ABB RELION 620 Series Technical Manual page 521

1MRS757644 F
620 series
Technical Manual
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.
GUID-A97464D1-3085-4F27-B829-11EEC47CA654 V1 EN
Figure 260: The limits and operation of the fifth harmonic blocking when both
blocking and deblocking features are enabled using the Harmonic
deblock 5.H control parameter.
The fifth harmonic blocking has a hysteresis to avoid rapid fluctuation between
"TRUE" and "FALSE". The blocking also has a counter, which counts the required
Section 4
Protection functions
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515