ABB REG670 Applications Manual page 140

Section 7
Differential protection
134
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:
• If any comparison indicates an internal fault, then it is an internal fault.
• If any comparison indicates an external fault, then it is an external fault
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
1MRK 502 071-UEN -
Application manual