ABB 615 series Technical Manual page 513

1MAC059074-MB A
615 series ANSI
Technical Manual
phases. The ratio of the concerned phase is of most weight compared to the ratios of the
other two phases. In this protection relay, 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 Pickup 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
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 87T 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 Pickup value 5.H and if blocking is enabled through the Restraint mode
parameter, the operation of the biased stage of 87T 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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