ABB RELION REX640 Technical Manual page 641

1MRS759142 C
REX640
Technical Manual
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 algorithm does not
eliminate the blocking at inrush currents unless there is a fault in the protected area.
The feature can be enabled and disabled through the Harmonic deblock 2. setting.
Fifth harmonic blocking
The inhibition of the protection relay operation in situations of overexcitation is based
on the ratio of the fifth harmonic to the fundamental component of the differential
current (Id5f / Id1f). The ratio is calculated separately for each phase without
weighting factors. If the ratio exceeds the set value of Start value 5.H and blocking is
enabled through the Restraint mode setting, the operation of the biased stage of the
protection relay in the phase is blocked.
At dangerous levels of overvoltage which may cause damage to the transformer, the
blocking can be automatically eliminated. The blocking is removed if the ratio of the
fifth harmonic to the fundamental component of the differential current exceeds the
set value of the Stop value 5.H setting, and the blocking removal is enabled. The
deblocking feature can be enabled or disabled with the Harmonic deblock 5. setting.
Section 4
Protection functions
635