INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET)

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1 INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING & TECHNOLOGY (IJEET) International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 ISSN (Print) ISSN (Online) Volume 3, Issue 2, July September (2012), pp IAEME: Journal Impact Factor (2012): (Calculated by GISI) IJEET I A E M E A HYBRID POLYGONAL SV STRUCTURE FOR THE V/F CONTROL OF AN INDUCTION MOTOR WITH OPEN END WINDING ABSTRACT Snehaprabha T V Associate Professor M S Ramaiah Institute of technology Bangalore , India. Dr. Sanjay Lakshminarayanan Associate Professor M S Ramaiah Institute of technology Bangalore , India. Multilevel Inverters are preferred over conventional two level inverters in order to reduce inverter switching losses and to achieve output quality. The present study involves the harmonics analysis of an open end winding Induction Motor drive, using level PWM technique. The proposed multilevel inverter arrangement produces hexagonal voltage space vector structure in lower modulation region and a 12-sided polygonal space vector structure in the higher modulation region. The stator winding of the motor is fed from both ends with an asymmetric DC link voltage having ratio 1: This method eliminates 5 th &7 th order harmonics from the phase voltage and current. Other advantage is higher linear modulation range.fft analysis is proposed on the scheme to verify the reduction in harmonics in the output wave form. KEYWORDS- Dodecagonal Space Vector,FFT, level-pwm,mli, Open-end winding IM, SVPWM. I. INTRODUCTION Multilevel inverter [MLI] technology has emerged recently as a very important alternative in the area of high-power medium-voltage [MV] energy control. [1,2]. They are used for pipeline pumps in the petrochemical industry, fans in the cement industry, pumps in water pumping stations, traction applications in the transportation industry, steel rolling mills in the metals industry and so on[3]. In Sin PWM & conventional Space Vector PWM (SVPWM) drives the inverter operates at a high switching frequency to generate the reference vector. Obviously this operation produces high dv/dt stress, and more switching losses. But low switching frequency introduces 6n±1 (n=odd) harmonics in the motor output. This in turn 417

2 causes harmonic heating and torque pulsation in the motor drive. PWM with dodecagonal space vector structure is suggested for medium & high power drives as this method eliminates 5 th and 7 th order harmonics from the phase voltage. [4-6].With hybrid polygonal SV structure the advantages of both hexagonal& dodecagonal SV is made use of. To achieve higher power, a series of power semiconductor switches with several lower voltage dc sources in cascade are used in MLI.In high-power applications MLIs are preferred over two level inverters due to the attractive features listed below [3-7]. MLI can operate at lower switching frequencies, hence there is a reduction in switching losses. Fast switching speed of the semiconductor devices results in high dv/dt stresses in the output voltage waveform of the inverter.this high dv/dt can cause premature failure of the motor winding insulation due to partial discharges. But MLI operates with small steps of voltage and so dv/dt stresses are reduced. Moreover electromagnetic compatibility (EMC) problems can be reduced due to the small increment in voltage steps. From the literature and other journals it is studied higher the number of voltage levels lesser is the Total Harmonic Distortion (THD) in the output wave form.with higher number of levels, the synthesized output voltage gets more steps and produces the required reference more accurately.[8-11]. Multilevel converters produce lesser common mode (CM) voltage. There is also a reduction in cost as there is no need of transformers.[19-22]. As the number of levels is increased, the amount of switching devices and other components are also increased tremendously, making the inverter becoming more complex and costly. This is one of the disadvantages of multilevel inverters. Higher number of levels also means that the numbers of DC capacitors used are substantial, which could cause voltage imbalance among the DC capacitors that may results in overvoltage in one or more of the devices. Various inverter topologies, like NPC, cascaded H-bridge and flying capacitor based multilevel inverter topologies, have been proposed in the literature [12, 13]. MLI fed Induction Motor [IM] with open end winding is another topology that is discussed in literature. [14-19]. II. PROPOSED MLI SCHEME FOR AN OPEN-END WINDING INDUCTION MOTOR DRIVE MLI fed IM drive with an open end winding, where the neutral wire is disconnected, is a good choice for MV high power applications. It is possible to generate a three-level voltage profile from this proposed inverter power scheme. IM is fed with two-level inverters from both sides. By using asymmetric DC link voltages for the two inverters, more voltage space phasor levels can be achieved, similar to conventional multilevel inverter. In the present work, an open end winding induction motor drive with dodecagonal space vector structure is proposed in which all the 5 th and 7 th order (6n ± 1, n =odd) harmonics are eliminated for the entire speed range. The schematic of the open-end winding drive is shown in Fig.1 418

3 INV1 INV2 Fig.1 Proposed inverter power circuit The neutral point of the induction motor is disconnected and the 3 phase windings are fed from INV1 and INV2 as shown in Fig-1. DC-link voltage of inverter 1 is kv dc and DC link voltage of inverter 2 is 0.366kV DC. The factor k that shown in circuit is selected such that the radii of the 12-sided voltage space vector polygon are the same as that of the conventional hexagonal voltage space vector structure.. Fig.2 Voltage space vector locations of the two-level inverters [INV1&INV2] Fig.3 Voltage space phasor combinations from the scheme Individual phasor positions of the two level inverters (INV1,INV2) is given in Fig-2.With a DC link voltage ratio of 1: between the inverters, it can be observed that the positions of vector combinations 13,15, 24, 26, 35, 31, 46, 42, 51, 53, 62, 64 are 30 o separated.(fig-3).the tip of these vector positions is located at the vertices of a 12 sided polygon. By using these vectors all the 5 th and 7 th (6n± 1, n = 1, 3, 5 etc.) order harmonics can be cancelled from the motor phase voltage 419

4 III. ANALYSIS OF LEVEL SHIFTED CARRIER BASESD PWM The SVPWM schemes are based on hexagonal voltage space vectors that are vectors along the radii of the hexagon. Conventional SVPWM is actually a modified method of SPWM [3,7].Level shifted carrier based SVPWM concept is used for this study. Here the modulating wave is compared with higher frequency triangular waves. The number of carrier waves chosen depends on the number of voltage levels chosen. These triangles are in phase, but vertically disposed. In the present work, three triangles of ratio 0.366: 0.634: are generated from the modulating sine wave itself as it is necessary to synchronize both the waves for the proper operation. The sampled triplen modulating wave that is used for the study is shown in Fig-4. This fig corresponds to 35 Hz operation. The addition of a triplen voltage is done to ensure that the zero vector period is equally divided at the start and end of a cycle, which is one of the primary characteristics of SVPWM. The sampled modulating wave is compared with 3 triangular-waves. The switching logic is as follows: As long as the sampled value of the modulating wave for a phase is more than the corresponding carrier wave value, the switch is turned on. When the triangle value exceeds the modulating wave, the switch is turned off. When the modulating wave is above or below the amplitude of the carrier wave then the corresponding inverter is clamped. At 35 Hz operation as shown in Fig- 4, the modulating wave is bounded by all the 3 triangles. Fig-4 TABLE-1 gives the 4 voltage levels that can be generated from the proposed power circuit. As the inverter 1 and inverter 2 are feeding from opposite ends, the space vectors of inverter 2 should be subtracted from that of inverter 1 to get the resultant motor phase voltage vector. For example to generate voltage level 2 for the phase A-A, switch S11 of INV-1 & switch S11 of INV-2 should be in ON position. Pole voltages are generated by proper switching sequences of the inverters that are clear from the Tables

5 TABLE-2 Fig.5 pole voltage wave form at 50 Hz operation The voltage levels taken as per Fig-6 during the time interval 0.01 to 0.03 are [230,320,310, 301,302,203,103,013,023,032,031,130]and this can be compared with TABLE-2.The space vector numbers to generate the dodecagonal structure as shown in Fig-7 are[45,57,53,50,51,36,20,8,12,15,14,29], and the space vector numbers are given in TABLE- 6. Fig.6 Formation of the 12 SV positions TABLE-3 gives the inverter switch position for the phases A, B &C to generate Space Vector no-8 as per TABLE

6 TABLE-3 The generation of the 64 -Vector points from the proposed inverter power circuit with space Vector magnitude, angle & Pole voltage levels is shown in TABLE-6 in the APPENDIX. Fig.7 Hexagons & 12-sided polygon from the proposed power circuit TABLE-4 Space vector OA OB OC OD OE OF OH Magnitude and angle 0.366kV dc L kV dc L0 kv dc L kv dc L kV dc L 21.2 o 0.876kV dc L38.8 o 1.225kV dc L15 o Fig-7 shows the hexagonal and 12-sided space vector structures produced from the proposed inverter power circuit. There exist three hexagonal space vector diagrams with radii 0.366kV dc, 0.634kV dc, and k V dc. The radius of the12-sided polygonal is kv dc. Table -4 gives the details of the Space Vectors on the hexagons & the12-sided polygonal structure [Fig-7] that can be used to generate the reference SV. For realizing the desired reference vector, at lower modulation indices, adjacent vectors such as B,C, E lying on the hexagonal boundary can be switched as shown in Fig-7. Up to m= the desired reference vector is generated from the hexagonal structure of radius At higher modulation range, the vectors are selected from the outer hexagon and the extreme 422

7 12-sided polygonal locations, (e.g. G, H, C ) resulting in highly reduced 5 th and 7 th order harmonics. In the 12-step mode of operation, space vectors on the vertices of the 12-sided polygon are chosen (e.g. G, H, G) resulting in the elimination of all 6n ± 1, (n=odd) harmonics from the phase voltage. The benefit of such a scheme is to retain the advantages of a multilevel inverter topology. IV. SIMULATION STUDY The proposed topology and SVPWM technique is studied using simulink. The motor operates under no load and its parameters are given in the Appendix. V dc is taken as 505V. The steady state behavior of phase voltage and phase current along with their harmonic spectrum are analyzed here. The study presents the operation of the machine at 10Hz and 50 Hz. The overall switching frequency can be reduced by selecting the samples as given here. 4samples per sector up to 20 Hz operation, 3 samples per sector from 20 Hz to 30 Hz, 2 samples per sector from 30 Hz to 40 Hz and beyond 40 Hz, one sample per sector is selected in this study. With increase in modulation index, the switching frequency decreases and at 50Hz operation, the switching frequency of the inverter is only 600Hz. [12 50].This results in reduced switching losses of the inverter. The maximum phase peak fundamental of 0.658Vdc is possible in the 12-step mode of operation compared to 0.637Vdc in the 6-step mode of a conventional multi-level with hexagonal voltage space vector structure. (Vdc is the radius of the polygonal voltage structure).if constant V/f ratio is maintained, then for a rated frequency of 50Hz, the linear modulation is achieved up to a frequency of = 48.3Hz and for a hexagonal space vector diagram, the linear modulation is obtained up to a frequency of 43.3Hz.[50 cos30]. Figures 8-10 give the simulation result at 10 Hz operation of the motor. At 10 Hz operation, the reference vector lies entirely inside the innermost hexagon; thus switching happens within the innermost hexagon space vectors. The Pole voltage in each phase varies from 0 to 0.366kVdc as shown in Fig-8 Fig.8 Modulating & carrier wave, pole voltage at 10 Hz operation Fig-8shows the sampled modulating wave which is bounded by the 1 st carrier triangular waves of amplitude Fig-9 shows the simulation result of phase voltage & current. As expected the phase voltage waveform is similar to a conventional 2-level inverter. The FFT analysis of both the voltage and current is given in Fig-10.The phase voltage & current harmonic spectra shows the complete elimination of the 6n±1, (n=odd) harmonics. The harmonics in the waveform reside around 48 times the fundamental, as the switching 423

8 frequency is 4 samples /sector (12 4).The WTHD is & for the voltage and current respectively which is very well within the limit. Fig.9 Phase voltage & current wave at 10 Hz operation Fig.10 Harmonic spectra of phase voltage & current at 10 Hz Figures show the steady state operation of the inverter at 50 Hz, This is the 12-step mode of operation, where only one sample /sector is taken. As seen from the pole voltage waveform (Fig-8), all the inverters are switched once in a cycle. The phase voltage waveform is 12-step and its harmonic spectrum (Fig-10) is characterized by the complete elimination of 6n ± 1, (n=odd) harmonics. Fig.11 Modulating & carrier wave at 50 Hz Fig.12 Pole voltage &Phase voltage wave at 50 Hz operation 424

9 Fig.13 current & torque output at 50 Hz operation Fig.14 Harmonic spectra of phase voltage & current at 50 Hz A comparison of the phase voltage and phase current harmonic performances under three operating conditions in terms of WTHD is tabulated in Table 5. TABLE-5 V. CONCLUSION A simulation study is done on a 3 phase Induction motor with open end winding using Hybrid PWM space vector structure. The proposed topology is realized by two conventional twolevel inverters, fed from asymmetrical isolated dc voltage sources of value kv DC & 0.366k V DC as explained in Sec-II.In this work, a multilevel inverter topology is proposed which produces a hexagonal space vector diagram in lower-modulation region and a 12-sided polygonal space vector diagram in the over-modulation region extending to a final 12-step mode of operation. 425

10 The main contributions from this study are reviewed below The proposed power circuit uses only low voltage switching devices-0.366kv dc &1kV dc. It is very simple to fabricate as conventional two-level inverters are used. The space vector structure consists of three hexagonal structures (with radius 0.366, 0.634, 1) in the lower modulation region, an outer 12-sided polygonal structure and the outermost hexagonal structure. Because of the multilevel structure, switching on the hexagonal vector diagram at lower modulation indices, suppresses the lower order harmonics from the phase voltage. Explanation on the switching sequence and space vectors has also been given. For switching of the inverters, level-pwm technique is used and is explained in this paper. Throughout the modulation range, the switching frequency of the multilevel inverter is kept less than 1 khz. At 12-step operation, the switching frequency of the inverter is only 600Hz.By this approach the 6n±1, (n=odd) harmonics are totally eliminated. It is analyzed from TABLE-5 that the % WTHD in phase voltage & current is very negligible for all the operating conditions. So the torque pulsation and harmonic heating in the machine is reduced considerably. APPENDIX Induction Motor specifications: 3-phase, 400V, 50 Hz, 4 pole, R s = 2.08 Ω; R r = 1.19 Ω, L s = L r =0.28H; M = 0.272H, J=0.1 Kgm 2 ; 426

11 TABLE Vector points from the proposed inverter power circuit with space Vector magnitude, angle & Pole voltage levels 427

12 REFERENCES 1. Bimal K. Bose, Modern Power Electronics and AC Drives published by Prentice Hall 2. R. Krishnan, Electric Motor Drives-modeling, analysis and control, Pearson 3. Bin Wu, HIGH-POWER CONVERTERS AND AC DRIVES, IEEE Press,Hoes Lane, Piscataway, NJ J. Rodriguez, J. S. Lai, and F. Z. Peng, Multi-level inverters: A survey of topologies, controls, and applications, IEEE Trans. Ind. Electron., vol. 49, no. 4, pp , Aug J. Rodriguez, S. Bernet, B. Wu, J. O. Pontt, and S. Kouro, Multilevel voltage-sourceconverter topologies for industrial medium-voltage drives, IEEE Trans. Ind. Appl., vol 54, no. 6, Dec 2007, pp A. Nabae, I. Takahashi, and H. Akagi, A new neutral point clamped PWM inverter, IEEE Trans. Ind. Appl., vol. IA-17, no. 5, pp , Sep J.-S. Lai and F. Z. Peng, Multi-level converters A new breed of power converters, IEEE Trans. Ind. Appl., vol. 32, no. 3, pp , May/Jun Sanjay Lakshminarayanan, R. S. Kanchan, P. N. Tekwani, and K. Gopakumar, Multilevel inverter with 12-sided polygonal voltage space vector locations for induction motor drive, IEE Proc.-Electr. Power Appl., vol. 153, no. 3, May 2006, pp Sanjay Lakshminarayanan, Gopal Mondal, P.N Tekwani, K.K Mohapatra, and K.Gopakumar, Twelve-sided polygonal voltage space vector based multi-level inverter for an induction motor drive with common-mode voltage elimination, IEEE Transactions on Industrial Electronics, vol. 54, no. 5, Oct 2007, pp A. Das, K. Sivakumar, R. Ramchand, C. Patel, and K. Gopakumar, A combination of hexagonal and 12-sided polygonal voltage space vector PWM control for IM drives using cascaded two-level inverters, IEEE Trans. Ind. Electron. vol. 56, no. 5, pp , May A. Das, K. Sivakumar, R. Ramchand, C. Patel, and K. Gopakumar, A Pulsewidth Modulated Control of Induction Motor Drive Using Multilevel 12-Sided Polygonal Voltage Space Vectors, IEEE Trans. Ind. Electron. vol. 56, no. 7, pp , Jul R. Teodorescu; F. Blaabjerg; J. K. Pedersen; E. Cengelci; S. U. Sulistijo; B. O. Woo; P. Enjeti, Multilevel Converters - A Survey, EPE Conference Proceedings, EPE Pavelka J., High Voltage Multilevel Inverters, EPE-PEMC Conference Proceedings, EPE-PEMC Somasekhar, V.T., Gopakumar, K., Shivakumar, E.G., and Sinha, S.K.: A space vector modulation scheme for a dual two level inverter fed open-end winding induction motor drive for the elimination of zero sequence currents, EPE J., 2002, 12, (2), pp Shivakumar, E.G., Gopakumar, K., Sinha, S.K., Pittet, A., an Ranganathan, V.T.: Space vector PWM control of dual-inverter fed open-end winding induction motor drive, EPE J., 2002, 12, (1),pp Somasekhar, V.T., Gopakumar, K., Pittet, A., and Ranganathan, V.T.: PWM inverter switching strategy for a dual two-level inverter fed open-end winding induction motor drive with a switched neutral, IEE Proc., Electr. Power Appl., 2002, 149, (2), pp E. G. Shivakumar, V. T. Somasekhar, K. K. Mohapatra, K. Gopakumar, L. Umanand, and S. K. Sinha, A multi levelspace phasor based PWM strategy for an open-end winding induction motor drive using two inverters with differentdc-link voltages, in Proc. 4th IEEE Int. Conf. 428

13 18. V. T. Somasekhar and K. Gopakumar, Three-level inverter config-uration cascading two 2- level inverters, Proc. Inst. Electr. Eng. Electrical Power Applications, vol. 150, no. 3, pp , May V. T. Somasekhar, K. Gopakumar, M. R. Baiju, K. K. Mohapatra, and L. Umanand, A multilevel inverter system for an induction motor with open-end windings, IEEE Trans. Ind. Electron., vol. 52, no. 3,pp , Jun K. K. Mohapatra, K. Gopakumar, V. T. Somasakhar, and L. Umanand, A harmonic elimination and suppression scheme for an open-end winding induction motor drive, IEEE Trans. Ind. Electron., vol. 50, no. 6,pp , Dec V.T. Somasekhar, M.R. Baiju and K. Gopakumar, Dual two-level inverter scheme for an open-end winding induction motor drive with a single DC power supply and improved DC bus utilisation IEE Proc.-Electr. Power Appl., Vol. 151, No. 2, March H. S. Patel and R. G. Hoft, Generalized techniques of harmonic elimination and voltage control in thyristor inverters, IEEE Trans. Ind. Applicat., vol. IA-9, pp , May/June Ms. Snehaprabha T.V received BSc Engineering degree in Electrical Engineering from NSS College of Engineering, Calicut University, Kerala, India in the year 1981 and ME from UVCE, Bangalore, India, in She is currently an Associate Professor at M.S.Ramaiah Institute of Technology in EE Dept, Bangalore, India. She is having 30 years of teaching experience.at present she is pursuing towards her doctoral degree from JNTU, Hyderabad, India. Dr.Sanjay Lakshminarayanan received the B.Tech. degree from the Indian Institute of Technology (IIT), Kharagpur, India, in 1990 and the M.Sc. (Engg.) degree in electrical engineering in 1995 and the Ph.D. degree in 2007 from the Indian Institute of Science, Bangalore, India,in He has been in the industry for about ten years. He was with Grentel Technologies, Cochin, Hical Magnetics Pvt. Ltd, Bangalore, and GE Medical Systems, Bangalore. His research interests are in the area of power converters, PWM strategies, and motor drives. 429