Compensated Bank Neutral Voltage Unbalance (Ansi 59Nu) - GE C70 Instruction Manual
Capacitor bank protection and control system
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CHAPTER 9: THEORY OF OPERATION
9.1.4 Compensated bank neutral voltage unbalance (ANSI 59NU)
9.1.4.1 Operating principle
The neutral voltage unbalance function is applicable to ungrounded banks. Fundamentally, this function responds to an
overvoltage condition of the neutral-point voltage. If the capacitor bank and the power system voltages are balanced, the
neutral-point voltage is zero. If a capacitor element in the bank fails, then the bank becomes unbalanced and the neutral
voltage increases.
The operate signal for the neutral voltage unbalance protection is:
V
The restraint signal for the neutral voltage unbalance protection is:
In these equations, k
and k
AB
AC
following figure and are expressed in per-units of the nominal value of the neutral-point VT.
These equations involve phasors, not magnitudes. That is, the vector sum of the voltages is created by the protection
function implementing the method.
The neutral voltage unbalance protection operates when the operate signal is greater than the set pickup level and the
operate signal is greater than the set percentage of the restraint signal, all 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. In addition, a
slope characteristic is used to deal with measurement errors for the involved voltages under large system unbalances,
such as during a close-in external fault.
Figure 9-3: Compensated bank neutral overvoltage application
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
1
=
-- V
(
1 k
+
+
k
) 3V
–
OP
X
AB
AC
0
3
V
=
V
REST
represent capacitor bank unbalance ratio settings. The voltages are as defined in the
C
C
B
C
I
I
B
C
I
+ I
+ I
= 0
A
B
C
+
V
(
1 k
) V
+
(
1 k
)
–
–
B
AB
C
AC
+
V
X
0
C
A
V
I
A
V
X
834745A1.CDR
OVERVIEW
Eq. 9-25
Eq. 9-26
C
B
A
A
9
9-7
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