Capacitor Precautions

incremental switching, depending on the system reactive requirements and the amount of system voltage required. To remove a switched capacitor bank from service, you should open the control box and the automatic control lever. or control switch, should be placed in the OFF position. The circuit breaker or the switching device should then be tripped. To ensure the circuit breaker or switching device remains open, you should remove the fuses from the control circuit. Before it can be assumed that the capacitor bank has been de-energized, the position of the switching device should be inspected. On a circuit breaker, the position indicator should be checked. For oil switches, the position of the operating handle can be checked with a switch stick. CAPACITOR PRECAUTIONS.—Capacitors and transformers are entirely different in their operation. When a transformer is disconnected from the line, it is electrically dead. Unlike the transformer and other devices, the capacitor is not dead immediately after it is disconnected from the line. It has the peculiar property of holding its charge because it is essentially a device for storing electrical energy. It can hold this charge for a considerable length of time. There is a voltage difference across its terminals after the switch is opened. Capacitors for use on electrical lines, however, are equipped with an internal-discharge resistor. This resistor. connected across the capacitor terminals, will gradually discharge the capacitor and reduce the voltage across its terminals. After 5 minutes, the capacitor should be discharged. To be perfectly safe, however, proceed as follows: Before working on a disconnected capacitor, wait 5 minutes. Then test the capacitor with a high-voltage tester rated for the circuit voltage. If the voltage is zero, short-circuit the terminals externally using hot-line tools and ground the terminals to the case. Now you can proceed with the work. COUPLING CAPACITORS.—Communication signals in the form of high-frequency voltages are transmitted to the transmission lines through coupling capacitors. Some of the coupling capacitors are equipped with potential devices that make it possible to measure the voltage on transmission line circuits. The coupling-capacitor potential devices are accurate enough to be used for supplying voltage to protective relays but, unless they are specifically compensated, not accurate enough to supply voltage for meters designed for billing purposes. Figure 4-28 shows a coupling capacitor. Figure 4-28.—Coupling capacitor. TYPES OF SWITCHING DEVICES.—Switch- ing capacitors imposes severe duty on switching devices because of the differences in phase relationship between the current and voltage on a capacitor circuit. When a capacitor bank is energized. high transient overvoltages and high-frequency transient inrush currents may be produced. The magnitude of the transient overvoltages easily may be three times the rated line voltage, and transient inrush currents may approach the short-circuit current duty values. These factors are especially important when one or more capacitor banks already is energized and another one at the same location is switched on to the bus. The methods for determining the values of inrush current, transient overvoltage, and resonant frequency of the circuit are discussed in more detail in ANSI C37.99. IEEE Guide for Protection of Shunt Capacitor Banks. and ANSI C37.012, Application Guide for Capacitance Current Switching of AC High- Voltage 4-18