ABB RELION REX640, Техническое руководство, Страница: 722 / 1960

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 can be calculated.
Desensitized Low operatevalue Low operatevalue ABS Max winding
= +
( ttap Minwinding tap Step of tap
− ⋅
)
(Equation 169)
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 operate 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 operation 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, their subtraction in a delta
compensation results in a very small second harmonic component.
Because of the small second harmonic component, some action must 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 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 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 phase has the most weight compared to the ratios of the
other two phases (the weighting factors are 4, 1 and 1, where 4 is the factor of the
phase). The operation of the biased stage on the phase is blocked if the weighted
ratio of that phase is above the set
Start 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 switching of the power transformer onto 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 the different waveform and the different rate of change of
the normal inrush current and the inrush current containing the fault current. The
Protection functions 1MRS759142 F
722 REX640
Technical Manual