application of svpwm technique to three level voltage - The ...

International Journal of Engineering and Technology Volume 1 No. 1, October, 20112. MULTILEVEL INVERTERS ANDMODULATING TECHNIQUES2.1 Pulse Width Modulation(PWM) TechniquesA power electronic inverter is essentially a devicefor creating a variable AC magnitude and frequency outputfrom a DC input. The frequency of the output voltage orcurrent is readily established by simply switching for equaltime periods to the positive and negative DC bus andappropriately adjusting the half cycle period. However thevariable frequency ability is accompanied by acorresponding need to adjust the amplitude of fundamentalcomponent of the output waveform as the frequencychanges i.e., voltage control. One of the widely utilizedstrategies for controlling the AC output of powerelectronic converters is the PWM [4] Technique. Thisvaries the duty cycle of the inverter switches at a highfrequency to achieve a target average low-frequencyoutput voltage or current.Modulation theory has been a major research areain power electronics for over three decades and continuesto attract considerable attention and interest. On the otherhand, there have been a number of clear trends in thedevelopment of PWM concepts and strategies since 1970s,addressing the main objectives of reduced harmonicdistortion and increased output magnitudes for a givenswitching frequency and the development of modulationstrategies to suit different converter topologies.Principle of PWMFig. 2.1 illustrates the circuit model of a singlephaseinverter with a center-tapped grounded DC bus andFig. 2.2 illustrates the principle of PWM.Fig. 2.2 Pulse Width Modulation(PWM)From Fig. 2.2 the inverter output voltage is determined inthe following1. When , = /22. When , = /2M= , ………(1)……..(2)3. MODULATION TECHNIQUES FORDIODE CLAMPED MULTILEVELINVERTER3.1 Third Harmonic Injected PWMFig. 2.1 Circuit Model of Single - Phase InverterThe reference ac waveform is not sinusoidal asillustrated in Fig. 3.1 but consists of both fundamentalcomponent and a third harmonic component. As a result,the resulting peak to peak amplitude of the resultingreference function does not exceed the dc supply voltage, but the fundamental component is higher than theavailable supply . The presence of exactly the samethird harmonic component in each phase results in aneffective cancellation of the third harmonic component atthe neutral terminal and all sinusoidals with peakamplitude. This is approximately 15.5% higher inamplitude than that achieved by the sinusoidal PWM.Therefore, the third harmonic PWM provides betterutilization of the dc supply voltage.Copyright IJET © 2011 - IJET Publications UK

International Journal of Engineering and Technology Volume 1 No. 1, October, 2011= +j …….(8)4.2 PRINCIPLE OF SPACE VECTORMODULATIONAn inverter is now-a-days commonly used invariable speed ac motor drives to produce a variable, threephase ac output voltage from a DC voltage. Since ACvoltage is defined by two characteristics, amplitude andfrequency, it is essential to work out a strategy that permitscontrol over both these quantities. PWM controls theaverage output voltage in a sufficiently small period,called switching period, by producing pulses of variableduty-cycles [3]. Here, sufficiently small means theswitching is small compared to the desired output voltagewhich may be considered as equal to desired.Fig. 3.1 Third Harmonic Injected PWM with TriangularCarriers for Multilevel Inverter4. SPACE VECTOR MODULATION(SVM)4.1 INTRODUCTIONThe space vector constituted by the pole voltages ,and is defined as:= + .exp [j (2π/3)] + .exp [j (4π/3)]…(3)The relationship between the phase voltages , ,and pole , and is given by:Fig. 4.1 Three-phase two-level PWM inverterSince + + =0;= + ;…..(4)= + ;…….(5)= + ;……(6)= ( )/3 …..(7)Where is the common mode voltageFrom Eqns. (4), (5) and (6) it is evident that phase voltages, , also result in the same space vector .The space vector can also be resolved into tworectangular components namely and as in Eqn. (7).It is customary to place the α-axis along the A-phase axisof the motor.Hence:= …(9)Also, the relationship between switching variablevector [a b c] t and line-line voltage vector [ ] tcan be expressed in Eqn. (10)= ……(10)As illustrated in Fig. 4.2 there are eight possiblecombinations of on and off patterns for the three upperpower switches [11]. The on and off states of the lowerpower devices are opposite to the upper one and so areeasily determined once the states of the upper powertransistors are determined. According to Eqns.(4),(5),(6),the switching vectors, output line to neutral voltage, andCopyright IJET © 2011 - IJET Publications UK

International Journal of Engineering and Technology Volume 1 No. 1, October, 2011output line-line voltages in terms of DC link are givenin table 4.1 and Fig. 4.2 shows the eight inverter voltagevectors ( to )Table 4.1 Switching vectors, line to neutralvoltages and line to line voltagesVoltageVectorSwitchingVectorsa b cLine to neutralvoltageLine to linevoltage0 0 0 0 0 0 0 0 01 0 0 2/3 -1/3 -1/3 1 0 -11 1 0 1/3 1/3 -2/3 0 1 -10 1 0 -1/3 2/3 -1/3 -1 1 00 1 1 -2/3 1/3 1/3 -1 0 10 0 1 -1/3 2/3 2/3 0 -1 11 0 1 -2/3 1/3 1/3 1 -1 01 1 1 0 0 0 0 0 0Fig. 4.3 illustrates the basic circuit for the three-levelDC3LI. The circuit employs 12 power switching devicesand 6 clamping diodes (D 1 -D 6 )and the DC bus voltage issplit into three-levels(+V dc /2, 0,-V dc /2). Thus, the voltagestress of the switching device is greatly reduced. Theoutput phase voltage V ao has three different states: +V dc /2,0, -V dc /2. Here take phase A as an e.g., for voltage. Forvoltage +V dc /2, S a1 and S a2 need to be turned on. We candefine these states as 2, 1, and 0, respectively [12].Theswitching variable S a in table 4.4 ,is similar to three-phasetwo-level inverter, the switching states of each bridge legof three-phase three-level inverter is described by usingswitching variables S a , S b and S c .The difference is that, inthree-level inverter, each bridge leg has three differentswitching states.Table 4.4 Switching variables of phase AV ao S a1 S a2 S' a2 S' a1 S a+V dc /2 ON ON OFF OFF 20 OFF ON ON OFF 1-V dc /2 OFF OFF ON ON 0Using switching variable S a and DC bus voltageV dc , the output phase voltage V ao is obtained as follows:V an =(S a -1)*V dc /2 ………(11)And the output line voltage of phase A and B can beexpressed as follows:V ab = V ao - V bo = 1/2*V dc (S a -S b ).....(12)4.4 SPACE VECTOR PWM FOR THREELEVEL INVERTERFig. 4.2 Inverter voltages vectors ( to )4.3 OPERATION OF THREE-PHASETHREE-LEVEL INVERTERThere are altogether 27 switching states in aDC3LI. They correspond to 19 voltage vectors whosepositions are fixed. These space voltage vectors can beclassified into four groups, where the first groupcorresponds to 3 zero vectors or null vectors (V0, V7,V14), the second group consists of large voltage vectors(V15-V20), the third group consists of medium voltagevectors (V8-V13) and finally the fourth group consists ofsmall voltage vectors (V1-V6). The last three groups canbe distinguished into three hexagons illustrated in Fig. 4.6.Fig. 4.3 Power circuit for Three-phase three-level inverterFig. 4.4 Space Vector hexagonCopyright IJET © 2011 - IJET Publications UK

International Journal of Engineering and Technology Volume 1 No. 1, October, 2011The plane can be divided into 6 major triangularsectors (1-6). Each major section represents pi/3 of thefundamental cycle. Within each major sector, there are 4minor triangular sectors. There are totally 24 minor sectorsin the plane and the vertices of these sectors represent thevoltage vectors.The modulation ratio of three-phase three-level inverter isrepresented as follows:M= / (2/3V d ) = 3 /2V d ……(15)Table 4.5 The switching states (27 states) for a threelevelinverterTHESWITCHING STATESS a S b S c VS 1 0 0 0 V 0S 2 1 1 1 V 7S 3 2 2 2 V 14S 4 1 0 0 V 1S 5 1 1 0 V 2Fig. 4.5 Space Vector hexagon displaying switching statesS 6 0 1 0 V 3S 7 0 1 1 V 4S 8 0 0 1 V 5S 9 1 0 1 V 6S 10 2 1 1 V 1S 11 2 2 1 V 2Fig. 4.6 Three-level inverters hexagons(a) Small hexagon (b) Medium hexagon (c) Big hexagonIn three-phase three-level inverter, when the rotatingvoltage vector falls into one certain sector, adjacentvoltage vectors are selected to synthesize the desiredrotating voltage vector based on the vector synthesisprinciple, resulting in three-phase PWM waveforms. Bythe examination of the phase angle and the magnitude of arotating reference voltage vector V*, the sector whereinV* resides can be easily located.From table 4.5, each small voltage vector and zero voltagevector have 2 and 3 redundant switching states,respectively. This will be analyzed in the later section.X = T x /T s ; Y = T y /T s ; Z = T z /T s …..(13)Based on the principle of vector synthesis, the followingequations can be written:X+Y+Z=1 ... (13.1)S 12 1 2 1 V 3S 13 1 2 2 V 4S 14 1 1 2 V 5S 15 2 1 2 V 6S 16 2 1 0 V 8S 17 1 2 0 V 9S 18 0 2 1 V 10S 19 0 1 2 V 11S 20 1 0 2 V 12S 21 2 0 1 V 13S 22 2 0 0 V 15S 23 2 2 0 V 16S 24 0 2 0 V 17S 25 0 2 2 V 18S 26 0 0 2 V 19V x *X +V y *Y+ V z *Z =…….(14)S 27 2 0 2 V 20Copyright IJET © 2011 - IJET Publications UK

International Journal of Engineering and Technology Volume 1 No. 1, October, 2011Where is the magnitude of the referencevoltage vector , which rotates with an angular speed ofω=2f in d-q coordinate plane and 2/3 V dc is magnitude ofthe large voltage vector, e.g., V 13 .states: + V dc /2, 0, and -V dc /25. SIMULATION RESULTS ANDDISCUSSIONFig. 5.1 illustrates the Line Voltage of DC3LI withSPWM and Fig. 5.2 illustrates the THD spectrum ofDC3LI with SPWM.In this modulation technique thefundamental voltage is 233.3 V and THD is 29.89%.5.1.2 Space Vector Pulse Width Modulation(SVPWM)5.1 FOR A DIODE CLAMPED THREE-LEVELINVERTER5.1.1. Sinusoidal Pulse Width Modulation (SPWM)Fig. 5.3 Line Voltage of DC3LI with SVPWMFig. 5.1 Line Voltage of DC3LI with SPWMMODULTIONTECHNIQUESDC3LITHDFundamentalComponentSinusoidal PWM 29.95% 200.1 VSVPWM 20.31% 173.1.7 VFig. 5.2 THD spectrum of DC3LI with SPWMFig. 5.4 THD spectrum of DC3LI with SVPWMCopyright IJET © 2011 - IJET Publications UK

International Journal of Engineering and Technology Volume 1 No. 1, October, 2011Fig. 5.4 illustrates the Line Voltage of DC3LI withSVPWM and the THD spectrum of DC3LI with SVPWM.In this modulation technique, the fundamental voltage is312.7 V and THD is 23.20%.6. CONCLUSIONS AND FUTURE SCOPE6.1 ConclusionsDiode Clamped Multi-Level Inverter topologies aredeveloped with 3-levels for various modulation techniquesi.e., Sinusoidal PWM, Space Vector PWM and DC3LItopology. Space Vector PWM technique gives lesser THDcompared to that of Sinusoidal PWM.6.2 Scope For Future WorkThe presented simulink model can be setupexperimentally using semi conductor devices likeTransistor, Thyristor, MOSFET etc., and controllingstrategies by using any of the following devices likemicroprocessor, microcontroller, digital signal processorand FPGA. The controlling strategies can be implementedusing m-file programming with FPGA for betterprocessing speed and performance.ACKNOWLEDGEMENTAuthors acknowledge the support, encouragementand facilities provided by the Electrical&ElectronicsEngineering Department and management of BharatInstitute of Engineering & Technology(BIET),Mangalpally, ibrahimpatnam,Hyderabad,AP, Indiain carryout the presented study/research work.REFERENCES[1] R. Teodorescu, F. Beaabjerg, J. K. Pedersen, E.Cengelci, S. Sulistijo, B. Woo, and P. Enjeti,―Multilevel converters — A survey,‖ in Proc.European Power Electronics Conf. (EPE’99),Lausanne, Switzerland, 1999, CD-ROM.[2] A. Nabae, I. Takahashi, and H. Akagi, ―A newneutral-point clamped PWM inverter,‖ IEEETrans. Ind. Applications., vol. IA-17, pp. 518–523, Sept./Oct. 1981.[3] T. A. Meynard and H. Foch, ―Multi-levelchoppers for high voltage applications,‖ Eur.Power Electron. Drives J., vol. 2, no. 1, p. 41,Mar.1992.[4] C. Hochgraf, R. Lasseter, D. Divan, and T. A.Lipo, ―Comparison of multilevel inverters forstatic var compensation,‖ in Conf. Rec. IEEE-IAS Annu. Meeting, Oct. 1994, pp. 921–928.[5] P. Hammond, ―A new approach to enhancepower quality for medium voltage ac drives,‖IEEE Trans. Ind. Applications., vol. 33, pp.202–208, Jan./Feb. 1997.[6] E. Cengelci, S. U. Sulistijo, B. O. Woom, P.Enjeti, R. Teodorescu, and F. Blaabjerge, ―Anew medium voltage PWM inverter topologyfor adjustable speed drives,‖ in Conf. Rec.IEEE-IAS Annu. Meeting, St. Louis, MO, Oct.1998, pp. 1416–1423.[7] R. H. Baker and L. H. Bannister, ―Electricpower converter,‖ U.S. Patent 3 867 643, Feb.1975.[8] R. H. Baker, ―Switching circuit,‖ U.S. Patent 4210 826, July 1980.[9] ―Bridge converter circuit,‖ U.S. Patent 4 270163, May 1981.[10] P.W. Hammond, ―Medium voltagePWMdriveand method,‖ U.S. Patent 5 625 545, Apr.1997.[11] F. Z. Peng and J. S. Lai, ―Multilevel cascadevoltage-source inverter with separate DCsources,‖ U.S. Patent 5 642 275, June 24, 1997.[12] P.W. Hammond, ―Four-quadrant AC-AC driveand method,‖ U.S. Patent 6 166 513, Dec.2000.[13] M. F. Aiello, P. W. Hammond, and M. Rastogi,―Modular multi-level adjustable supply withseries connected active inputs,‖ U.S. Patent 6236 580, May 2001.[14] RODRÍGUEZ et al.: MULTILEVELINVERTERS 737 ―Modular multi-leveladjustable supply with parallel connectedactiveinputs,‖ U.S. Patent 6 301 130, Oct. 2001.AUTHOR’S BIOGRAPHYDr. JBV Subrahmanyam is a Doctorate in ElectricalEngineering from JNTU-Hyderabad, India, with twodecades of rich experience in teaching, training, research,industry, projects and consultancy. He published 15research papers in reputed international journals and 20papers in international and national conferences.Hisresearch interest is in automation of power systems. He isan expert in condition monitoring of industrial equipmentthrough modern diagnostic techniques. He implementedCopyright IJET © 2011 - IJET Publications UK

International Journal of Engineering and Technology Volume 1 No. 1, October, 2011the latest GPS and GIS technologies in many powerutilities in India successfully. He executed manyinternational and national level technical projectseffectively funded by Power Finance Corporation,Ministry of Power, Government of India, APDRP, DRUM,USAID and DFID-UK.Mr. Sankar is a faculty in electrical engineeringdepartment of HITS, Hyderabad, India, with many years ofrich experience in teaching, training, research. Hisresearch interest is in automation of power systems.Copyright IJET © 2011 - IJET Publications UK