ABB RED615 Technical Manual page 396

Section 4
Protection functions
390
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 IED, 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.
1YHT530004D05 D
615 series
Technical Manual