Arrillaga, J. “Power Quality”
The Electrical Engineering Handbook
Ed. Richard C. Dorf
Boca Raton: CRC Press LLC, 2000
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)
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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.
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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.
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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.
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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.
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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.
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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.
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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 ms)
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.
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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.
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In the remaining categories, it is necessary to perform real time continuous monitoring; examples of loads
producing non-stationary distortion are arc furnaces and rolling mills.
Most of the instruments commercially available are not designed specifically for power system application,
i.e., they are not multi-phase and cannot process continuous information. At the time of writing, the only
system capable of multi-channel three-phase real time continuous monitoring is CHART [Miller and Dewe,
1992] which, although originally designed for harmonic monitoring, is capable of deriving continuous infor-
mation of other power quality indexes such as flicker. It is based on the Intel Multi-bus II architecture and the
RMX 386 operating system. An illustration of the system, shown in Fig. 62.14, includes remote data conversion
modules, digital fiber optic transmission, GPS synchronization, central parallel processing, and ethernet-
connected PCs for distant control and display.
62.3 Power Quality Conditioning
A common device in current use to ensure supply continuity for critical loads is the UPS or uninterruptible
power supply. For brief interruptions, the UPS are of the static type, using batteries as the energy source and
involving a rectifier/inverter system. A block diagram of a typical UPS is shown in Fig. 62.15 [Heydt, 1991].
FIGURE 62.14 Simultaneous measurement of voltages and currents in a three-phase line.
FIGURE 62.15 Uninterruptible power supply.
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In the next few years power quality enhancements, in terms of reduced
interruptions and voltage variations, can be expected by the application
of power electronic controllers to utility distribution systems and/or at
the supply end of many industrial and commercial customers.
Among the solutions already available are the solid state circuit breaker,
the static condensers (or statcon), and the dynamic voltage restorer [Hin-
gorani, 1995].
In a solid state circuit breaker, thyristors connected back-to-back form
an ac switch module, several of which are, in turn, connected in series to
acquire the required voltage rating. The breaker will interrupt the circuit
at the first zero of the ac current. This means a delay of a few milliseconds,
which should be acceptable for most applications.
Figure 62.16 shows a simplified illustration of a statcon which is made
up of GTOs (Gate Turn Off) or similar devices such as insulated-gate
bipolar transistors (IGBTs) or MOS-controlled thyristors (MCTs). The
converter is driven by a dc storage device such as a dc capacitor, battery,
or superconducting magnetic storage, and an ac transformer.. The dynamic
voltage restorer, shown schematically in Fig. 62.17, turns a distorted wave-
form, including voltage dips, into the required waveform. The device injects
the right amount of voltage by way of a series-connected transformer into
the distribution feeder between the power supply side and load side.
The dynamic voltage restorer is similar to the statcon, with a trans-
former, converter, and storage, except that the transformer is connected
in series with the busbar feeding the sensitive load. Compensation occurs in both directions, making up for
the voltage dips and reducing the overvoltage. The response is very fast, occurring within a few milliseconds.
The capacity of the dc storage capacitor, in both the statcon and the dynamic voltage restorer, determines
the duration of the correction provided for individual voltage dips. It can be a few cycles or seconds long. To
enhance the load support capability, a storage battery with a booster electronic circuit can be connected in
parallel with the capacitor.
Superconducting magnetic energy storage can be very effective to provide power for short periods. When
the storage is not supporting the load, the converter will automatically charge the storage from the utility
system, to be ready for the next event.
FIGURE 62.17 Dynamic voltage restorer.
FIGURE 62.16Static condenser.
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Defining Terms
Distortion: Any deviation from a perfectly sinusoidal wave.
Disturbance: Any sudden change in the intended power, voltage, or current supply.
FFT (fast Fourier transform): Efficient computation of the discrete Fourier transform.
GPS (global positioning satellite): Used for time stamping and synchronization of multi-measurements at
different geographical locations.
Harmonic instability: Extreme distortion of the voltage waveform at a particular frequency that causes inverter
maloperation.
HVdc: High voltage direct current transmission.
Ripple control signal: A burst of pulses at a fixed non-harmonic frequency injected into the power system for
the purpose of load management control.
Sequence components: Three symmetrical sets of voltages or currents equivalent to an asymmetrical three-
phase unbalanced set.
THD (total harmonic distortions): The ratio of rms value of the harmonic content to the rms value of the
generated frequency (in %).
Uncharacteristic harmonics: Static converter harmonics of orders different from Pk ± 1 where P is the pulse
number.
Related Topic
5.3 Distortion
References
J. Ainsworth, “The phase-locked oscillator. A new control system for controlled static convertors”, Trans. IEEE,
PAS-87, pp. 859-865, 1968.
J. Arrillaga, High Voltage Direct Current Transmission, London: IEE-Peter Peregrinus, 1983.
J. Arrillaga, D.A. Bradley, and P.S. Bolger, Power System Harmonics, London: John Wiley & Sons, 1985.
S. Chen, A.R. Wood, and J. Arrillaga, “HVdc converter transformer core saturation instability: a frequency
domain analysis”, IEE Proc.—Gener. Transm. Distrib, 143(1), 75-81, 1996.
A. Greenwood, Electrical Transients in Power Systems, New York: Wiley Interscience, 1971.
J. Heydt, Electric Power Quality, Stars in a Circle Publications, 1991.
N.G. Hingorani, “Introducing custom power”, IEEE Spectrum, 41-48, June 1995.
International Electrotechical Commission Group, IEC TL 77, 1993.
A.J. Miller and M.B. Dewe, “Multichannel continuous harmonic analysis in real time”, Trans. IEEE Power
Delivery, 7(4), 1913-1919, 1992.
Further Information
Electric Power Quality by J. Heydt and Power System Harmonics by J. Arrillaga et al. are the only texts discussing
the topic, though the latter is currently out of print. Two international conferences take place biennially
specifically related to Power Quality; these are PQA (Power Quality: end use applications and perspectives) and
the IEEE sponsored ICHQP (International Conference on Harmonics and Quality of Power).
Important information can also be found in the regular updates of the IEC and CENELEC standards, CIGRE,
CIRED, UIC, and UNIPEDE documents and national guides such as the IEEE 519-1992.
Finally, the IEE and IEEE Journals on Power Transmission and Delivery, respectively, publish regularly
important contributions in this area.
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