
IMinNegSeq ), no directional comparison is made in order to avoid the possibility to produce a
wrong decision.
This magnitude check, guarantees stability of the algorithm when the power transformer is
energized. In cases where the protected transformer can be energized with a load connected
on the LV side (e.g. a step-up transformer in a power station with directly connected auxiliary
transformer on its LV side) the value for this setting shall be increased to at least 12%. This is
necessary in order to prevent unwanted operation due to LV side currents during the
transformer inrush.
The setting
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. If a fault is detected,
that is, START 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 (
is increased internally.. This assures response times of the differential protection below one
power system cycle (below 20 ms for 50 Hz system) for all more severe internal faults. Even for
heavy internal faults with severely saturated current transformers this differential protection
operates 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.
68
NegSeqROA represents the so-called Relay Operate Angle, which determines the
1MRK 504 158-UEN A
IMinNegSeq ) is settable in the
IMinNegSeq )
Application manual
M15266-269 v10