ABB RELION REX640 Technical Manual page 604

Section 4
Protection functions
598
The position value is available through the Monitored data view on LHMI or through
other communication tools in the tap position indication function. When the quality of
the TAP_POS value is not good, the position information in TAP_POS is not used but
the last value with the good quality information is used instead. In addition, the
minimum sensitivity of the biased stage, set by the Low operate value setting, is
automatically desensitized with the total range of the tap position correction. The new
acting low operate value is
Desensitized Low operate value = Low operatevalue ABS MaxWi
GUID-8A2EC8BB-A1EF-4393-BC71-D3EE0D53C083 V1 EN
Second harmonic blocking
The transformer magnetizing inrush currents occur when energizing the transformer
after a period of de-energization. The inrush current can be many times the rated
current and the halving time can be up to several seconds. To the differential
protection, the inrush current represents a differential current, which would cause the
differential protection to operate almost always when the transformer is connected to
the network. Typically, the inrush current contains a large amount of second
harmonics.
Blocking the operation of the TR2PTDF biased low stage at a magnetizing inrush
current is based on the ratio of the amplitudes of the second harmonic digitally filtered
from the differential current and the fundamental frequency (Id2f /Id1f).
The blocking also prevents unwanted operation at the recovery and sympathetic
magnetizing inrushes. At the recovery inrush, the magnetizing current of the
transformer to be protected increases momentarily when the voltage returns to normal
after the clearance of a fault outside the protected area. The sympathetic inrush is
caused by the energization of another transformer running in parallel with the
protected transformer already connected to the network.
The ratio of the second harmonic to a fundamental component can vary considerably
between the phases. Especially when the delta compensation is done for a Ynd1
connected transformer and the two phases of the inrush currents are otherwise equal
but opposite in phase angle, the subtraction of the phases in a delta compensation
results in a very small second harmonic component.
Some measures have to be taken in order to avoid the false tripping of a phase having
too low a ratio of the second harmonic to the fundamental component. One way could
be to always block all the phases when the second harmonic blocking conditions are
fulfilled in at least one phase. The other way is to calculate the weighted ratios of the
second harmonic to the fundamental component for each phase using the original
ratios of the phases. The latter option is used here. The second harmonic ratios
I_2H_RAT_x are given in Monitored data.
The ratio to be used for second harmonic blocking is, therefore, calculated as a
weighted average on the basis of the ratios calculated from the differential currents of
the three phases. The ratio of the concerned phase is of most weight compared to the
ratios of the other two phases. In this protection relay, if the weighting factors are four,
one and one, four is the factor of the phase concerned. The operation of the biased
n n ding tap Minwinding tap Step of tap
(
+ −
1MRS759142 C
⋅ )
(Equation 109)
REX640
Technical Manual