# POWER QUALITY EBOOK

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Maintaining a stable level of power quality in the distribution network is a growing challenge due to increased use of power electronics converters in domestic. In the present day deregulated power market electric power quality issues can be used on all reading devices; Immediate eBook download after download. Editorial Reviews. About the Author. Alexander Kusko is the corporate vice president of Failure eBook features: Highlight, take notes, and search in the book; Length: pages; Optimized for larger screens. Kindle e-Readers. Kindle (5th Generation).

Voltage Distortion 33 example, we assume there are no lightning arresters or insulation break- down to limit the transient voltages. These transients can occur due to res- onances during switching. A circuit capable of exhibiting this phenom- enon is shown in Figure 3. A power supply bus is shown with the bus having inductance L. A capacitor bank labeled C1 is connected at one end of the bus. This capacitor bank may be in place, for instance, for power factor improvement or for voltage sag improvement.

The resultant resonance will be underdamped, and the cur- rent in the capacitor bank may look something like that in Figure 3. Capacitor bank switching. The capac- itor bank may be for power factor correction, or for some other reason.

We see in Figure 3. The top trace is the instantaneous line-voltage, while the bottom trace is the current in the capac- itor. Voltage Distortion 35 capacitor bank is switched. The ripple in the load voltage is due to the capac- itor current that rings at the resonant frequency of the LC circuit.

Interruptions can be a result of control malfunction, faults, or improper breaker tripping.

The top trace is the rms line-voltage. The bottom trace is the first mil- liseconds of the interruption. With a finite line inductance, there is a finite switchover time from diode pair to diode pair. If we assume the line inductance is zero, how will this circuit operate? Looking at the rectifier output voltage Figure 3.

In this circuit, D1 and D4 are on for the positive half-wave of the sine wave, and D2 and D3 are on for the negative half-wave. At the sine wave zero crossing, the switchover from diode pair to diode pair occurs instantaneously. The effect of the inductor is to introduce a finite switchover time from one pair of diodes turning off to the other pair turning on.

During this switchover time, all four diodes are on and the output of the rectifier is zero. This effect is shown in Figure 3. This notching adds unde- sirable harmonics to the load voltage, and also reduces the average value of the load voltage. Note the notching in the output voltage wave- form. Welders Rolling Residences Internal A.

A varying source of harmonic cur- rents includes welders and capacitor banks. Voltage variation created by this setup couples to residential lighting through the distribution system.

The harmonics produced by an arc furnace are unpredictable due to the variation of the arc during metal melting. We see that during ini- tial melting, the harmonic content both even and odd harmonics of the line-voltage are relatively high. During the latter part of the arc furnace melt cycle, the arc is more stable and the harmonic current has dimin- ished. Flicker is the human perception of light intensity variation. Table 4. October 24, A voltage imbalance can cause a reverse-rotating airgap field in induc- tion machines, increasing heat loss and temperature rise.

Summary To summarize voltage distortion types and causes, see Figure 3. Disturbance type Description Causes Narrow pulse with fast rise and Load switching, fuse clearing, utility Impulse exponential or damped oscillatory switching, arcing contacts, lightning decay; 50 V to 6 kV amplitude, 0.

Cornick and H. Redl, R. Chapter 4 Harmonics and Interharmonics In this chapter, we shall discuss harmonics frequency components that are integer multiples of the fundamental line frequency and interharmonics high-frequency components. For most of what we shall do in this chapter, the fundamental frequency used will be 60 Hz.

Background As we mentioned in previous chapters, harmonics can adversely affect the operation of cables, capacitors, metering, and protective relays.

To summarize, a brief listing of some systems and the effects of harmon- ics is shown in Table 4. Periodic Waveforms and Harmonics The notion that any periodic waveform can be broken up into a series of sine waves at the proper amplitudes and phase relationships was first worked out by Joseph Fourier, the French mathematician and physicist [4.

For instance, a square wave Figure 4. Also, note that the square wave has only odd harmonics that is, harmonics of the order 1, 3, The spectrum of the square wave is shown in Figure 4. Likewise, a triangle wave Figure 4. The square wave has only odd harmonics.

This makes sense since the triangle wave more closely resembles a pure sine wave than a square wave, and therefore has fewer harmonics than the square wave. Shown are the first har- monic at 60 Hz top trace , third and fifth harmonics, and the total waveform bottom trace that is the sum of the three harmonics. Shown in Figure 4. Another waveform often encountered in power systems is the trape- zoidal waveform Figure 4.

This waveform models a switching wave- form with a finite risetime and falltime. The Fourier series for this waveform is given by [4. It can be shown that the two corner frequencies f1 and f2 are found by [4. The power dissipation in both cases is the same.

For a sine wave of peak value Vpk, the rms value is Vpk Vrms 5! The rms value of a waveform can be interpreted by considering power dissipation. Looking at Figure 4. The power dissipation in both loads is the same at W. Remember that the rms value of a periodic waveform is the square root of the average value of the square of the waveform over a period. Irms 5 I Pure sine wave A pure sine wave Figure 4. The rms value of this waveform is Irms 5 Ipk i t Ipk t Figure 4.

In this case, i t is the inductor current. In this case, the rms value of the current is ipp Irms 5 2! In this case, i t is the capacitor current. Pulsating waveform The rms value of a pulsating waveform Figure 4. The rms value of the total waveform made up of the sum of i t Ipk Figure 4. Irms 5 2I1,rms 2 1 I2,rms 2 1 I3,rms 2 1??? Total Harmonic Distortion Total harmonic distortion or THD is a measure of how much harmonic content there is in a waveform.

Crest Factor Crest factor is another term sometimes used in power systems analysis, and represents the ratio of the peak value to the rms value of a waveform. For a sine wave Figure 4. Thus, the crest factor is 1. For a square wave Figure 4. Example 4. A truncated square wave. The total waveform for this example is 4 4 4 4 vstd 5 a b sinsvtd 1 a b sins3vtd 1 a b sins5vtd 1 a b sins7vtd p 3p 5p 7p The rms value of the first harmonic is 4 V1,rms 5 5 0. Harmonics and Interharmonics 55 Truncated square wave 1.

The THD is Neutral current in three-phase systems. The three-phase voltages have the form: The phase currents are: V ia 5 a b sinsvtd R V ib 5 a b sinsvt 2 d R V ic 5 a b sinsvt 2 d R The neutral current is the vector sum of the three-phase currents.

Referring to the phase current phasor diagram Figure 4. Nonlinear loads. Power-line harmonics are created when nonlinear loads draw nonsinusoidal current from a sinusoidal volt- age source. These har- monics can result in neutral current that exceeds the individual phase current. Therefore, the magnitudes of the current in each phase are equal to one another. Mathematically, we can express the current in phases a, b, and c as: We see that I1 is the amplitude of the fundamen- tal, and the Ins are the amplitudes of the odd harmonics.

There are phase shifts, denoted by un, for each of the harmonics as well. In many three-phase circuits, the third harmonic is the dominant harmonic. In three-phase systems where there are third-harmonic currents, the degree phase shift for the funda- mental results in a degree phase shift for the third harmonic.

This means that the third-harmonic currents from each phase conductor are in phase with one another, and that the neutral current is equal to the sum of the third-harmonic amplitudes from each of the phases, or: Note that the peak of the 60 Hz fundamental is 1. Shown in the bottom trace of Figure 4.

Next, we add up the sum of the phase currents to get the total neutral current. The top traces show the fundamental and third harmonic currents. The bottom trace is the vector sum. Note that the vector sum of the fundamental of the neutral current is zero. The vector sum of the third-harmonic neutral current is three times that of the third-harmonic amplitude of each phase. Total harmonic distortion.

The THD for this waveform is found simply by: Effects of load current harmonics on load voltage and THD. Figure 4. VLOAD 0. Next, we add a load current that draws a fifth-harmonic current of 50 A Figure 4.

The fifth-harmonic current results in significant load volt- age distortion. The resultant waveform Figure 4. Harmonics cause many detrimental effects in equipment.

References [4. New York: Dover Publications, Inc. Mardiguian, M. Erickson and D. Maksimovic, Fundamentals of Power Electronics, 2nd ed, Springer, Harmonics are generated by rectifiers, line-frequency converters, and nonlinear magnetics. Interharmonics are created by high-frequency switching power supplies. Background A typical setup that shows how harmonic currents can affect power quality is shown in Figure 5.

An AC voltage source is displayed, with its associated line reactance, Xs, and resistance, Rs. This AC source can be single-phase or three-phase. The line inductance depends on the length of the line and the geometry of the conductors.

The line resist- ance, on the other hand, depends on the length of the wire and the wire gauge used. The AC source voltage then supplies a nonlinear load that draws harmonic current. Typically, this harmonic source is a rectifier or other converter. In Figure 5. Note that the voltage labeled Vpcc for voltage at the point of common coupling or PCC has harmonic components due to the harmonic current Ih drawn by the load running through the line impedance. If this voltage at the PCC feeds additional equipment other than the harmonic generating cir- cuit, the resulting voltage distortion can disrupt operation of the equip- ment if the harmonic distortion is too high.

In low-power applications using single-phase power, rectifiers are used as the front-end of switch- ing power supplies and small motor drives. A single-phase, full-wave rectifier with current source load is shown in Figure 5. This circuit is an idealized model of systems where the load draws approximately constant current. In this simplified model, the line current is a square wave. Note that the line current drawn by this rectifier circuit is very harmonic-rich, with a THD of Another rectifier is the full-wave rectifier with capacitive filter Figure 5.

In this type of circuit, the diodes are only on for a frac- tion of a 60 Hz cycle, and the capacitor charges near the peaks of the input sine wave voltage. Example 5. Line-current harmonics.

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Line current is a square wave. With a F bus capacitor, the load voltage ripple is about V peak-peak Figure 5. Note that the line current waveform has significant harmonic distortion Figure 5.

As shown in Figure 5. In Table 5. The THD for the rectifier with a F capacitive filter is approximately per- cent, and the peak-peak output ripple is 25 V. This result illustrates one trade-off with this type of rectifier. Three-phase power labeled phases a, b, and c is full-wave rectified by the six-pulse rectifier.

The rectified voltage is filtered by the high-voltage bus capacitor, Cbus, generating a DC voltage, which is used by the subsequent inverter. The three-phase inverter generates the three-phase currents necessary to drive the motor. A six-pulse rectifier is shown in Figure 5.

Assuming that the load approximates a current source with very large load inductance , the line current drawn from the rectifier shows a THD of 31 percent, and an absence of a third-harmonic and all triplen harmonics Figure 5. We can show that the harmonic amplitudes of the phase currents of the ideal six-pulse rectifier with current source load, IL, are [5. The resultant phase current waveform Figure 5. This is because the pulse topology elimi- nates the 5th, 7th, 17th, and 19th harmonics, leaving the 11th, 13th, 23rd, and 25th harmonics.

The line waveform is rectified by a power-factor corrected boost converter. The power factor correction circuit switches at a frequency much higher than line frequency. Usually, the line side has passive filter- ing to help reduce the high-frequency harmonics drawn from the AC line. Figure 5. The induc- tor has N turns; a mean path length, lc; core permeability, mc; and a core cross- sectional area, Ac.

From magnetic circuit analysis, we can find the inductance of this structure as: Equivalently, saturation means the permeability of the core decreases with a corresponding decrease in the inductance at high current levels. This nonlinear property of the magnetic core requires that the current has harmonic distortion, as shown in Figure 5. Other Systems that Draw Harmonic Currents High-frequency switch-mode power supplies are covered extensively in Chapter 7.

Other sources of harmonic currents are adjustable speed drives ASDs , motors, and arc furnaces. Harmonics can cause many detrimental effects, including resonances with power factor correction capacitors, the heating of neutral conductors, the false tripping of relaying equipment, and the heating of capacitors.

References [5. Maksimovic, Fundamentals of Power Electronics, second edi- tion, Springer, Chapter 6 Power Harmonic Filters In this chapter, we will discuss methods of reducing harmonic distortion in line voltages and currents through the use of filters.

Filters can be implemented with either passive components capacitors and magnetics or active filters. Introduction Industrial and commercial power systems usually incorporate power capacitors to improve the power factor and provide reactive power for voltage support [6. When the system includes sources of harmonic cur- rent, such as power electronic converters or adjustable speed drives ASDs , the capacitors may be used in power harmonic filters to mini- mize the harmonic voltage applied to the system load at the point of common coupling PCC.

The current harmonics produced by power converters, usually polyphase rectifiers, can be reduced in one of three ways: When these measures do not reduce the current harmonics to an acceptable level, power harmonic filters can be introduced to obtain further reduction.

The current harmonics, of themselves, are seldom the problem, such as when the third harmonic produces overheating in the three-phase feeder neutral conductor. The problem occurs when a higher-order current harmonic is resonant with the capacitors and system reactance to pro- duce excessive voltages at the point of common coupling PCC.

A model of a distribution system powering a nonlinear load is shown in Figure 6. The utility is modeled as a source with impedance con- sisting of line resistance and line inductance. The source is then stepped down with a transformer. The resulting voltage typically V line-to- line in three-phase systems drives nonlinear loads such as motors and other equipment. Shown in Figure 6. The power-factor correction capacitor has been converted to a series-tuned passive filter.

In the nonlinear load, for example, an ASD requires a fun- damental frequency current for operation, but can be represented as the source of harmonic currents into the system.

The harmonic cur- rents and voltages are described as follows: A motor load, nonlinear load, and a filter reactor are part of a power factor correction circuit. The standard does this by means of the Tables A very large load has an SCR of 10 and maximum harmonic volt- ages of 2. A very small load has an SCR of and max- imum harmonic voltages of 0.

In order to achieve the voltage harmonic limits of Table The har- monic current is shown in Figure 6. The current is the resultant of the converter current Ih and the filter current Ihf. In addition, the TDD total demand distortion1 must be less than Figure 6. Line reactor One of the simplest harmonic filters is the line reactor2 shown as the three-legged inductor in Figure 6. This magnetic component is often used in the line in series with motor controllers and other converters that draw significant harmonic current.

The reactor presents high impedance to high frequency currents while passing the fundamental. The theoretical waveform of the line current of the six-pulse con- verter rectifier is shown in Figure 6.

This first figure assumes no line inductance. When we add a line reactor, the inductance of the reac- tor causes the converter to exhibit a significant commutation time. The 2 The line reactor is basically a series inductor. The reactor does not reduce the 5th and 7th harmonics significantly, but does reduce the 11th harmonic and higher— for example, the 11th is reduced from 9.

The reactor will also reduce the magnitude of the short-circuit current within the converter. In Figure 6. For line reactor applications, the reac- tor is usually rated in the percent voltage drop at the rated load current. The diagram illustrates the typical reduction in harmon- ics and total harmonic distortion THD that can be accomplished through the use of line reactors.

Without the reactor, we see a THD of The filter is usually placed as shown in Figure 6. The capacitors of the filter also provide reactive power at the fundamental frequency 60 Hz for power factor correction.

The filter is usually made up of one or more sections, as shown in Figure 6. The single-tuned RLC filter for each harmonic frequency is the most common. The impedance Z of the single-tuned section shown in Figure 6.

The resistance R is due to the winding loss and the core loss of the inductor. The quality factor, or Q of an inductor, is given by: The series resonant circuit has a dip in its series impedance at resonance at the frequency where the inductive impedance and capacitive impedance exactly cancel each other out.

For other than resonance, the magnitude of Z is given by: The impedances of the L and C at resonance are selected for the example as 0. Example 6. A series resonant filter used on an AC line is shown in Figure 6. Inductor Ls models the inductance of the source. A harmonic filter designed to attenuate fifth-harmonic components for an adjustable speed drive4 application is shown in Figure 6. In 4 In the power world, capacitors are often specified not by the capacitance value, but by the VAr rating.

VAr stands for volt-amperes reactive. Power Harmonic Filters 87 Figure 6. Harmonic filters have also been used to reduce har- monic interference with telephone systems. The filter is designed to attenuate higher-order harmonics such as the 5th, 7th, and 11th that are generated by the nonlinear load.

Generally, the filter components are tuned a few percent below the harmonic frequency [6. In this design example, each filter section is tuned 4 percent below the filtered harmonic. The series resonant frequencies of the three series resonant circuits are 1 1 f1 5 5 5 Hz 2p 2L1C1 2p 2s 3 ds 3 d 1 1 f2 5 5 5 Hz 2p 2L2C2 2p 2s 3 ds 3 d 1 1 f3 5 5 5 Hz 2p 2L3C3 2p 2s 3 ds 3 d These are the frequencies at which we expect significant attenua- tion, as evidenced in the PSPICE plot of Figure 6.

We also see peak- ing at frequencies below the three series-resonant frequencies. The harmonic generating load is modeled as a current source of value Ih. This filter provides mimima at the 5th, 7th, and 11th harmonics. Note the circuit of Figure 6. First, we need to determine what the IEEE limits are for line har- monic currents, using Table From Table Therefore, both the 5th and 7th harmonics violate this standard.

We also violate the TDD specification, which is 12 percent. The line current is shown in Figure 6. A spectrum of the line current Figure 6. A hypothetical nonlinear load draws fundamental 60 Hz and harmonic currents from an AC source.

The AC source is modeled as an ideal V source in series with a H inductance and a line resistance of 0. The load draws harmonic currents with the strength shown in Table 6. We can calculate the expected total harmonic distortion of the load volt- age using the calculated values shown in Table 6. We have found the magnitude of the line impedance at each harmonic frequency, and then the voltages at the harmonics are calculated.

So, the THD of the load voltage is In the frequency domain, we see harmonic distortion, as expected, at the 5th, 7th, and 11th harmonics, as shown in the Fourier spectrum of Figure 6. We can reduce the harmonic distortion in the load voltage using the circuit of Figure 6. The three series-resonant circuits are tuned 4 percent below the 5th, 7th, and 11th harmonics.

Looking at the spectrum Figure 6. TABLE 6. There is no unique solution to the design problem, so in each case a careful trade-off analysis must be performed. Practical con- siderations include the following: The harmonic filter sections are tuned below the harmonic frequency to prevent the filter frequency from shifting upward if one or more capacitors fail and their fuses blow.

Typical orders are 4. Capacitors are protected by fuses in small groups to minimize the effect of fuse blowing. The whole filter can be divided into assemblies, each protected by a circuit breaker. Filters provide fundamental frequency reactive power vars. Portions of the filter can be switched off at times of light load to limit overvoltage.

Capacitors and inductors must be specified so that the combination of ratings L and C does not result in resonance at an undesired frequency. In other words, we do not want positive peaks in the filter impedance curves to correspond with harmonic frequencies.

The current rating of the inductors and the voltage rating of the capacitors must include the fundamental and harmonic com- ponents. Filters should be located electrically close to the nonlin- ear load that produces the harmonic currents.

A change in system impedance or component variations due to aging or temperature can result in some detuning of the har- monic filter.

Especially at high power levels, the cost of magnetic and capacitive components can be high. High- frequency switching devices, including the metal-oxide semiconductor field-effect transistor MOSFET and insulated gate bipolar transistor IGBT have emerged in recent years with high current and voltage ratings.

These devices switch on and off with fast switching speeds. Thus, high-frequency converters can be designed with good power deliv- ery efficiency using them. An alternative to passive filters are active filters, where power electronics components are used to actively inject harmonics to cancel harmonics in the line current.

This method has been used in the past in lower power electronics applications [6. The diagram of one type of active harmonic filter is shown in Figure 6. Note that the nonlinear load draws harmonic current, Ih, from the power source. The active compensator senses the harmonic current and injects a compensation current, Ic, which cancels the harmonic current.

The net supply current, Is, contains only the fundamental. The compensator switches at a very high frequency compared to the fundamental fre- quency—hence, the VA rating of the energy storage devices in the com- pensator can be minimized. Purported advantages of active filters are [6. The goal is to have the supply current be the only fundamental harmonic. The load draws har- monic current, Ih, and the active compensator injects a current, Ic, to cancel the harmonic content in the line current.

The filter can be tuned under microprocessor control if, for instance, the system impedance changes. The high switching speed of devices allows energy storage elements capacitors and inductors to be of smaller weight and volume. They are more flexible in application compared to pas- sive filters.

Of course, the purported advantages of active filters must be weighed against extra design time and cost.

In this method, harmonic reduction and reactive power compensation is shared between a passive filter and a modest active filter. Typically, the active filter section is rated at a few percent of load kVA. Summary In this chapter, we have discussed power harmonic filters, both passive and active. Passive filters can be as simple as a line reactor, or as complicated as a multisection filter with individual sections tuned to resonant fre- quencies.

Active filters afford design flexibility and smaller physical size. In all cases, filters are designed so that IEEE limits are met. McGranaghan and D. Gonzales and J. LaWhite and M. Chen, F. Blaabjerg, and J. Rivas, L. Moran, J. Dixon, and J. Fujita and H. Bhattacharya, P. Cheng, and D. Chapter 7 Switch Mode Power Supplies In this chapter, we shall examine high-frequency switching power supplies.

Typically, switching frequencies are in the few-kiloHertz range to upwards of a megahertz or higher. Due to the high switching speed of these power supplies, and also due to the fast rising and falling edges of voltages and currents, these converters create significant high-frequency harmonics. Background Switch mode power supplies are used extensively in consumer and industrial equipment such as personal computers and battery chargers.

These fast switching frequencies have a detrimental effect on electromagnetic compatibility EMC because of conducted electromagnetic interference EMI to the power line. In each case, there is switching going on at frequencies much higher than the line frequency.

In recent years, the United States and Europe have placed stringent requirements on the harmonic pollution injected into power lines. In this chapter, we will discuss high-frequency power supply issues and other technical chal- lenges related to harmonic injection and EMI.

The input AC source is rectified, and the resultant rectified voltage is chopped at a high fre- quency to produce the desired DC output s. The load presented by the input rectifier is a large capacitor, so the line current is therefore highly discontinuous, as shown in the typical waveform of Figure 7.

A typ- ical spectrum of the input current is shown in Figure 7. Numerous types of switching power supplies are used in offline appli- cations, and we will discuss some representative examples. Figure 7. The switch is ON for a fraction of a switching cycle of D percent. A full-wave rectifier at the front end of the flyback converter converts the AC input waveform to a DC waveform, which is filtered by the bus capac- itor, CBUS.

The voltage across the bus capacitor is used to periodically ener- gize the primary side of the isolation transformer. When the switch turns off, energy is dumped to the output capacitor. This process repeats itself over and over at the switching frequency, fsw.

This switching action gen- erates frequency components at multiples of the switching frequency fsw to be drawn from the line through the full-wave rectifier. The bus capacitor will have some series inductance, and at high frequencies some of the switch current can bypass the bus capacitor and enter the utility line.

By properly controlling the high-frequency switch, the first harmonic of the line current is forced to be sinusoidal and in-phase with the line voltage, improving power factor to be near percent.

The push-pull and forward converters are shown in Figure 7. Also note that the characteristic drain current risetime and falltime is tR and that TD is the pulse width of the drain current. The spectrum of the drain current includes a number of impulses at multiples of the switching frequency.

In Chapter 4, we showed that f1 and f2 are found by: The switching period is T, the pulse width is TD, and the risetime of the current pulse is tr. Example 7. Spectrum of boost converter current.

The spectral lines occur at multiples of kHz. From kHz to Above Note that the fundamental switching frequency is kHz, and that the overall spectrum of the switch current is as shown in Figure 7. Boost current spectrum with slower risetime and falltime. Above 6. Slower switching of the MOSFET causes the higher frequency harmonics to roll-off faster, but incurs a penalty in power supply efficiency.

Testing for conducted EMI Conducted EMI is the terminology used for the harmonic pollution that high-frequency circuits put onto the utility line. These standards are discussed in Chapter 2. The LISN provides a controlled impedance in this case 50 ohms through which we measure the harmonic current injected onto the util- ity line by the device-under-test D. We then measure the voltage across the resistor using a spectrum ana- lyzer.

This network shunts high-frequency currents from the switching power supply to ground through C2 and C3 , while allowing line frequency currents to pass through.

Inductors L1 and L2 present high-impedance to high-frequency currents, further forcing con- ducted EMI to shunt to ground. In Figure 7. Summary Switching power supplies are ubiquitous components in consumer and industrial equipment. References [7. Chapter 8 Methods for Correction of Power-Quality Problems In this chapter we discuss methods of correction of power- quality problems. Methods discussed include filters, transformers, uninterruptible power supplies UPSs and compensators.

Introduction The first manifestation of a power-quality problem is a disturbance in the voltage waveform of the power source from a sine wave, or in the ampli- tude from an established reference level, or a complete interruption. The disturbance can be caused by harmonics in the current or by events in the supply system.

The disturbance can last for a fraction of a cycle mil- liseconds to longer durations seconds to hours in the voltage supplied by the source. The disturbance usually becomes evident through mis-oper- ation of the equipment supplied by the source, or because of a complete shutdown.

Certain power-quality problems, such as current or voltage waveform distortion, will also appear as a result of a survey of a facility using appropriate instruments, as described in Chapter For example, the standard can be for limits on rms volt- age deviation in the form of an amplitude versus time chart, such as the classical CBEMA curve as described in Chapter 2.

Power-quality standards are discussed in Chapter 2. Each problem may require a different correc- tion method that will be discussed in this chapter, and in the following chapters. Power-quality problems, particularly line-voltage disturbances, can originate at four levels of the system that delivers electric power, namely [8.

Includes service equipment and building wiring In addition, the problems can be caused by the equipment supplied with electric power—for example, power-electronic converters. Redundancy at all levels of the electric-power system reduces the incidence and duration of line-voltage disturbances.

Correction Methods Correction methods include the following: The supply system can be designed to reduce source impedance, to separate loads, and to avoid harmonic resonances.

Power-harmonic filters are installed to correct continuous voltage dis- tortion produced by nonlinear loads in power systems, such as adjustable speed drives. Dynamic voltage compensators function to correct short- time voltage waveform sags. The available time duration of the correc- tion depends on whether supplementary energy storage means, such as batteries, are incorporated in the compensator. A UPS provides an independent power source to the load from an electronic inverter.

When utility power is interrupted, batteries serve as the energy source. Correction methods for voltage disturbances are classified by whether or not they require a stored energy source, such as a battery, flywheel, fuel cell, or other means. As listed earlier, filters require no stored energy.

UPSs always require stored energy. Voltage disturbances versus correction methods Before disturbances in power quality at a site can be corrected, the dis- turbance must be anticipated or identified. The objective of the correction must be established, and the correction method selected. The amplitude, waveform, and duration of voltage disturbances can be determined by measurement at the site, or by reports of typical disturbances made at the site or at other similar sites.

The impact on the operation of equipment at the site is another measure of disturbances. An example of the distribu- tion of voltage sags in low-voltage networks is shown in Figure 8. Equipment built to operate within the voltage tolerance envelope is supposed to operate without interruption for voltage sags within the given amplitude and time duration envelope. For sags in Region A, namely 1 cycle to 30 cycles and 70 percent remaining voltage, the equipment will be out of the envelope and may be interrupted.

However, the interruption rate shown in Figure 8. For sags in Region B, namely zero voltage for up to 1 cycle, and 70 percent remaining ITI CBEMA curve revised Percent of nominal voltage RMS or peak equivalent Prohibited region Voltage tolerance envelope applicable to single-phase volt equipment B No interruption in function region 90 80 70 40 No damage region 0 c 0.

Regions A and B correspond to the dis- tribution in Figure 8. Methods for Correction of Power-Quality Problems voltage for 1 cycle to 30 cycles, the equipment should not be interrupted, even though the rate of sags can be up to six per site per year. The theory being that if the source voltage is corrected to the acceptance regions of these curves, then equipment designed to operate in these regions will operate without interruption with the corrected voltage.

An introduction to correction methods is given in Table Correction methods are listed for typical voltage distur- bances and time durations. Manufacturers offer variations of the end-use equipment, which can withstand ranges of voltage sag and time duration. Furthermore, pro- tected loads, like computers, can be divided and supplied by multiple cor- rection sources—for example, UPSs [8.

Reliability The reason for correcting power-quality problems is to insure the reli- ability of the equipment supplied by electric power from the system in which the problems occur. Although specific equipment, such as a bat- tery-inverter UPS, is given reliability numbers, like mean-time to fail- ure MTBF , data centers supplied by multiple UPSs are classified in terms of availability by using tier designations, as follows [8.

Tier I is composed of a single path for power and cooling dis- tribution, without redundant components, providing Unavailability, Tier II is composed of a single path for power and cooling dis- tribution, with redundant components, providing Tier III is composed of multiple active power and cooling dis- tribution paths, but only one path active, has redundant components, and is concurrently maintainable, providing Unavailability, 1.

Tier IV is composed of multiple active power and cooling distribution paths, has redundant components, and is fault toler- ant, providing Unavailability, 0. The unavailability is 5. Additional descriptions of equipment for data centers will be described in Chapter 9. Surge 0. Two remedies can be employed: A twelve-pulse rectifier circuit is shown in Figure 8. The two six-pulse bridges are supplied from delta and wye secondary windings of the supply transformer to obtain the degree phase shift between the source voltages to the rectifier bridges.

The result- ant line currents are shown in Figure 8. The 5th and 7th harmonics are eliminated; the lowest order harmonic is now the 11th.

The effect of PWM in the line current by switching the devices in a six-pulse rectifier is shown in the waveforms of Figure 8. Equipment subject to source voltage sags will respond in one of the following ways: The bridges are connected in series [8. Figure 8. The design of electric-power supply systems The correction methods for the following two types of power-quality problems that depend upon the design of the system include the following: An illustration of how a system is modeled is shown in Figure 8.

To calculate the currents and voltages, the system is reduced to the form shown in Figure 8. For example, for the fifth harmonic value of ih, the reactances of XL and XC are calculated for Hz. Obviously, resonance will occur when the inductor reactance XL equals the capacitive reactance XC at or near the harmonic frequency.

The design of an electric-power system to reduce the effect of voltage sags and surges that originate within the facility includes the following steps: The filters can be passive, tuned to a fixed frequency that is usually slightly below the harmonic frequency. The filters can be built as active filters—that is, to be tunable to account for changes in the system impedances and loads.

The subject of power-harmonic filters is addressed in Chapter 6. Utilization-dynamic voltage compensators The dynamic voltage compensator corrects voltage sags by inserting a voltage component between the power source and the load to maintain the required load voltage.

The power for the correction is usually taken from the source, but supplementary energy storage is sometimes used. The inserted voltage component is shaped in amplitude and waveform by a controller. Compensation is usually limited to 12 cycles for dips to zero source voltage, and to 2 seconds s for dips to 50 percent source volt- age. One circuit concept is shown in Figure 8.

Because it acts for a short time and has no energy storage, the compensator is smaller and lower in cost than a battery-inverter UPS.

However, it cannot compen- sate for long-term outages—for example, a duration of minutes. The sub- ject is treated in Chapter Uninterruptible power supplies The most commonly used equipment to protect critical loads from power- quality problems is the battery-inverter UPS. The concept is shown in Figure 8.

The basic parts of the module are the battery, the inverter, and the input rectifier, which also serves as the battery charger. In addition, a high speed bypass switch is incorporated to provide power to the load if the inverter fails.

The modules can be operated in systems for higher ratings— for example, up to 10, kW. Details are offered in Chapter 9. Transformers Transformers provide service to, and within, a facility from the utility source, typically These transformers can also correct power-quality problems due to har- monic currents. In addition, constant voltage transformers utilizing fer- roresonance are used to correct for short-term and long-term sags in source voltage down to a remaining voltage of 70 percent for local loads.

Utility substation transformers utilize under-load tap changers to cor- rect for slow deviations in voltage [8. A common application of transformers is the input to a pulse rec- tifier, as shown in Figure 8.

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The transformer has a wye and delta sec- ondary to supply each of the rectifier bridges. The degree phase shift between the two secondary voltages serves to cancel the fifth and sev- enth harmonics of the primary current. Another use of transformers is to cancel harmonics, as shown in Figure 8.

The two secondary wind- ings are zig-zag to provide 30 degrees of phase shift between secondary line-to neutral voltages for the two sets of equal-power loads.

The same effect can be achieved with wye-delta secondary windings with half the loads operating at V and half at V, single-phase. A third example Load No. The third-harmonic currents are prevented from trav- eling back to the source in the neutral conductor. The constant voltage transformer CVT utilizes a leakage-reac- tance transformer with a saturable magnetic core and a capacitor to obtain relatively constant output voltage in the face of input voltage sags down to a remaining voltage of 70 percent at full load and to 30 per- cent at a 25 percent load.

The characteristic curves for time in cycles and percent load are shown in Figure 8. The response occurs within a half cycle and is not limited in time.

## Electric Power Quality

The transformers are rel- atively large and heavy, typically twice the size of an ordinary single- phase transformer of the same kVA rating. They are built up to kVA single-phase and can be banked for three-phase operation. The circuit for the CVT is shown in Figure 8.

The rise compensates for the decline in Vo [8. Structures of leakage reactance transformers are shown in Figure 8. Standby power systems The battery-powered UPS can only deliver power to its critical load for the ampere-hour discharge time of the batteries. Typical times are in the range of 3 to 10 minutes for the UPS delivering rated power.

A simplified diagram is shown in Figure 8. Large systems requiring continuous power supply from UPSs are termed data centers. The description of reliability includes the Tier con- cept for definitions of availability time, while Chapter 13 provides details of standby power systems. Summary Deviations in source voltage and current for critical load equipment must be corrected to insure reliable operation of the equipment.

Methods of correction include filters, compensators, special transformers, and battery-inverter UPSs. The UPS modules are utilized singly and in groups as a function of the requirements of the load. Engine-genera- tor sets serve to extend the operating time of batteries in UPS. References [8. Ferreira, and D. Turner IV, J. Seader, and K. Gray and F. Enjeti, and B. Jimichi, H. Fujita, and H. Blume, G. Camilli, A.

Boyajian, and V. Montsinger, Transformer Engineering, John Wiley, Santosa, and H. Methods for Correction of Power-Quality Problems [8. Kusko and T. Press, Kusko and S. In this chapter, we will use the term UPS for the specific equipment. The UPS was developed in parallel with digital computers and other IT equipment to provide a reliable uninterruptible electric power source to create a standard the electric-utility industry could not provide.

The computer industry would not have developed without some type of UPS, which either used engine-generator sets or static inverters employing power electronic devices. If the concept of an independent UPS had not been developed, then all electronic equipment sensitive to disturbances in source voltage would have had to incorporate energy storage means—for example, batteries to counteract such disturbances in the source voltage.

Both the battery charger rectifier and the inverter utilize power semiconductor devices, which are switched to convert power from AC to DC, or DC to AC. The theory of operation is described in Chapter 5.

The devices are usually pulse-width modulated to shape the line-current waveform of the rectifier, as well as the output voltage of the inverter to sine waveforms. Additional elimination of input and output har- monics is done with filters. The Delta UPS is more efficient than the double-conversion UPS because the load power is supplied directly from the line nearly all of the time.

The inverter and battery still have to be sized for the longest utility outage time. History The silicon-controlled rectifier SCR was introduced in An early publication by Fink, Johnston, and Krings in described three-phase static inverters for essential loads up to kVA [9. Modern UPS modules are considerably more reliable. The concept of uninterruptible system availability is described in Chapter 8.

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The SCRs require forced commutation to turn off the current—that is, to open the switch. The IGBTs are controlled by the gate voltage. The forced commutation for SCRs required addi- tional circuits and operations that increased the failure rates and reduced the reliability of the inverters. It was a rotary machine set consisting of a diesel engine, clutch, elec- tric motor, flywheel, and generator [9.

When utility power was avail- able, the motor drove the generator. When utility power failed, the engine would be started while the flywheel and the generator slowed. At a certain speed, the clutch was engaged so the engine could drive the generator to restore and maintain its synchronous speed. A motor- generator which is a UPS, but employs batteries to provide energy when utility power fails, is shown in Figure 9.

Types of UPS Equipment Commercially available UPS equipment can be categorized into the fol- lowing types, depending upon the manufacturer and the application. Power transistors, IGBTs. Utility power is continually protected by the S3K2U series while internal battery power is maintained for deep sag conditions.

The built-in protection for under and over voltage conditions of the S3K2U units includes low-energy lightning surge protection on the input power source. They are provided with an input circuit protector and a pair of surge protected data line connectors RJ The S3K2U series include an automatic bi-weekly test function to ensure the capability of the battery to supply power in deep sag situations.

Should the battery fail this test, the UPS will display a warning to indicate that the battery needs to be replaced. Figure 9. Each model is available in various input and output voltage combination and offers assorted output voltage and receptacle or hard wired configurations.

Electric Power Quality has evolved from the researches carried out by the authors. The key features of the book can be highlighted as follows: the contents focuses, on one hand, different power quality issues, their sources and effects and different related standards, which are required for students, researchers and practising engineers and, on the other hand, measurement techniques for different power quality parameters, the content level is designed in such a way that the concepts of different power quality issues in modern power system are built up first, followed by some existing and new measurement methods.

Surajit Chattopadhyay has obtained B. And Ph. He has been involved in research work on power quality in the Department of Applied Physics. He has authored 35 papers published in international and national journals and conferences. Three of his papers have been selected as best paper in international level. He has industrial experience on computer interfacing in electrical applications and for last eight years he has been involved in teaching profession in degree and post graduate level.

His field of interest includes power system protection, power quality and computer interfacing in electrical applications. He has coauthored one book on Basic Electrical Engineering. Madhuchhanda Mitra has obtained B. Degree in Physics Honours, B.The line inductance depends on the length of the line and the geometry of the conductors. He has authored 35 papers published in international and national journals and conferences. When utility power was avail- able, the motor drove the generator.

Chapter 10, Dynamic Voltage Compensators, is a description of low- cost equipment to prevent the most frequent short-time line-voltage dips from affecting sensitive equipment. PAGE 1. In this case, the rms value of the current is ipp Irms 5 2!

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