GE C70 Instruction Manual page 447

8 THEORY OF OPERATION
V V V
A B C
C
834748A1.CDR
Figure 8–6: NEUTRAL CURRENT UNBALANCE APPLICATION
The neutral current unbalance protection uses the following operate signal.
The neutral current unbalance protection uses the following restraint signal.
In the above equations, k represents the vector capacitor bank positive sequence inherent unbalance factor setting, and
1
k * represents the complex conjugate of k
1
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:
GE Multilin
I
A
I
B
I
C
I I
A1 B1
C C
A1 B1 C1
I
N1
I
N1
I I
= –
OP DIF
I
REST
. The currents are as defined in the figure above, and are in per-unit values of
1
C70 Capacitor Bank Protection and Control System
I I I
C1
A2 B2
C C
A2 B2
I
N2
I
DIF
window CT
I
N2
I
DIF
V
X low ratio CT
*
k I k I
–
1 1 1 2
I
=
0
8.1 OVERVIEW
I
C2
C
C2
grounded
configuration
ungrounded
configuration
(EQ 8.54)
(EQ 8.55)
8-13
8