Arrillaga, J. "Power Quality The Electrical Engineering Handbook Ed. Richard C. Dorf Boca raton crc Press llc. 2000
Arrillaga, J. “Power Quality” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000
62 Power Quality 62.1 Power Quality Disturbances Periodic Waveform Distortion. Voltage Fluctuations and Flicker Brief Interruptions, Sags, and Swells Unbalances. Transients Jos arrillaga 62.2 Power Quality Monitoring University of Canterbury 62.3 Power Quality Conditioning New Zealand Ideally, power should be supplied without interruptions at constant frequency, constant voltage and with perfectly sinusoidal and, in the case of three-Phase, symmetrical waveforms. Supply reliability constitutes a recognized independent topic, and is not usually discussed under power quality. The specific object of power quality is the"pureness"of the supply including voltage variations and waveform distortion. Power system disturbances and the continually changing demand of consumers give rise to voltage variations Deviation from the sinusoidal voltage supply can be due to transient phenomena or to the presence of non- The power network is not only the main source of energy supply but also the conducting vehicle for possible interferences between consumers. This is a subject that comes under the general heading of electromagnetic compatibility(Emc) EMC refers to the ability of electrical and electronic components, equipment, and systems to operate satisfactorily without causing interference to other equipment or systems, or without being affected by other operating systems in that electromagnetic environment. EMC is often perceived as interference by electromagnetic radiation between the various elements of a system The scope of EMC, however, is more general and it also includes conductive propagation and coupling by capacitance, inductance(self and mutual) encompassing the whole frequency spectrum a power quality problem is any occurrence manifested in voltage, current, or frequency deviation that results in failure or misoperation of equipment. The newness of the term reflects the newness of the concern. Decades ago, power quality was not a worry because it had no effect on most loads connected to electric distribution systems. Therefore, power quality can also be defined as the ability of the electrical power system to transmit and deliver electrical energy to the consumers within the limits specified by EMC standards 62.1 Power Quality Disturbances Following standard criteria [IEC, 1993], the main deviations from a perfect supply ar periodic waveform distortion(harmonics, interharmonics) voltage fluctuations, flicker short voltage interruptions, dips(sags), and increases(swells) three-phase unbalance The main causes, effects and possible control of these disturbances are considered in the following sections. c 2000 by CRC Press LLC
© 2000 by CRC Press LLC 62 Power Quality 62.1 Power Quality Disturbances Periodic Waveform Distortion • Voltage Fluctuations and Flicker • Brief Interruptions, Sags, and Swells • Unbalances • Transients 62.2 Power Quality Monitoring 62.3 Power Quality Conditioning Ideally, power should be supplied without interruptions at constant frequency, constant voltage and with perfectly sinusoidal and, in the case of three-phase, symmetrical waveforms. Supply reliability constitutes a recognized independent topic, and is not usually discussed under power quality. The specific object of power quality is the “pureness” of the supply including voltage variations and waveform distortion. Power system disturbances and the continually changing demand of consumers give rise to voltage variations. Deviation from the sinusoidal voltage supply can be due to transient phenomena or to the presence of nonlinear components. The power network is not only the main source of energy supply but also the conducting vehicle for possible interferences between consumers. This is a subject that comes under the general heading of electromagnetic compatibility (EMC). EMC refers to the ability of electrical and electronic components, equipment, and systems to operate satisfactorily without causing interference to other equipment or systems, or without being affected by other operating systems in that electromagnetic environment. EMC is often perceived as interference by electromagnetic radiation between the various elements of a system. The scope of EMC, however, is more general and it also includes conductive propagation and coupling by capacitance, inductance (self and mutual) encompassing the whole frequency spectrum. A power quality problem is any occurrence manifested in voltage, current, or frequency deviation that results in failure or misoperation of equipment. The newness of the term reflects the newness of the concern. Decades ago, power quality was not a worry because it had no effect on most loads connected to electric distribution systems. Therefore, power quality can also be defined as the ability of the electrical power system to transmit and deliver electrical energy to the consumers within the limits specified by EMC standards. 62.1 Power Quality Disturbances Following standard criteria [IEC, 1993], the main deviations from a perfect supply are • periodic waveform distortion (harmonics, interharmonics) • voltage fluctuations, flicker • short voltage interruptions, dips (sags), and increases (swells) • three-phase unbalance • transient overvoltages The main causes, effects and possible control of these disturbances are considered in the following sections. Jos Arrillaga University of Canterbury (New Zealand)
Distorted shape h harmonic FIGURE 62.1 Example of a distorted sine wave Periodic waveform distortion Harmonics are sinusoidal voltages or currents having frequencies that are whole multiples of the frequency at which the supply system is designed to operate(e.g, 50 Hz or 60 Hz). An illustration of fifth harmonic distortion is shown in Fig. 62. 1. When the frequencies of these voltages and currents are not an integer of the fundamental they are termed interharmonics. Both harmonic and interharmonic distortion is generally caused by equipment with non-linear voltage/cur ent characteristics In general, distorting equipment produces harmonic currents that, in turn, cause harmonic voltage drops across the impedances of etwork. Harmonic currents of the same frequency from different sources add orally. The main detrimental effects of harmonics are [Arrillaga et al., 1985 maloperation of control devices, main signalling systems, and protective relays extra losses in capacitors, transformers, and rotating machines additional noise from motors and other apparatus telephone interference The presence of power factor correction capacitors and cable capacitance can cause shunt and series resonances in the network producing voltage amplification even at a remote point from the distorting As well as the above, interharmonics can perturb ripple control signals and at sub-harmonic levels can cause flick To keep the harmonic voltage content within the recommended levels, the main solutions in current use are the use of high pulse rectification(e. g, smelters and HVdc converters) passive filters, either tuned to individual frequencies or of the band-pass type active filters and conditioners The harmonic sources can be grouped in three categories according to their origin, size, and predictability i.e., small and predictable( domestic and residential), large and random(arc furnaces), and large and predictable Small Sources The residential and commercial power system contains large numbers of single-phase converter-fed power upplies with capacitor output smoothing, such as TVs and PCs, as shown in Fig. 62. 2. Although their individual rating is insignificant, there is little diversity in their operation and their combined effect produces considerable dd-harmonic distortion. The gas discharge lamps add to that effect as they produce the same harmonic components. c 2000 by CRC Press LLC
© 2000 by CRC Press LLC Periodic Waveform Distortion Harmonics are sinusoidal voltages or currents having frequencies that are whole multiples of the frequency at which the supply system is designed to operate (e.g., 50 Hz or 60 Hz).An illustration of fifth harmonic distortion is shown in Fig. 62.1. When the frequencies of these voltages and currents are not an integer of the fundamental they are termed interharmonics. Both harmonic and interharmonic distortion is generally caused by equipment with non-linear voltage/current characteristics. In general, distorting equipment produces harmonic currents that, in turn, cause harmonic voltage drops across the impedances of the network. Harmonic currents of the same frequency from different sources add vectorially. The main detrimental effects of harmonics are [Arrillaga et al., 1985] • maloperation of control devices, main signalling systems, and protective relays • extra losses in capacitors, transformers, and rotating machines • additional noise from motors and other apparatus • telephone interference • The presence of power factor correction capacitors and cable capacitance can cause shunt and series resonances in the network producing voltage amplification even at a remote point from the distorting load. As well as the above, interharmonics can perturb ripple control signals and at sub-harmonic levels can cause flicker. To keep the harmonic voltage content within the recommended levels, the main solutions in current use are • the use of high pulse rectification (e.g., smelters and HVdc converters) • passive filters, either tuned to individual frequencies or of the band-pass type • active filters and conditioners The harmonic sources can be grouped in three categories according to their origin, size, and predictability, i.e., small and predictable (domestic and residential), large and random (arc furnaces), and large and predictable (static converters). Small Sources The residential and commercial power system contains large numbers of single-phase converter-fed power supplies with capacitor output smoothing, such as TVs and PCs, as shown in Fig. 62.2.Although their individual rating is insignificant, there is little diversity in their operation and their combined effect produces considerable odd-harmonic distortion. The gas discharge lamps add to that effect as they produce the same harmonic components. FIGURE 62.1 Example of a distorted sine wave
A FIGURE 62.2 Single-phase bridge supply for a TV set. time(ms) harmonic FIGURE 62.3 Current waveform(a)and harmonic spectrum(b)of a high efficiency lamp Figure 62. 3 illustrates the current waveform and harmonic spectrum of a typical high efficiency lamp. The total harmonic distortion (THD of such lamps can be between 50 and 150% Large and Random Sources The most common and damaging load of this type is the arc furnace. Arc furnaces produce random variations of harmonic and interharmonic content which is uneconomical to eliminate by conventional filters. c 2000 by CRC Press LLC
© 2000 by CRC Press LLC Figure 62.3 illustrates the current waveform and harmonic spectrum of a typical high efficiency lamp. The total harmonic distortion ( THD) of such lamps can be between 50 and 150%. Large and Random Sources The most common and damaging load of this type is the arc furnace. Arc furnaces produce random variations of harmonic and interharmonic content which is uneconomical to eliminate by conventional filters. FIGURE 62.2 Single-phase bridge supply for a TV set. FIGURE 62.3 Current waveform (a) and harmonic spectrum (b) of a high efficiency lamp
FIGURE 62.4 Typical frequency spectra of arc furnace operation. (a)During fusion;(b) during refining. t Figure 62.4 shows a snap-shot of the frequency spectra produced by an arc furnace during the melting and tin t of the These loads also produce voltage fluctuations and flicker. Connection to the highest possible voltage level and the use of series reactances are among the measures currently taken to reduce their impact on power quali Static Converters Large power converters, such as those found in smelters and HVdc transmission, are the main producers of harmonic current and considerable thought is given to their local elimination in their design The standard configuration for industrial and HVdc applications is the twelve-pulse converter, shown in Fig. 62.5. The"characteristic harmonic currents for the configuration are of orders 12 K+ l and their amplitudes are inversely proportional to the harmonic order, as shown by the spectrum of Fig. 62. 6(b) which correspond to the time waveform of Fig. 62.6(a). These are, of course, maximum levels for ideal system conditions,i.e with an infinite(zero impedance)ac system and a perfectly flat direct current (i. e, infinite smoothing reactance) When the ac system is weak and the operation not perfectly symmetrical, uncharacteristic harmonics appear [Arrillaga, 1983] While the characteristic harmonics of the large power converter are reduced by filters, it is not economical reduce in that way the uncharacteristic harmonics and, therefore, even small injection of these harmo urrent can, via parallel resonant conditions, produce very large voltage distortion levels CB1 FIGURE 62.5 Twelve-pulse converter. c 2000 by CRC Press LLC
© 2000 by CRC Press LLC Figure 62.4 shows a snap-shot of the frequency spectra produced by an arc furnace during the melting and refining processes, respectively. These are greatly in excess of the recommended levels. These loads also produce voltage fluctuations and flicker. Connection to the highest possible voltage level and the use of series reactances are among the measures currently taken to reduce their impact on power quality. Static Converters Large power converters, such as those found in smelters and HVdc transmission, are the main producers of harmonic current and considerable thought is given to their local elimination in their design. The standard configuration for industrial and HVdc applications is the twelve-pulse converter, shown in Fig. 62.5.The “characteristic” harmonic currents for the configuration are of orders 12 K ± 1 and their amplitudes are inversely proportional to the harmonic order, as shown by the spectrum of Fig. 62.6(b) which correspond to the time waveform of Fig. 62.6(a). These are, of course, maximum levels for ideal system conditions, i.e., with an infinite (zero impedance) ac system and a perfectly flat direct current (i.e., infinite smoothing reactance). When the ac system is weak and the operation not perfectly symmetrical, uncharacteristic harmonics appear [Arrillaga, 1983]. While the characteristic harmonics of the large power converter are reduced by filters, it is not economical to reduce in that way the uncharacteristic harmonics and, therefore, even small injection of these harmonic currents can, via parallel resonant conditions, produce very large voltage distortion levels. FIGURE 62.4 Typical frequency spectra of arc furnace operation. (a) During fusion; (b) during refining. FIGURE 62.5 Twelve-pulse converter