ABB RELION REX640 Technical Manual page 1121

1MRS759142 F
Figure 628: Functional module diagram
Natural unbalance Compensation
A standard double-Y-connected shunt capacitor bank configuration is shown in
Figure 629
measured on the common neutral connecting the two balanced parts of a shunt
capacitor bank, that is, between star point 1 and star point 2.
Figure 629: 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
unb
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
REX640
Technical Manual
I_A
I3P
I_UNB
BLOCK
. The fundamental frequency component of an unbalance current is
_ _
= ∠ I UNB − ∠ I A
Natural
Level
unbalance
compens-
detector 1
ation
Blocking
logic
Level
detector 2
Protection functions
Timer
OPERATE
t
t
START
Alarm
ALARM
control
(Equation 285)
1121