ABB RELION REX640 Technical Manual page 605

1MRS759142 C
REX640
Technical Manual
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.
If the peak value of the differential current is very high, that is I
the second harmonic blocking is desensitized (in the phase in question) by increasing
it proportionally to the peak value of the differential current.
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 a different waveform and a different rate of change of the normal inrush
current and the inrush current containing the fault current. The algorithm does not
eliminate the blocking at inrush currents, unless there is a fault in the protected area.
The feature can also be enabled and disabled with the Harmonic deblock 2.H
parameter.
Fifth harmonic blocking
The inhibition of TR2PTDF operation in the situations of overexcitation is based on
the ratio of the fifth harmonic and the fundamental component of the differential
current (Id5f/Id1f). The ratio is calculated separately for each phase without
weighting . If the ratio exceeds the setting value of Start value 5.H and if blocking is
enabled through the Restraint mode parameter, the operation of the biased stage of
TR2PTDF in the concerned phase is blocked. The fifth harmonic ratios I_5H_RAT_x
are given in Monitored data.
At dangerous levels of overvoltage, which can cause damage to the transformer, the
blocking can be automatically eliminated. If the ratio of the fifth harmonic and the
fundamental component of the differential current exceeds the Stop value 5.H
parameter, the blocking removal is enabled. The enabling and disabling of deblocking
feature is also done through the Harmonic deblock 5.H parameter.
Section 4
Protection functions
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