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

CHAPTER 9: THEORY OF OPERATION
9.1.3.4 Auto-setting
While a capacitor bank can 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 signal to zero in equation 9.7 or 9.8 and solves for the match factor k
successive voltage measurements. 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 balanced state, wherein the operating signal ought to be zero. Following the auto-set command, the
protection measures changes from the state that existed at the time the auto-set command executed.

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:
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.
C70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL
represent capacitor bank unbalance ratio settings. The voltages are as defined in the
OVERVIEW
using the average of several
A
Eq. 9-25
Eq. 9-26
9
9-7