ABB RELION 620 Series Technical Manual page 434

Section 4
Protection functions
428
Second harmonic blocking
Transformer-magnetizing inrush currents occur when the transformer is energized after a
period of de-energization. The inrush current may be many times the rated current, and the
half-life can be up to several seconds. To the differential protection relay, the inrush
current represents a differential current that causes the protection relay to trip almost
always when the transformer is connected to the network. Typically, the inrush current
contains a large amount of second harmonics.
The blocking of the biased low stage of the protection relay at magnetizing inrush current
is based on the ratio of the amplitudes of the second harmonic to the fundamental
frequency component of the differential current (Id2f / Id1f).
The blocking also prevents an unwanted trip at recovery and a sympathetic magnetizing
inrush. At 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 the 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 them in a delta compensation results in a very
small second harmonic component.
Because of the small second harmonic component, some action needs to be taken to avoid
false tripping of the phase having a too low ratio of the second harmonic to the
fundamental component. One way could be to block all phases always when the second
harmonic blocking conditions are fulfilled at least in one phase. The other way is to
calculate weighted ratios of the second harmonic to the fundamental for each phase using
the original ratios of the phases. The latter option is used here.
The ratio to be used for the second harmonic blocking is 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 the most weight compared to the ratios of the other two
phases (weighting factors are four, one and one, where four is the factor of the phase
concerned). The trip of the restrained differential on the concerned phase is blocked if the
weighted ratio of that phase is above the set Pickup value 2.H blocking limit and if the
blocking is enabled through the Restraint mode setting.
Using separate blocking for the individual phases and weighted averages calculated for
the separate phases provides a stable blocking scheme at connection inrush currents.
The connection of the power transformer against a fault inside the protected area does not
delay the operation of the tripping, because in such a situation the blocking based on the
second harmonic of the differential current is prevented by a separate algorithm based on
1MAC504801-IB E
620 series ANSI
Technical Manual