Analysis of Discontinuous Space Vector PWM Techniques for a Five-phase Voltage Source Inverter

Transcription

1 Analysis of Discontinuous Space Vector PWM Techniques for a Five-phase Voltage Source Inverter Atif Iqbal Mohd. Arif Khan Sk. Moin Ahmed M. Rizwan Khan Haitham Abu-Rub* atif2004@gmail.com, arif.md27@gmail.com, moin.nt@gmail.com, rizwan_eed@rediffmail.com, haitham.abu-rub@qatar.tamu.edu Department of Electrical Engineering, Aligarh Muslim University, Aligarh, , India *Electrical & Computer Engineering Programme, Texas A&M University at Qatar, Doha, Qatar Abstract: This paper develops discontinuous space vector PWM (DPWM) techniques for a five-phase two-level voltage source inverter (VSI). Space vector model of a five-phase VSI shows that re exist 32 space vectors with three different lengths forming three concentric decagons. Application of outer most and middle set of space vectors to implement Space vector PWM yield nearly sinusoidal output. Thus proposed DPWM utilises same set of space vectors to implement discontinuous modulation techniques. Performance is evaluated in terms of total harmonic distortion (THD) in output phase voltages. A significant reduction in switching losses is observed by adopting proposed PWM schemes. The simulation and experimental results are provided to validate concept. I. INTRODUCTION Multi-phase (more than three phases) motor drives have gained much popularity in recent years and a number of research papers have been published. The main reason of interest in such drive systems are inherent advantages offered by multi-phase motors such as reduction in amplitude and increase in frequency of torque pulsation, reduction in rotor current harmonics, reduction in dc link current harmonics, reduction in current per phase without increasing voltage per phase leading and increasing torque per ampere for same volume machine. Keeping in view se advantages application of multi-phase motors are coming up mainly in high power ranges such as ship propulsion, electric and hybrid vehicles, aircraft fuel pump applications etc [-6]. Multiphase motors need invariably some sort of power electronic converter for ir supply as phases more than three is not available from grid. The most common choice is a multi-phase voltage source inverter. Appropriate PWM techniques are required to control drive system fed using multi-phase inverters. The requirements that are imposed on multi-phase inverters depend upon multi-phase machine type; distributed winding or concentrated winding. Strictly speaking, sinusoidal voltages/currents are required to be produced by multi-phase inverters feeding distributed winding multi-phase machines. However, more often harmonic injection schemes are used to enhance torque production from a concentrated winding machine [7-0]. A number of PWM techniques are investigated and 222 presented in literature for multi-phase voltage source inverters. The most popular are Space Vector Pulse Width Modulation (SVPWM) because of easiness of digital implementation and better DC bus utilisation, when compared to ramp-comparison sinusoidal PWM method [6, ]. Anor PWM method known as Discontinuous PWM is widely used for three-phase VSIs because it offers reduced number of switching and consequently reduced switching losses [2]. This aspect becomes extremely important when dealing with high power drive system, as even a small saving in switching losses means a large amount of overall power saving and thus enhanced energy efficiency of motors. However, little attention has been paid on development of such PWM strategy for multiphase VSIs. Simulation approach is used in [3-4] and preliminary studies are provided for discontinuous PWM for a five-phase VSI. In contrast this paper provides comprehensive analysis of discontinuous SVPWM for a five-phase VSI using simulation and experimental approach. This paper takes up issue of space vector PWM for a five-phase VSI in discontinuous mode. It is shown that number of switching and consequently switching losses can be greatly reduced by tiding one or more inverter legs to eir positive or negative. This paper utilises large and medium length space vectors to implement discontinuous SVPWM, providing sinusoidal output phase voltages. This paper furr illustrates leg voltages, common mode voltages and amount of reduction in switching due to each scheme. Moreover, two novel methods are proposed in this paper, which yield better results compared to existing schemes of [3-4]. The simulation and experimental results are provided to support findings. II. PROPOSED DISCONTINUOUS SPACE VECTOR PWM There exist 30 active space vectors and two zero space vectors spanning over ten sectors (each sector spans for 36 degrees) in a five-phase VSI. To obtain symmetrical SVPWM, four active and zero vectors are employed in each sector, with one zero vector (00000) applied at start of a sampling period and anor zero vector () applied at end of sampling period. It is

2 possible to move position of active voltage pulses within half switching interval, to eliminate one zero output voltage pulse. Modulation strategies using this concept are termed as discontinuous modulation. This method causes one or more inverter leg to tied to eir positive or negative dc bus. This scheme is mainly important for high power applications where switching losses are considerable. Six different Discontinuous space vector PWMM techniques are proposed and presented in this section. However, all schemes essentially just rearrange e placement of zero space voltage vectors within each half carrier or carrier interval. The method where t 3 ( ) is kept zero for complete fundamental cycle is termed as DPWMMIN and method where t 0 (00000) is kept zero for one fundamental cycle is termed as. Rest of four methods splits sectors and arranges zero space vectors in various fashions are termed as DPWM0, DPWM, DPWM2 and DPWM3. The time of application of various space vectors are given in [8]. The fundamental output voltage magnitude and nature of phase voltage waveforms remains sinusoidal as that of continuous SVPWM [8]. The switching waveforms for DPWMMIN and are shown in Fig. 6. For rest of methods, similar switching pattern follows. In case DPWMMIN, lower switch remains switched on for whole fundamental period and in, upper switch remains on. There are ten sectors and ten switches in a five-phase VSI thus one leg remains idle in two sectors i.e. for 72, upper switch of a leg is inoperative in one sector and lower one in or one sector. In DPWMMIN each leg is tied to DC bus for two subsequent sectors. For instance in DPWM0 upper switch of leg D is tied to lower dc bus in sector and upper switch of same leg is tied to upper dc bus in sector 6. Thus number of switching is now reduced by one fifth compared to ir continuous counterpart and consequently a corresponding reduction in switching losses. III. SIMULATION RESULTS All proposed schemes are simulated using Matlab/Simulink and resulting waveforms are shown in Fig 2. Part (a) of Fig 2 shows placement of zero vectors in different sectors and part (b) of Fig 2 shows waveforms for each schemes where V a leg (average leg voltage), V a (average phase voltage), V nn (average voltage between neutral points) called common mode voltage. The switching frequency is taken as 5 khz and fundamental frequency is chosen as 50 Hz. The referencee input is varied from 0 to maximumm obtainable ( (p.u.)) [8]. Sa T0/2 T/2 T2/2 T/2 T2/2 T2/2 T/2 T2/2 T/2 T0/2 Sb Sc Sd Se V0 V V2 V V V2 V V0 T3/2 Sa T2/2 T3/2 (i) T2/22 T3/2 T3/2 T2/2 T3/2 T2/2 T3/2 Sb Sc Sd Se V3 V2 V3 V3 V3 V3 V2 V3 (ii) Fig.. Switching waveform (i) Sector, DPWMMIN (ii) Sector 2, 223 DPWMMIN Basic schemes designated as DPMWMMIN and DWPWMMAX tied each leg to negativee dc rail and positive dc rail, respectively and thus name MIN and Max are given to m. These schemes are thus not recommended for use as y may damage eir lower or upper switches of VSI. To avoid this situation threee more techniques namely DPWM0, DPWM and DPWM2 are suggested in which DPWMMIN and are applied alternatively in each sector so that each leg can be kept inoperative alternately providing

3 a symmetric switching. Each of three methods proposed here are suitable to feed different types of loads. For DPWM0 discontinuous period is 8 lagging DPWM DPWM 0 DPWM 2 224

4 β V4(0 0 0) V3( 0 0) V5(0 0) V4(0) V3(0000) V2( 0 0 0) V5(0000) (0) T0 = 0 V6(0 0 0) V6(0) V(0000) V( 0 0 ) α V7(0000) V20(0) V7(0 0 ) V8(0) V9(0000) V0( ) V8(0 0 0 ) V9( 0 0 ) DPWM 3 Fig. 2. Simulation Waveforms for different DSVPWM schemes phase voltage and thus most suitable load will be one operating at cos( 8 ) power factor leading. This is because of fact that in range of discontinuous period current is maximum and number of switching is minimum offering lower switching losses. Similarly for DPWM suitable load power factor is cos 8 lagging. It is furr observed that for DPWM ( ) and DPWM2 discontinuity is splitted in two halves and thus two different loads may be catered are cos 9,cos 24 power factor lagging and or leading. ( ) ( ) THD THD of Large and Medium Vectors DPWMMIN THD THD DPWM0 THD DPWM THD DPWM2 THD DPWM3 THD IV. PERFORMANCE EVALUATION Parameters that is taken in consideration for performance evaluation of proposed DSVPWM methods namely Total Harmonic Distortion (THD) is defined in [2] by equations (), V 2 THD = n n= 3,5,7.. V () Where V n represents n th order harmonic component and V represent fundamental output phase voltages. The lower order harmonic contents (upto 25 th order) are considered for calculation of THD. The simulation is carried out to determine se performance indices for complete range of modulation index. The resulting THD are shown in Fig. 3. It is seen from Fig. 3 that at low modulation index THD of is lowest compared to all or schemes and DPWM has highest THD. Since may not be employed for implementation purposes, thus next best scheme is DPWM2. At high modulation index value of THD is smaller compared to value at lower modulation index, which implies that output voltage is very near to sinusoidal. DPWM has lowest THD and DPWM0 and DPWM3 has highest THD at high modulation in dex Modulation Index Fig. 3. THD with Modulation index for different DPWM V. EXPERMENTAL IMPLEMENTATION Experimental set up is prepared in laboratory to implement proposed discontinuous space vector PWM techniques. A five-phase voltage source inverter is developed using intelligent power module from VI Micro systems, Chennai, India. Texas Instrument DSP TMS320F282 is used as processor to implement control algorithm. Since this DSP may coded in C or C++ +, it is more user friendly and y have dedicated 6 hardware PINS to generate desired PWM signals. The PWM circuits associated with compare units make it possible to generate up to eight PWM output channels (per Event Manger) with programmable dead band and polarity. This DSP is specifically meant for use in motor drive purposes and it can control up to 8-phase two-levell inverter. The control code is written in C++ + language in Code composer studio 3. which runs in a PC. The control signal generated by PC is transferred to DSP board through RS 232 cable connected in parallel printerr port of PC. The DSP board is connected to Power Module through dedicated control cable. The DSP interfacing circuit along with required A/D and D/A converter is built on DSP board itself procured from VI Micro systems. The experimental results for

5 scheme and DPWMMIN are shown in Fig. 4 and Fig. 5. The experimental results match well with simulation results. Fig. 4. Experimental result for scheme Fig. 5. Experimental result for scheme DPWMMIN VI. CONCLUSION The paper present PWM technique termed as Discontinuous Space Vector PWM for a five-phase voltage source inverter. This paper utilises large and medium vectors to synsise input reference in discontinuous mode. The proposed method of PWM offers a reduction in overall number of switching and consequently switching losses. The DSVPWM is seen to reduce number of switching to /5 th compared to continuous SVPWM and consequently switching losses reduces by same factor. Alternatively inverter switching frequency can be enhanced by 20% keeping same inverter losses. Six different schemes are proposed and presented. The analysis is done on basis of THD in output phase voltages. It can be concluded that provide lowest THD for low modulation index. However, this method is not recommended for practical implementation as this may shorten life of inverter. DPWM2 offers next best result and thus it may be used for implementation. At high modulation index DPWM is recommended for use. The viability of proposed schemes is validated using simulation and experimental results. VII. REFERENCES [] G.K.Singh,Multi-phase induction machine drive research a survey, Electric Power System Research, vol. 6, 2002, pp [2] M.Jones and E.Levi; A literature survey of state-of--art in multiphase ac drives, Proc. 37 th Int. Universities Power Eng. Conf. UPEC, Stafford, UK, 2002, pp [3] R. Bojoi, F. Farina, F. Profumo and Tenconi, Dual three induction machine drives control-a survey, IEEE Tran. On Ind. Appl.,vol. 26, no. 4, pp , [4] E. Levi, R.Bojoi, F. Profumo, H.A. Toliyat and S. Williamson, Multi-phase induction motor drives-a technology status review, IET Elect. Power Appl. Vol., no. 4, pp , July [5] E.Levi, Guest editorial, IEEE Trans. Ind. Electronics, vol.55, no. 5, May 2008, pp [6] E.Levi, Multi-phase Machines for variable speed applications, IEEE Trans. On Ind. Elect. vol. 55, no. 5, May 2008, pp [7] H.Xu, H.A.Toliyat and L.J.Petersen; Rotor field oriented control of a five-phase induction motor with combined fundamental and third harmonic injection, Proc. IEEE Applied Power Elec. Conf. APEC, Anaheim, CA, 200, pp [8] R.Shi, H.A. Toliyat and A.El-Antably, Field oriented control of five-phase synchronous reluctance motor drive with flexible 3 rd harmonic current injection for high specific torque, Conf. Rec. IEEE IAS, Annual meeting, Chicago, IL, USA, 200, pp [9] M.J. Duran, F. Salas, M.R. Arahal, Bifurcation Analysis of fivephase induction motor drives with third harmonic injection, IEEE Trans. On Ind. Elect. vol. 55, no. 5, pp , May [0] M.R. Arahal, M.J. Duran, PI tuning of Five-phase drives with third harmonic injection, Control Engg. Practice, 7, pp , Feb [] D. Dujic, M. Jones, E. Levi, Generalised space vector WPM for sinusoidal output voltage generation with multiphase voltage source inverter, Int. Journal of Ind. Elect. And Drives, vol., NO [2] G.D.Holmes, T.A.Lipo, Pulse Width Modulation for Power Converters - Principles and Practice, IEEE Press Series on Power Engineering, John Wiley and Sons, Piscataway, NJ, USA, [3] M.A. Khan and A. Iqbal, Discontinuous space vector PWM for a five-phase VSI with higher dc bus utilisation, Proc. IEEE INDICON 2007, 6-8 Sept, 2007, Bangalore, India, CD-ROM paper. [4] X.F.Zhang, F.Yu, H.S.Li and Q.G. Song, A Novel Discontinuous Space Vector PWM Control for Multiphase Inverter Proc. Int. Symp. Power Electronics, Electrical Drives Automation and Motion SPEEDAM, Taormina, Italy, 2006, CD-ROM paper S8-6. [5] A. Iqbal, S. Moinuddin, Space vector model of a five-phase voltage source inverter, Proc. IEEE International Conf. On Industrial Technology (ICIT06), 5-7Dec Mumbai, India,pp [6] A.Iqbal, E.Levi, Space vector modulation scheme for a five-phase voltage source inverter, Proc. European Power Electronics (EPE) Conf., Dresden, Germany, 2005, CD-ROM paper no pdf. [7] P.S.N.deSilva, J.E.Fletcher, B.W.Williams, Development of space vector modulation strategies for five-phase voltage source inverters, Proc. IEE Power Electronics, Machines and Drives Conf. PEMD, Edinburgh, UK, 2004, pp [8] A. Iqbal, and E. Levi, Space vector PWM techniques for sinusoidal output voltage generation with a five-phase voltage source inverter, Elect. Power Component and Systems, vol. 34, no. 2, 2006, pp [9] A. Hava, R. J. Kerkman and T.A. Lipo, A high performance generalised discontinuous PWM algorithm, IEEE Trans. Ind. Appl. Vol. 34, no. 5, sept/oct Acknowledgment: Authors fully acknowledge support for this work provided by CSIR standard research grant no. 22(0420)/07/EMR-II. 226