Circuit Breakers Rated on a Symmetrical Current Basis.
Switching devices, as discussed below, have a separate capacitive switching rating for the reasons mentioned above; and the switching rating of the device must be at least 135 percent of the capacitor bank rating to which the switching device is connected. This 135 percent rating is a minimum specified by the National Electrical Code(NEC)® and includes allowance for operation at overvoltage, allowance for capacitance manufacturing tolerance, and allowance for harmonic components above the fundamental frequency.
CONSTRUCTION. - A capacitor unit consists of two aluminum foil strips, or plates, with a thin high- grade insulating paper or a synthetic film placed between them. The strips, or plates, are compactly wound and connected in groups, each of which is connected to a terminal. There is no contact between the two metal surfaces. When these two surfaces are connected to a source of power, energy is stored in the capacitor. The capacitor remains charged at, or above: full-line voltage when disconnected from the source of power until a discharge path is provided between the terminals. Capacitors have a built-in discharge resistor designed to drain off or reduce this residual charge. NEC® requires capacitors rated 600 volts or more to be discharged to a residual voltage of 50 volts or less in 5 minutes. Since the built-in resistor has the disadvantage that it cannot be visually inspected for an open circuit, it should not be relied upon for positive drain off of the residual charge. The wound plates and discharge resistor of a capacitor are enclosed in a welded sheet steel or stainless steel container, which is hermetically sealed to protect the capacitor from deterioration due to entrance of foreign material or moisture. The contents are vacuum dried and are usually impregnated with a dielectric fluid. As of 1 October 1977, dielectric fluids containing polychlorinated biphenyls (PCBS) can no longer be installed. The connecting leads from the capacitor are brought up through the bushings to a joint at the top directly under the brazed terminal. The bushings supplied on capacitors are usually made of porcelain. As of 1 October 1988, existing PCB capacitors in unrestricted areas must be removed.
TYPES OF INSTALLATIONS. - The greatest electrical benefits are derived from capacitors connected directly at the loads. This connection permits maximum loss reduction and released line capacity. However, economics and physical limitations are usually the governing factors. Capacitors may be divided into two classes: primary capacitors and secondary capacitors. Primary capacitors are those rated 2,400 volts and above. Secondary capacitors are those used on the low- voltage side of distribution transformers or at motor terminals and are normally rated 600 volts and below. The three most common types of power capacitor installation are as follows: pole mounted, metal enclosed, and open rack.
Pole Mounted. - Pole-mounted capacitors are packaged as a complete unit containing all necessary items for a switched distribution capacitor bank installation. The banks consist of an aluminum- or steel-mounting frame that supports the capacitor units, interconnecting wiring, and capacitor switches. Overcurrent, protection is usually provided by group fuses.
Metal Enclosed. - Metal-enclosed capacitor banks consist of a factory-assembled group of individual capacitor units mounted in a protective housing complete with bus connections, controls, and protective and switching equipment within the enclosure. Personnel safety and compactness are the major benefits. Each capacitor unit normally is protected by an individual current-limiting fuse.
Open Rack. - An open-rack capacitor installation (fig. 4-29) is a field-assembled group of capacitor units mounted in an open-rack structure without enclosing plates or screens. Open-rack installations normally are made up of several stack type of capacitors connected in parallel to provide desired kvar capacity. All the units in a given stacking unit are normally connected in parallel with the steel frame forming one terminal and the insulated bus forming the other. For open-rack installations the capacitor units are protected by individual fuses, group fuses or relays, and a circuit breaker.
CAPACITOR MAINTENANCE. - All switched capacitor banks should be inspected and checked for proper operation once each year before the time period when they are automatically switched on and off to meet system requirements. A suggested reading source for capacitor maintenance is The Lineman's and Cableman's Handbook.
Capacitor-bank oil switches should be maintained on a schedule related with the type of on/off controls installed at each bank. The maximum number of open and close operations between maintenance of the switches normally should not exceed 2,500.Continue Reading