The electrical engineering handbook
Arrillaga, J. “Power Quality” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000 © 2000 by CRC Press LLC 62 Power Quality 62.1Power Quality Disturbances Periodic Waveform Distortion•Voltage Fluctuations and Flicker• Brief Interruptions, Sags, and Swells• Unbalances• Transients 62.2Power Quality Monitoring 62.3Power 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 non- linear 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) © 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/cur- rent 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. © 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. © 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. © 2000 by CRC Press LLC An example of uncharacteristic converter behavior is the presence of fundamental frequency on the dc side of the converter, often induced from ac transmission lines in the proximity of the dc line, which produces second harmonic and direct current on the ac side. Even harmonics, particularly the second, are very disruptive to power electronic devices and are, therefore, heavily penalized in the regulations. The flow of dc current in the ac system is even more distorting, the most immediate effect being asymmetrical saturation of the converters or other transformers with a considerable increase in even harmonics which, under certain conditions, can lead to harmonic instabilities [Chen et al., 1996]. Another common example is the appearance of triplen harmonics. Asymmetrical voltages, when using a common firing angle control for all the valves, result in current pulse width differences between the three phases which produce triplen harmonics. To prevent this effect, modern large power converters use the equidistant firing concept instead [Ainsworth, 1968]. However, this controller cannot eliminate second harmonic amplitude mod- ulation of the dc current which, via the converter modulation process, returns third harmonic current of positive sequence. This current can flow through the converter transformer regardless of its connection and penetrate far into the ac system. Again, the presence of triplen harmonics is discouraged by stricter limits in the regulations. Voltage Fluctuations and Flicker This group includes two broad categories, i.e., •step voltage changes, regular or irregular in time, such as those produced by welding machines, rolling mills, mine winders, etc. [Figs. 62.7(a) and (b)]. •cyclic or random voltage changes produced by corresponding variations in the load impedance, the most typical case being the arc furnace load (Fig. 62.7(c)). FIGURE 62.6 Twelve-pulse converter current. (a) Waveform; (b) spectrum. © 2000 by CRC Press LLC Generally, since voltage fluctuations have an amplitude not exceeding ± 10%, most equipment is not affected by this type of disturbance. Their main disadvantage is flicker, or fluctuation of luminosity of an incandes- cent lamp. The important point is that it is impossible, in practice, to change the characteristics of the filament. The physiological discomfort associated with this phenomenon depends on the amplitude of the fluc- tuations, the rate of repetition for voltage changes, and the duration of the disturbance. There is, however, a perceptibility threshold below which flicker is not visible. Flicker is mainly associated with the arc furnaces because they draw different amounts of current each power cycle. The upshot is a modulation of the system voltage magnitude in the vicinity of the furnace. The mod- ulation frequency is in the band 0 to 30 Hz, which is in the range that can cause noticeable flicker of light bulbs. The flicker effect is usually evaluated by means of a flickermeter (IEC Publication 868). Moreover, the amplitude of modulation basically depends on the ratio between the impedance of the disturbing installation and that of the supply network. Brief Interruptions, Sags, and Swells Voltage Dips (SAGS) A voltage dip is a sudden reduction (between 10 and 90%) of the voltage, at a point in the electrical system, such as that shown in Fig. 62.8, and lasting for 0.5 cycle to several seconds. Dips with durations of less than half a cycle are regarded as transients. A voltage dip may be caused by switching operations associated with temporary disconnection of supply, the flow of heavy current associated with the start of large motor loads or the flow of fault currents. These events may emanate from customers’ systems or from the public supply network. The main cause of momentary voltage dips is probably the lightning strike. In the majority of cases, the voltage drops to about 80% of its nominal value. In terms of duration, dips tend to cluster around three values: 4 cycles (the typical clearing time for faults), 30 cycles (the instantaneous reclosing time for breakers), and 120 cycles (the delayed reclosing time of breakers). The effect of a voltage dip on equipment depends on both its magnitude and its duration; in about 42% of the cases observed to date they are severe enough to exceed the tolerance standard adopted by computer manufacturers. Possible effects are: •extinction of discharge lamps •incorrect operation of control devices •speed variation or stopping of motors •tripping of contactors •computer system crash or measuring errors in instruments equipped with electronic devices •commutation failure in HVdc converters [Arrillaga, 1983] FIGURE 62.8 Voltage sag. FIGURE 62.7Voltage fluctuations. © 2000 by CRC Press LLC Brief Interruptions Brief interruptions can be considered as voltage sags with 100% amplitude (see Fig. 62.9). The cause may be a blown fuse or breaker opening and the effect an expensive shutdown. For instance, a five-cycle interruption at a glass factory has been estimated as $200,000, and a major computer center reports that a 2-second outage can cost approximately $600,000. The main protection of the customer against such events is the installation of uninterruptible power supplies or power quality conditioners (discussed later). Brief Voltage Increases (SWELLS) Voltage swells, shown in Fig. 62.10, are brief increases in rms voltage that sometimes accompany voltage sags. They appear on the unfaulted phases of a three-phase circuit that has developed a single-phase short circuit. They also occur following load rejection. Swells can upset electric controls and electric motor drives, particularly common adjustable-speed drives, which can trip because of their built-in protective circuitry. Swells may also stress delicate computer components and shorten their life. Possible solutions to limit this problem are, as in the case of sags, the use of uninterruptible power supplies and conditioners. Unbalances Unbalance describes a situation, as shown in Fig. 62.11, in which the voltages of a three-phase voltage source are not identical in magnitude, or the phase differences between them are not 120 electrical degrees, or both. It affects motors and other devices that depend on a well-balanced three-phase voltage source. FIGURE 62.9 Voltage interruption. FIGURE 62.10 Voltage swell. FIGURE 62.11 Voltage unbalance. © 2000 by CRC Press LLC The degree of unbalances is usually defined by the proportion of negative and zero sequence components . The main causes of unbalance are single-phase loads (such as electric railways) and untransposed overhead transmission lines. A machine operating on an unbalanced supply will draw a current with a degree of unbalance several times that of the supply voltage. As a result, the three-phase currents may differ considerably and temperature rise in the machine will take place. Motors and generators, particularly the large and more expensive ones, may be fitted with protection to detect extreme unbalance. If the supply unbalance is sufficient, the “ single-phasing ” protection may respond to the unbalanced currents and trip the machine. Polyphase converters, in which the individual input phase voltages contribute in turn to the dc output, are also affected by an unbalanced supply, which causes an undesirable ripple component on the dc side, and non- characteristic harmonics on the ac side. Transients Voltage disturbances shorter than sags or swells are classified as transients and are caused by sudden changes in the power system [Greenwood, 1971]. They can be impulsive, generally caused by lightning and load switching, and oscillatory, usually due to capacitor-bank switching. Capacitor switching can cause resonant oscillations leading to an overvoltage some three to four times the nominal rating, causing tripping or even damaging protective devices and equipment. Electronically based controls for industrial motors are particularly susceptible to these transients. According to their duration, transient overvoltages can be divided into: •switching surge (duration in the range of ms ) •impulse, spike (duration in the range of m s ) Surges are high-energy pulses arising from power system switching disturbances, either directly or as a result of resonating circuits associated with switching devices. They also occur during step load changes. Impulses in microseconds, as shown in Fig. 62.12, result from direct or indirect lightning strokes, arcing, insulation breakdown, etc. Protection against surges and impulses is normally achieved by surge-diverters and arc-gaps at high voltages and avalanche diodes at low voltages. Faster transients in nanoseconds due to electrostatic discharges, an important category of EMC, are not normally discussed under Power Quality. 62.2 Power Quality Monitoring Figure 62.13 illustrates the various components of a power quality detection system, i.e., voltage and current transducers, information transmission, instrumentation, and displays. The most relevant information on power quality monitoring requirements can be found in the document IEC 1000-4.7. This document provides specific recommendations on monitoring accuracy in relation to the operating condition of the power system. With reference to monitoring of individual frequencies, the maximum recommended relative errors for the magnitude and phase are 5% and 5 o , respectively, under normal operating conditions and with constant voltage or current levels. However, such precision must be maintained for voltage variations of up to 20% (of nominal value) and 100% (peak value). For current measurements, the precision levels apply for overcurrents of up to 20% and peaks of 3 times rms value (on steady state) and 10 times the nominal current for a 1-sec duration. Errors in the frequency response of current transformers occur due to capacitive effects, which are not significant in the harmonic region (say, up to the 50th harmonic), and also due to magnetizing currents. The FIGURE 62.12Impulse. © 2000 by CRC Press LLC latter can be minimized by reducing the current transformer load and improving the power factor; the ideal load being a short-circuited secondary with a clamp to monitor the current. Alternative transducers are being proposed for high frequency measurements using optical, magneto-optical, and Hall effect principles. The iron-core voltage transformers respond well to harmonic frequencies for voltages up to 11 kV. Due to insulation capacitance, these transformers are not recommended for much higher voltages. The conventional capacitive voltage transformers (CVTs) are totally inadequate due to low frequency resonances between the capacitive divider and the output magnetic transformer; special portable capacitive dividers, without the output transformers, are normally used for such measurements. Again, alternative transducer principles, as for the current transformer, are being proposed for future schemes. The signal transmission from the transducers to the control room passes through very noisy electromagnetic environments and the tendency is to use fiber optic cables, designed to carry either analog or digital samples of information in the time domain. The time domain information is converted by signal or harmonic analyzers into the frequency domain; the instrumentation is also programmed to derive any required power quality indexes, such as THD (total harmonic distortion), EDV (equivalent distortion voltage), EDI (equivalent distortion current), etc. The signal processing is performed by either analog or digital instrumentation, though the latter is gradually displacing the former. Most digital instruments in existence use the FFT (Fast Fourier Transform). The pro- cessing of information can be continuous or discontinuous depending on the characteristic of the signals under measurement with reference to waveform distortion. Document IEC 1000-4.7 lists the following types: • quasi stationary harmonics • fluctuating harmonics • intermittent harmonics • interharmonics Only in the case of quasi stationary waveforms can the use of discontinuous monitoring be justified; examples of this type are the well-defined loads such as TV and PC sets. FIGURE 62.13 Power quality monitoring components. . 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. change the characteristics of the filament. The physiological discomfort associated with this phenomenon depends on the amplitude of the fluc- tuations, the