Cross-Blocking Between Phases; External/Internal Fault Discriminator - ABB RET650 Applications Manual

1MRK 504 169-UEN A
6.1.3.5
6.1.3.6
Transformer protection RET650 2.2 IEC
Application manual

Cross-blocking between phases

Basic definition of the cross-blocking is that one of the three phases can block
operation (that is, tripping) of the other two phases due to the harmonic pollution of
the differential current in that phase (waveform, 2nd or 5th harmonic content). In
the algorithm the user can control the cross-blocking between the phases via the
setting parameter CrossBlockEn. When parameter CrossBlockEn is set to On, cross
blocking between phases will be introduced. There are no time related settings
involved, but the phase with the operating point above the set bias characteristic
will be able to cross-block the other two phases if it is self-blocked by any of the
previously explained restrained criteria. As soon as the operating point for this
phase is below the set bias characteristic cross blocking from that phase will be
inhibited. In this way cross-blocking of the temporary nature is achieved. It should
be noted that this is the default (recommended) setting value for this parameter.
When parameter CrossBlockEn is set to Off, any cross blocking between phases
will be disabled.

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 (IMinNegSeq) is settable in the 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 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
Section 6
Differential protection
M15266-269 v10
M15266-317 v7
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