Compensated Bank Neutral Voltage Unbalance (Ansi 59Nu) - GE C70 Instruction Manual

8.1 OVERVIEW
In practice, k is no less than 2.0 as the tap is no more than half way up the phase string. Comparing equation 8.16 with
A
equation 8.24, it can be seen that V
duces a operating signal no smaller than the same failure in the upper sub-string. Thus V
case sensitivity. A failure resulting in a 0.01 pu capacitance change in the leg capacitance results in an operating signal of
at least 0.01 pu of bus phase-to-ground voltage.
d) AUTO-SETTING
While a capacitor bank may be designed to have a tap at say the mid-point or the one-third point, manufacturing tolerances
result in the actual tap ratio being slightly different from the design target. To prevent a spurious component in the operating
signal, the match factor settings must correspond to the actual rather than the design tap ratio. As a convenient alternative
to manually determining the optimum match factor settings, the relay can automatically calculate these settings from its
own measurements while the capacitor is in-service, as described in the Commands chapter. The C70 sets the operate sig-
nal to zero in equation 8.7 or 8.8 and solves for the match factor k
surements. This technique has the further advantage that it to a large degree compensates for instrumentation error.
However, the assumption made here is that when the auto-set command is executed, the capacitor is in an acceptably bal-
anced state, wherein the operating signal ought to be zero. Following the auto-set command, the protection will be measur-
ing changes from the state that existed at the time the auto-set command executed.
a) 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. Should a capacitor element in the bank fail, the bank will become unbalanced and the neutral
voltage will increase.
The operate signal for the neutral voltage unbalance protection is:
The restraint signal for the neutral voltage unbalance protection is:
In the above equations, k
figure below, 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 func-
tion 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
8 slope characteristic is used to deal with measuring errors for the involved voltages under large system unbalances such as
during a close-in external fault.
8-6
(pu) ≥ V
OP(2A) OP(1A)

8.1.4 COMPENSATED BANK NEUTRAL VOLTAGE UNBALANCE (ANSI 59NU)

1
-- - V
V ( 1 k k
= + +
OP X AB AC
3
V
REST
and k represent capacitor bank unbalance ratio settings. The voltages are as defined by the
AB AC
C70 Capacitor Bank Protection and Control System
(pu), and thus that an element failure in the lower substring pro-
using the average of several successive voltage mea-
A
) 3V V ( 1 k ) V ( 1 k
– + – +
0 B AB C
V V
= +
X 0
8 THEORY OF OPERATION
(pu) represents the worse
OP(1A)
)
–
AC
GE Multilin
(EQ 8.25)
(EQ 8.26)