GE C70 Instruction Manual page 427

8 THEORY OF OPERATION
Figure 8–3: COMPENSATED BANK NEUTRAL OVERVOLTAGE APPLICATION
b) BALANCED CASE
To understand neutral voltage unbalance protection, we can start with Kirchhoff's current law for the neutral node of the
bank:
I I
+
A
This expression can be rearranged as follows.
 
V C
–
X A
 
V
–
X
Multiplying both sides by –1 / C
As the left hand side of this equation is equal to zero, we can subtract it from the expression inside the absolute value
brackets in equation 8.25 to get:
1

V -- - V 1 k k
= + +
OP X AB
3
C
1

B
-- - V ------ -
k
= – +

X AB
3 C
A
The unbalance ratio k-values that reflect the initial or inherent bank unbalance are chosen as:
Therefore, as seen from the previous two equations, the initial operating signal will be zero.
GE Multilin
C C
B
C
I I
B
C
I + I + I = 0
A B C
 
I jC V V jC
+ = – +
B C A A X
 
V C C C V
– + + +
X A B C A

C C V C V C
+ + + + +
B C A A B A
 
C C C C V V
+ + + +
A B C A A B
and substituting the sum of the phase voltages with 3V
A
C C
+
 
B C
V 1 -------------------- - 3V V
+ – +
 
X 0 B
C
A
 3V  
V 1 k V
– + – +
AC 0 B AB C
C C
  
C B
------- ------ - k
k V
– + – +
  
AC B AB
C C
A A
C
B
------ - , k
k
=
AB
C
A
C70 Capacitor Bank Protection and Control System
C
B
A
C
A
V
A
I
A
V
X
834745A1.CDR
  
V V jC V V
– + –
B B X C C X
C V C V C 0
+ + =
A B B C C
V C V C V C V C
+ – +
C A B B B A C
   
V V C C V C
+ + – +
C B B A C C
C C
   
B C
1 ------ - V 1 ------- 0
– + – =
   
C
C C
A A
C C
+
 
B C
  V
1 k 1 -------------------- -
– – + +
 
AC X
C
A
C
 
C
------- k
V
–
 
C AC
C
A
C
C
-------
=
AC
C
A
8.1 OVERVIEW

0
(EQ 8.27)
=
V C 0
– = (EQ 8.28)
C C A

C 0
– =
A
yields:
0
(EQ 8.29)
C C
  V 
B C
3V V 1 ------ - 1 -------
– – – –
  
0 B C
C C
A A
(EQ 8.30)
(EQ 8.31)
8


8-7