ABB ret650, Applications Manual

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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
(
IMinNegSeq ) 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.
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 algorithm stability during external faults.
However, at the same time the differential function is still capable of tripping
quickly for evolving faults.
The principle of the internal/external fault discriminator can be extended to
autotransformers and transformers with three windings. If all three windings are
connected to their respective networks then three directional comparisons are
made, but only two comparisons are necessary in order to positively determine the
position of the fault with respect to the protected zone. The directional comparisons
which are possible, are: W1 - (W2+W3) and W2 - (W1+W3). The rule applied by
the internal/external fault discriminator in case of three-winding power
transformers is:
Section 6
1MRK 504 169-UEN A
Differential protection
82 Transformer protection RET650 2.2 IEC
Application manual