CHAPTER 9: THEORY OF OPERATION OVERVIEWC70 CAPACITOR BANK PROTECTION AND CONTROL SYSTEM – INSTRUCTION MANUAL 9-799.1.4 Compensated bank neutral voltage unbalance (ANSI 59NU)9.1.4.1 Operating principleThe neutral voltage unbalance function is applicable to ungrounded banks. Fundamentally, this function responds to anovervoltage condition of the neutral-point voltage. If the capacitor bank and the power system voltages are balanced, theneutral-point voltage is zero. If a capacitor element in the bank fails, then the bank becomes unbalanced and the neutralvoltage increases.The operate signal for the neutral voltage unbalance protection is:Eq. 9-25The restraint signal for the neutral voltage unbalance protection is:Eq. 9-26In these equations, kAB and k AC represent capacitor bank unbalance ratio settings. The voltages are as defined in thefollowing 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 protectionfunction implementing the method.The neutral voltage unbalance protection operates when the operate signal is greater than the set pickup level and theoperate 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 thestandard hysteresis approach to deal with chattering of the operating signal at the boundary of operation. In addition, aslope 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 applicationV OP13-- V X 1 k AB k AC+ +( ) 3V 0– V B 1 k AB–( ) V C 1 k AC–( )+ +=V REST V X V 0+=834745A1.CDRVA - VXVAABCVXIC I B I AIA + IB + IC = 0CC CB CA
