ABB Relion 620 Series, Technical Manual

Page: 784 / 1224

Figure 412: Double-Y-connected capacitor bank
The phase angle of the measured fundamental frequency component of the
unbalance current I_UNB is synchronized by using the phase current I_A as a
reference.
∠ = ∠ − ∠
I I UNB I A
unb
_ _
(Equation 147)
In a three-phase star-connected capacitor bank circuit, there may be some amount
of natural unbalance current flowing through the neutral, which is primarily due
to capacitor manufacturing tolerances. The natural unbalance current must be
compensated for before using the measured unbalance current for the 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 > CUBPTOC. By selecting Record
unbalance with value “Record”, the measured unbalance current
I
Unb
is considered
as the natural unbalance current
I
NatUnb
and is stored as a reference. The amplitude
and angle of the recorded natural unbalance current I_AMPL_NAT and I_ANGL_NAT
are available in the monitored data view.
Once the natural unbalance current is recorded during further executions of the
function, the natural unbalance current is subtracted from the measured unbalance
current
I
Unb
to obtain the compensated unbalance current
I
CompUnb
as shown in
Figure 413
.
Protection functions 1MRS757644 H
784 620 series
Technical Manual