ABB RET670 Applications Manual page 110

Section 3
IED application
104
up transformers in power stations) should be provided with an overexcitation
protection based on V/Hz to achieve a trip before the core thermal limit is reached.
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 Enabled, 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. In should be noted that this is the default
(recommended) setting value for this parameter. When parameter CrossBlockEn is set
to Disabled, any cross blocking between phases will be disabled.
External/Internal fault discriminator
The internal/external 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.
1MRK504116-UUS C
Application manual