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Words 2640

Pages 11

SUBMITTED BY

LEKHASRI.M

KARTHIKA DEVI.G

ERODE SENGUNTHAR ENGINEERING COLLEGE,

Erode-57.

Mail id: eseclekha.eee@gmail.com karthierd.g@gmail.com

Contact no: 9791758935 8012178062

Abstract:

A lot of topologies of pulse-width modulated (PWM)-AC voltage controllers for single-phase and threephasesystems are proposed up to date. PWM-AC controllers have important advantages comparedwith the phase-controlled AC voltage controllers using thyristors and triacs. This article presents anovel control technique for application to PWM-AC controllers with ability of generating smallerharmonics. In the proposed control method, both the traditional AC voltage controllers and the PWMAC. controllers are combined, smaller THD values are obtained and switching losses are minimized.Thus, the harmonic pollution in the power system will be reduced and the power quality will beincreased. In order to investigate the proposed controller performance, computer simulations andexperimental studies are performed. Obtained results are compared with those of the conventionalPWM-AC controller.

Key words: Power quality, AC choppers, PWM-AC controllers, AC sector-control voltage converters

INTRODUCTION

There are three basic types of AC to AC converters. Thesimplest ones, the AC voltage controllers, allow controllingthe output voltage only, while the output frequency is the same as the input frequency. The second one is the

cycloconverter. In cycloconverters, the output frequency can be controlled, but it is at least one order of magnitude lower than the input frequency. In both the AC voltage controllers and cycloconverters, the maximum available output voltage approaches the input voltage. The last one is the matrix converter. Because of having no inherent limits on the output frequency, matrix converters are mostversatile, but the maximum available output voltage isabout 15% lower than the input voltage (Bose, 2002).

AC voltage controllers are widely used to obtain variable AC voltage from fixed AC source. Triacs orthyristors are usually employed as the power controlelements of such controllers. Such techniques offer someadvantages as simplicity and ability of controlling largeamount of power economically. However, delayed firingangle causes discontinuity and plentiful harmonics in loadcurrent, the size of the passive circuit becomes bulky andalso a lagging power factor occurs at the AC side eventhough the load is completely resistive especially whenthe firing angle increases. Due to the waveform distortion,specifically at large delay angles or low cyclic pulsation,applications are often restricted to industrial heating,lighting control and starting or low range speed control ofinduction motors (Balci and Hocaoglu, 2005).The switching mode power conversion gives highefficiency, but the main disadvantage is that harmonicsare generated at both the supply and load sides due to the nonlinearity of switches. The harmonic currents generated by the power electronics related-equipment flowthrough the utility system and cause various power quality problems.

Most of the power switches have different operating conditions; thus, they generate differrentorder and different amplitude harmonics. Theseissues have motivated industry to better solutions thatresult in lower total harmonic current distortion (THD) tocomply with IEEE Standard 519 - 1992 and lowerelectromagnetic interference (EMI).

To cope with the harmonic pollution many alternativemethods are proposed in the past. One of the well-knownmethods is the PWM control technique. The advantagesof PWM technique to be gained include nearly sinusoidalinput-output current/voltage waveforms, better inputpower factor, better transient response, elimination of thelow order harmonics and consequently, smaller inputoutputfilter parameters. On the other hand, control byswitching is often accompanied by extra losses due to theswitching losses (Bech, 2000; Nabil et al., 1999; Youm etal., 1999)In this study, a novel control technique for application toPWM-AC controllers with ability of generating smaller harmonics is proposed. In the proposed control method,both the traditional AC voltage controllers and the PWMACcontrollers have been combined. As the results of thiscombination, smaller THD values are obtained andswitching losses resulting from PWM switching areminimized. Thus, the harmonic pollution in the powersystem will be reduced and the power quality will beincreased. The proposed method is firstly investigated by simulations and then, the experimental study is realized.Several characteristics such as the THD values, rmsvalues and first harmonic’s amplitude of the outputvoltage waveforms are evaluated; 3-dimensional harmonic analysis is performed. These results arecompared with those of the conventional PWM method.This type of control technique is used in integral-cycle ACvoltage controller before and good results have been reported in that study (Nigim and Heydt, 2002).

THEORETICAL CHARACTERISTICS OF THETRADITIONAL AC VOLTAGE CONTROLLERS, PWM TECHNIQUE AND THD

There are several topologies of the AC voltage controllers; the mostcommons of which are phase-angle controlled AC voltagecontrollers, section-controlled AC voltage controllers and sectorcontrolledAC voltage controllers. The phase-angle control can beperformed by using triacs with ease. When triac is driven ata > 0 ;the rest of positive and negative half-wave cycles, provided for theload, have correspondingly smaller root-mean-square value. Timeprofile of output voltage of triac, driven at a = 90° in publicnetwork, is shown in Figure 1a. The section control, as opposed tothe phase-angle control, can be performed by switching on while.AC voltage passes zero and switching off at a certain angle value.Time profile of output voltage of the section control having0 £a £ 90

The PWM technique, different from other AC voltage controllerstotally, can be performed by cutting out nume-rous slices of mainvoltage within each switching cycle of the converter. Switchingsignal can be obtained by com-paring an isosceles triangle carrierwave with a suitable DC voltage.The points of intersection determine the switching points ofpower devices. The switching frequency must be higher than themain frequency. Duty cycle (ratio) of a switch is defined as thefraction of the switching cycle during which the switch is on. Timeprofile of output voltage of the conventional PWM technique,operating at a switching frequency of 1.8 kHz and a duty cycle of50%, is given in Figure 1d. The main voltage of the public networkis shown by the dotted line in Figures 1a, b, c and d.Conventionally, there are a few methods to represent harmonicanalysis results. These methods use two-dimensional (2D) graphic area. For some applications with different operating conditions or parameters to be examined together, 2D graphics sometimes causes con-fusion especially when displayed too many harmonicsat the same graphic area. To cope with this problem, threedimensional (3D) graphic area representing harmonic analysis results clearly is developed in this study. 3D method represents theharmonics as a surface in 3D space. Such an approach can express harmonic analysis more clearly than 2D can. For thisreason, this method can be prefered for some applications.Harmonic analysis results of the AC voltage controllers in 3D spaceare given in Figure 2. The three dimentions are: the triggering angle(or duty-cycle) in Degree (or Percentage), the amplitudes of harmonics in Amperes (orVoltages) and the harmonic orders (or frequencies).Harmonic effectiveness of a non-linear load can be expressed with total-harmonic-distortion (THD). Classical AC voltagecontrollers have large THD values especially at small totalconductionangles. Note that the term THD used here is somewhatgeneralized from the usual formulation: the THD of a periodic signal is effectively the energy in the harmonics divided by the energy inthe fundamental (Equation 1). The term “energy” is used as in thesense of signal processing theory.where, the indices represent harmonic orders of a periodic waveforms. An extended definition of THD, given in Equation 2, canbe applied in periodic or non-periodic waveforms.

-200

Proposed PWM-AC controller

The main factor for harmonic producing in traditional AC voltagecontrollers is the discontinuity of the load voltage. In the proposed method, the discontinuity of the load voltage has been minimized byusing PWM technique in the discontinuous section in which the loadvoltage is zero. The duty cycle of PWM is not selected as aconstant value; it is directly proportional to the total conductionangle of the traditional AC voltage controllers. For example, if thetotal conduction angle is 45°, the duty cycle of PWM should be 25%which is calculated from 45°/180°; if the angle is 90°, the duty cyclemust be 50% (90°/180°). As a result, if the conduction angle isincreased from 0 to 180°, the duty cycle of PWM will be increasedfrom 0 to 100% proportionally. Graphical representations of outputvoltagewaveforms of the proposed PWM-AC controller with ohmicload for total-conduction angles of 45 and 90 degree[pic]

The schematic of power circuit used in the proposed PWM-ACvoltage controller. The circuit topology is wellknown for power community. The load is connected in series withBasic parameters’ graphs of the simulationcircuit, (a) line voltage, (b) sector-control pulses only(total conduction angle = 60°), (c) PWM pulses only, (d)S1 switching pulses, (e) S2 switching pulses and (f)resultant load voltage.the power source. The series switch S1 is used to connect ordisconnect the load terminals to the supply, that is, they regulate the power delivered to the load. The parallel switch S2 provides a freewheeling path for the load current to discharge its stored energywhen the series switch is turned off. The switches S1 and S2 arefully controlled power switches capable of conducting current in both directions (bi-directional). Such switches can be assembled from power transistors (BJT, MOSFET, IGBT, etc.) and diodes.

There are three ways to obtain a bi-directional switch: the diodeembedded, the two-common-emitter and the two-common-collector.

In this work the diode-embedded switches are employed as a power switch because it requires only one gate driver and oneactive switch. It also simplifies the driving circuitry.Simulation studies are realized with SimCad package program.Basic parameters’ graphs of simulation circuit of proposed methodfor sector control with a total-conduction angle of 60° and a PWMfrequencyof 2 kHz voltage. Naturally, ifthe load type is selected as purely resistive, switch-S2 can beeliminated. Sector-control strategy has less harmonic effectivenessthan both phase-angle con-trol and section control have (Altintas,2005). Therefore, the study is focused on it. Also, the circuitry usedfor obtaining switching signals for sector control is very simple.

SIMULATION RESULTS

A resistive load of 10 W is used as a load in simulationstudies. When compared to the other load types, theresistive load used with power electronics equipmentproduces many harmonics due to not having energystorageproperty and discontinuity of the load current.Because of waveform similarities between phase-anglecontrol and section control, only phase-angle control hasbeen examined. Thus, the proposed method has beenapplied to the phase-angle control and sector control,individually.

In simulation, the circuitry is firstly set up, the totalconductionangle and PWM frequency are adjusted andthe switching patterns are obtained. And then, outputvoltagewaveform of the converter is measured andstored in a Dat file. This procedure is repeated for a conduction-angle interval of 10°. Finally, the stored in the Datafile are transferred to the Matlab package program. Inorder to represent harmonics in 3D space, a special Matlab program is developed. The developed program FFT built-in-functions calculates the harmonics’amplitudes and expresses them in 3D space as afunction of harmonic frequency and conduction angle. Inaddition to this, the program computes and sketches theTHD value, rms value of the output-voltage waveformand amplitude of the fundamental harmonic as a functionof duty cycle.conventional PWM technique, phase-angle control with PWM technique and sector-control with PWM technique, respectively. PWM frequency is selected as 4 kHz in alloperations. the conventionalPWM technique produces high order harmonics; highorder harmonics are related to the switching operationand occur around switching frequency and multiples ofswitching frequency. In the same way, the proposedmethod However, the proposed method’s harmonics aresmaller than that of the conventional PWM, especially atlower conduction angles in phase-angle with PWMmethod and at higher conduction angles in sector-control with PWM method.

The proposed method has lower THD valuesthan the conventional PWM does.Comparison of the conventional PWM and the phaseanglecontrol with PWM. rms values of the output voltagewaveforms and. A disadvantage of the proposedmethod is to generate a few lower order harmonics athigher conduction angles. Also, comparison of the conventional In the samemanner, the disadvantage of the method is to generate a1080 Sci. Res. Essays

EXPERIMENTAL RESULTS

In experimental study, only sector-control with PWM method is examined. To illustrate the performance of the method, the proposed AC voltage converter is loaded with a resistive load of 4.84 kW. A PWM frequency of 4 kHz is employed in switching operations and no capacitive filtering is employed. For a specific workingcondition, sector-control pulses, S1 switching pulses Load current ismeasured and stored in a Dat file and harmonic analysisis performed with built-in functions of Matlab.Harmonic analysis results for total-conduction angles of150 and 120° .The main observation is that higher fundamentalcurrent is supplied as compared with the classical PWM method. As seen from the figures, while the classicalPWM technique has harmonics at the multiples of theswitching frequencies as usual, the proposed method hassuppressed these harmonics effectively. When total conduction angle of the voltage controller is decreasedup to 120°, the proposed voltage controller keeps workingproperly. However, switching harmonics and a few loworderharmonics come into existence slowly. A simplecapacitive filter across the load will reduce the magnitude.Capacitive filtering and freewheeling the current inthe proposed method is practically required to attenuatethe un-wanted harmonics and to allow transient energy topass through during the OFF period. The capacitor size isreadily determined using the switching frequency andrequired impedance of the capacitor. A path for freewheelingcurrent can be easily obtained using a voltagesnubber element in parallel with the switching element.

Conclusions

One of the well known and widespread methods forharmonic elimination is PWM technique. In this paper, anovel method for harmonic reduction in PWM-AC voltagecontroller has been proposed. In the proposed method,classical AC voltage controllers and conventionalPWMACvoltage controllers have been combined. As theresults of this combination, the proposed method has itsown merits and demerits. High-order harmonics areeffectively suppressed, thus, smaller high-pass LC filterwill be used in the filter circuitry. Smaller THD values areobtained (when compared with the conventional PWM technique); as a consequence, power quality improvement is realized. By using classical AC voltage controllersin the proposed method, the switching losses in conventionalPWM method are decreased proportionally correspondingto the total-conduction angle of AC controllers.

The main disadvantage of the proposed method is togenerate a few lower order harmonics at higher conductionangles in phase-angle control with PWM method,at lower conduction angles in sector-control with PWMmethod.According to the simulation and experimental results,sector-control with PWM method has better performancethan phase-angle control with PWM method does. So,sector-control with PWM method is recommended to thepower community. This method can be of practical valuein replacing the conventional counterparts that are usedin controlling heating elements and speed control of smallfractional horsepower induction motors.

REFERENCES

Altintas A (2005). A Comparative Study on AC Voltage Controllers inTerms of Harmonic Effectiveness. Erciyes University, J. Ins. Sci.

Tec., 21: 79-86.

Balci ME, Hocaoglu MH (2005). Effects of Source Voltage HarmonicDistortion on Power Factor Compensation in Triac Controlled AC

Chopper Circuits, IEEE PEDS. pp: 1119-1204.

Barros J, Diego RI (2006). A New Method for Measurement ofHarmonic Groups in Power Systems Using Wavelet Analysis in the

IEC Standard Framework. Elec. Power Syst. Res., 76(4): 200-208.

Bech MM (2000). Random Pulse-Width Modulation Techniques for

Power Electronic Converters, Ph.D. thesis, Aaborg University,

Denmark.

Bose BK (2002). Modern Power Electronics and AC Drives, Prentice-

Hall Inc., N.J.

IEEE Standard 519-1992 (1993). IEEE Recommended Practices and

Requirements for Harmonic Control in Electric Power Systems. IEEE,

New York, NY.

Nabil AA, Amei K, Sakui M (1999). A New Configuration of Single-

Phase Symmetrical PWM AC Chopper Voltage Controller. IEEE

Trans. Ind. Electron, 46: 942-952.

Nigim KA, Heydt GT (2002). Power Quality Improvement Using Integral-

PWM Control in an AC/AC Voltage Converter. Elec. Power Syst. Res.

,63(1): 65-71.

Altintas 1083

Youm JH, Kwon BH (1999). Switching Technique for Current-Controlled

AC-to-AC Converters. IEEE Trans. Ind. Electron., 46: 309-318.

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