Branch 1
Branch 3
Branch 2
Branch 4
I_UNB_A
I_UNB_B I_UNB_C
Figure 634: H-Bridge-connected capacitor bank
The phase angles of three-phase measured unbalance currents are synchronized
using the phase current I_A as reference. The phase-A unbalance current phase
angle can be calculated with the equation.
∠ = ∠ − ∠I I_UNB_A I_A
Unb_A
(Equation 287)
A three-phase H-bridge-connected capacitor bank circuit may have some amount of
natural unbalance current flowing through the common points of H-bridge, which
may primarily be due to capacitor manufacturing tolerances. Therefore, the natural
unbalance current must be compensated for before using the measured unbalance
current for function operation.
The natural unbalance current needs to be recorded when there is no fault in
the capacitor banks, and it is initiated through the command
Record unbalance
available under menu path Control> HCUBPTOC. The natural unbalance currents
are recorded for all the phases at the same time by setting command
Record
unbalance
to “Record all phases” or each natural unbalance current can be
recorded separately for individual phases by setting command
Record unbalance
to “Record phase A” or “Record phase B” or “Record phase C”. By selecting
Record
unbalance
with value “Record”, the measured three-phase unbalance currents
(
I I I
Unb_A Unb_B Unb_C
, , ,
) are considered the natural unbalance currents for each
phase respectively and stored as a reference. The amplitude and angle of the
phasewise-recorded natural unbalance currents I_NAT_AMPL_A, I_NAT_ANGL_A,
I_NAT_AMPL_B, I_NAT_ANGL_B, I_NAT_AMPL_C and I_NAT_ANGL_C are available in
the monitored data view.
Once a natural unbalance current is recorded, the phasewise natural unbalance
current is subtracted from the corresponding phase-measured unbalance current
(
) during further executions of the function to obtain the compensated
unbalance current (
) .
1MRS759142 F
Protection functions
REX640
Technical Manual
1133
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