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.
5 th harmonic block
Start value 5. H
Stop value 5. H
hysteresis
hysteresis
t
harmonic content / % of fundamental
1
0
Figure 409: The fifth harmonic blocking limits and the operation when both
blocking and deblocking features are enabled through the Harmonic deblock 5.
setting
Fifth harmonic blocking has a hysteresis to avoid rapid fluctuation between TRUE
and FALSE. The required consecutive fulfillments of the condition can be counted
using the blocking counter. When the condition is not fulfilled, the counter is
decreased (if >0).
Also fifth harmonic deblocking (based on the higher limit,
Stop value 5.H
, if enabled
with
Harmonic deblock 5.
) has a hysteresis and a counter which counts the required
consecutive fulfillments of the condition. When the condition is not fulfilled, the
counter is decreased (if >0).
Waveform blocking
The biased low stage can always be blocked with waveform blocking; it cannot
be disabled with
Restraint mode
. This algorithm has two parts. The first part is
intended for external faults while the second is intended for inrush situations. The
algorithm has criteria for the low-current periods of differential current during the
inrush and external fault.
1MRS759142 F
Protection functions
REX640
Technical Manual
723
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