Achieving optimum capacitor bank protection and control

TVA applies shunt capacitor banks to their 161 kV system to regulate the local substation bus voltage over a range of light to heavy load and load switching conditions. The substation capacitor bank configuration may consist of up to 6 separately switched capacitor stacks. The entire substation bank is switched with a circuit breaker. The voltage level is monitored and used to switch in and out capacitor stacks as required for correct VAR compensation. The bus voltage is regulated to operate within a defined bandwidth around nominal voltage. There are two operating band widths that are used depending on load conditions, narrow band (NB) and wide band (WB). The number of capacitor bank stacks and bandwidths are determined by System Operations and Planning. Capacitor stack switching is either done manually or automatically. Automatic control must address several issues. Some of these are hunting - trying to find a stable voltage within an operating bandwidth, bandwidth operation transfer, auto to manual transfer and balanced switching operations of up to six capacitor bank capacitor switches over the life of the bank.