ABB RELION 620 Series Technical Manual page 520

Section 4
Protection functions
514
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
stage on the concerned phase is blocked if the weighted ratio of that phase is above the
set blocking limit Start value 2.H and if blocking is enabled through the Restraint
mode parameter.
Using separate blocking for the individual phases and weighted averages calculated
for the separate phases provides a blocking scheme that is stable at the connection
inrush currents.
1MRS757644 F
620 series
Technical Manual