GE C70, Instruction Manual

Page: 447 / 708

GE Multilin C70 Capacitor Bank Protection and Control System 8-13
8 THEORY OF OPERATION 8.1 OVERVIEW
8
Figure 8–6: NEUTRAL CURRENT UNBALANCE APPLICATION
The neutral current unbalance protection uses the following operate signal.
(EQ 8.54)
The neutral current unbalance protection uses the following restraint signal.
(EQ 8.55)
In the above equations, k
1
represents the vector capacitor bank positive sequence inherent unbalance factor setting, and
k
1
* represents the complex conjugate of k
1
. The currents are as defined in the figure above, and are in per-unit values of
the nominal current of the bank source ground current channel.
These equations involve phasors, not magnitudes; that is, the vector sum of the currents is created by the protection func-
tion implementing the method. The protection operates when the operate signal is greater than the set pickup level for the
set pickup delay.
Sensitivity is the key performance parameter. The applied comparator uses a simple integration method in addition to the
standard hysteresis approach, to deal with chattering of the operating signal at the boundary of operation.
b) BALANCED CASE
The currents in the neutral current unbalance protection are driven by the individual admittances in each string:
834748A1.CDR
I
C1
I
A
I
DIF
window CT
I
N1
C
C1
I
B1
C
B1
I
A1
C
A1
I
C2
I
N2
C
C2
I
B2
C
B2
I
A2
C
A2
I
B
I
C
V
C
V
A
V
B
grounded
configuration
I
DIF
low ratio CT
I
N1
I
N2
V
X
ungrounded
configuration
I
OP
I
DIF
k
1
I
1
k
1
*
I
2
– – =
I
REST
I
0
=