Other Considerations

Where the loads contributing to power factor are relatively constant, and system load capabilities are not a factor, correcting at the main service could provide a cost advantage. When the low power factor is derived from a few selected pieces of equipment, individual equipment correction would be cost effective. Most capacitors used for power factor correction have built-in fusing; if not, fusing must be provided.

The growing use of ASDs (nonlinear loads) has increased the complexity of system power factor and its corrections. The application of pf correction capacitors without a thorough analysis of the system can aggravate rather than correct the problem, particularly if the fifth and seventh harmonics are present.

POWER QUALITY REQUIREMENTS6

The electronic circuits used in ASDs may be susceptible to power quality related problems if care is not taken during application, specification and installation. The most common problems include transient overvoltages, voltage sags and harmonic distortion. These power quality problems are usually manifested in the form of nuisance tripping.

TRANSIENT OVERVOLTAGES—Capacitors are devices used in the utility power system to provide power factor correction and voltage stability during periods of heavy loading. Customers may also use capacitors for power factor correction within their facility. When capacitors are energized, a large transient overvoltage may develop causing the ASD to trip.

VOLTAGE SAGS—ASDs are very sensitive to temporary reductions in nominal voltage. Typically, voltage sags are caused by faults on either the customer's or the utility's electrical system.

HARMONIC DISTORTION—ASDs introduce harmonics into the power system due to nonlinear characteristics of power electronics operation. Harmonics are components of current and voltage that are multiples of the normal 60Hz ac sine wave. ASDs produce harmon ics which, if severe, can cause motor, transformer and conductor overheating, capacitor failures, misoperation of relays and controls and reduce system efficiencies.

Compliance with IEEE-519 "Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems" is strongly recommended.

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