ABB RELION REX640, Техническое руководство, Страница: 687 / 1960

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 a 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 the phases in a
delta compensation results in a very small second harmonic component.
Some measures have to be taken in order to avoid the false tripping of a phase
having too low a ratio of the second harmonic to the fundamental component. One
way could be to always block all the phases when the second harmonic blocking
conditions are fulfilled in at least one phase. The other way is to calculate the
weighted ratios of the second harmonic to the fundamental component for each
phase using the original ratios of the phases. The latter option is used here. The
second harmonic ratios I_2H_RAT_x are given in Monitored data.
The ratio to be used for second harmonic blocking is, therefore, 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 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
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
r
>12×In, the limit
for 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
1MRS759142 F Protection functions
REX640
Technical Manual
687