Armonicas

POWER QUALITY AND POWER FACTOR CORRECTION

INTRODUCTION Since most loads in modern electrical distribution systems are inductive, there is an ongoing interest in improving power factor. The low power factor of inductive loads robs a system of capacity and can adversely affect voltage level. As such, power factor correction through the application of capacitors is widely practiced at all systemvoltages. As utilities increase penalties they charge customers for low power factor, system performance will not be the only consideration. The installation of power factor correction capacitors improves system performance and saves money. A number of manufacturers have catalogs and design manuals to assist in the application of their products. These publications provide guidance in the selectionand placement of capacitors and discuss general provisions that will affect the overall performance of the installation. Although the methodology for applying capacitors is relatively straight forward, there are a number of influencing factors that must be considered. To ensure that the capacitor installation does not create more problems than it solves, consideration must be given to non-linearloads, utility interaction and system configuration. PQ PROBLEMS RELATED TO POWER FACTOR CORRECTION It is ironic to think that as steps are being taken to improve the operating efficiency at a facility, those very steps may be adversely affecting the facility in other ways. This is sometimes the case when power factor correction capacitors are installed at a facility. As an example, generalapplication of capacitors on motors, when applied without regard to the connected system, can result in the inadvertent tuning of a system to a dominant harmonic. (The implications of this are discussed further below). Although “harmonic problems” are attributed to many power system problems, it is sometimes overly used. There are other ramifications associated with the use of power factor correctioncapacitors such as voltage rise and switching transients. Each of these power quality concepts will be discussed in turn. HARMONIC RESONANCE A common problem that occurs when power factor correction capacitors are installed on a system is harmonic resonance. When this occurs, the power system at a facility is tuned to a specific frequency due to a combination of the system inductance and the addedcapacitance. The system “resonates” at this

frequency, if there are loads at or near the installation that produce that harmonic. When this occurs, the normal flow of harmonic currents, from load to utility source, is altered. When the currents can flow normally, they combine with other load currents across the system. If the bulk of those loads are linear, there will not be a significantpercentage of distorted current. However, when the flow is altered by the installation of capacitors, distortion levels may rise, causing problems within a plant, at nearby utility customers or at system substations or currents may flow where they are not desired. When parallel resonant conditions exist, shunt capacitor banks appear to the harmonic source as being in parallel with the system sourcereactance (or short circuit reactance). When harmonic currents, from the harmonic source, flow through this high impedance circuit, high harmonic voltages develop. The high harmonic voltages can result in an overvoltage condition on the capacitors themselves and/or high voltage distortion. Overvoltage conditions can exceed the voltage rating of the capacitor and result in capacitor failure. Highvoltage distortion can result in the mis-operation or failure of equipment. When series resonant conditions occur, the capacitor appears to be in series with line impedance, as seen from the harmonic source. This presents a low impedance path to the flow of harmonic currents. Currents, then, will flow on the system in ways that were unintended. This can result in interference on communications...