APPLICATION OF SVPWM TECHNIQUE TO THREE LEVEL VOLTAGE SOURCE INVERTER

APPLICATION OF SVPWM TECHNIQUE TO THREE LEVEL VOLTAGE SOURCE INVERTER 1 JBV Subrahmanyam, 2 Sankar 1 Electrical & Electronics Engineering Dept.,Bharat Institute of Engineering &Technology, mangalpally, ibrahimpatnam, RR district, Hyderabad,AP,INDIA 501 510 2 Electrical & Electronics Engineering Dept Holymary institute of technology&science,kesara, RR district, Hyderabad,AP,INDIA Email: 1 jbvsjnm@gmail.com, 2 sankarmtech@gmail.com ABSTRACT The purpose of the study is to compute the Total Harmonic Distortion (THD) with the proposed latest Space Vector Pulse Width Modulation(SVPWM) technique and prove that the proposed technique gives lesser THD compared to that of Sinusoidal PWM.Multilevel inversion is a power conversion strategy in which the output voltage is obtained in steps thus bringing the output closer to a sine wave and reduces the Total Harmonic Distortion (THD). Multilevel inverter structures have been developed to overcome shortcomings in solid-state switching device ratings so that they can be applied to higher voltage systems. The multilevel Voltage Source Inverter (VSI) unique structure allows them to reach high voltages with low harmonics without the use of transformers. The general function of the multilevel inverter is to synthesize a desired ac voltage from several levels of dc voltages. In recent years, the multilevel inverters have drawn tremendous interest in the area of high-power medium-voltage energy control. Three different topologies have been proposed for multilevel inverters like Diode-Clamped Inverter (DCI), Capacitor Clamped Inverter (CCI) and Cascaded Multicell Inverter (CMI). The DCI is also called the Neutral- Point Clamped (NPC) inverter, when it was first used in a three-level inverter in which the mid-voltage level was defined as the neutral point. CCI is also called Flying Capacitor Inverter (FCI) and cascaded multicell is combination of individual small voltage sources, with separated dc sources. In addition, several modulation and control strategies have been developed or adopted for multilevel inverters including multilevel Sinusoidal Pulse Width Modulation (SPWM), and Space Vector Modulation (SVM). Key words: Total Harmonic Distortion, Sinusoidal PWM, Space Vector Pulse Width Modulation(SVPWM), Voltage Source Inverter (VSI) 1. INTRODUCTION Inversion is the conversion of DC power to AC power at a desired output voltage or current and frequency. A static semiconductor inverter circuit performs this electrical energy inverting transformation. The terms voltage-fed and current-fed are used in connection with the output from inverter circuits. A Voltage Source Inverter (VSI) is the one in which DC input voltage is essentially constant and independent of the load current drawn. The inverter specifies the load voltage while the drawn current shape is dictated by the load. The DC power input to the inverter is obtained from an existing power supply network (or) from a rotating alternator through a rectifier (or) a battery, fuel cell, photo voltage array (or) Magneto Hydro Dynamic (MHD) generator. Inverters are mainly classified as Voltage Source Inverters (VSI) and Current Source Inverters (CSI). A VSI is the one in which the DC source has small or negligible impedance. In other words, a VSI has stiff DC voltage source at its terminals. Because of low internal impedance, the terminal voltage of a VSI remains substantially constant with variations in load. It is therefore equally suitable to single motor and multi-motor drives. Any short circuit across its terminals causes current to rise very fast, due to the low time constant of its internal impedance. The fault current cannot be regulated by current control and must be cleared by fast acting fused links. On the other hand, the CSI is supplied with a control current from a DC source of high impedance. Typically a phase control thyristor rectifier feeds the inverter with a regulated current through a large series inductor. Thus load current rather than load voltage is controlled and the inverter output voltage is dependent upon the load impedance. Because of large internal impedance, the terminal voltage of a CSI changes substantially with a change in load. Therefore, if used in a multi-motor drive, a change in load on any motor affects other motors. Hence, CSIs are not suitable for multi-motor drives. MATERIALS AND METHODS This study was conducted in 2011 in the Electrical &Electronics Engineering Department of Bharat Institute of Engineering & Technology, Mangalpally, Hyderabad,AP, India

2. MULTILEVEL INVERTERS AND MODULATING TECHNIQUES 2.1 Pulse Width Modulation(PWM) Techniques A power electronic inverter is essentially a device for creating a variable AC magnitude and frequency output from a DC input. The frequency of the output voltage or current is readily established by simply switching for equal time periods to the positive and negative DC bus and appropriately adjusting the half cycle period. However the variable frequency ability is accompanied by a corresponding need to adjust the amplitude of fundamental component of the output waveform as the frequency changes i.e., voltage control. One of the widely utilized strategies for controlling the AC output of power electronic converters is the PWM [4] Technique. This varies the duty cycle of the inverter switches at a high frequency to achieve a target average low-frequency output voltage or current. Modulation theory has been a major research area in power electronics for over three decades and continues to attract considerable attention and interest. On the other hand, there have been a number of clear trends in the development of PWM concepts and strategies since 1970s, addressing the main objectives of reduced harmonic distortion and increased output magnitudes for a given switching frequency and the development of modulation strategies to suit different converter topologies. Principle of PWM Fig. 2.1 illustrates the circuit model of a singlephase inverter with a center-tapped grounded DC bus and Fig. 2.2 illustrates the principle of PWM. Fig. 2.2 Pulse Width Modulation(PWM) From Fig. 2.2 the inverter output voltage is determined in the following 1. When, = /2 2. When, = /2 M=, (1)..(2) 3. MODULATION TECHNIQUES FOR DIODE CLAMPED MULTILEVEL INVERTER 3.1 Third Harmonic Injected PWM Fig. 2.1 Circuit Model of Single - Phase Inverter The reference ac waveform is not sinusoidal as illustrated in Fig. 3.1 but consists of both fundamental component and a third harmonic component. As a result, the resulting peak to peak amplitude of the resulting reference function does not exceed the dc supply voltage, but the fundamental component is higher than the available supply. The presence of exactly the same third harmonic component in each phase results in an effective cancellation of the third harmonic component at the neutral terminal and all sinusoidals with peak amplitude. This is approximately 15.5% higher in amplitude than that achieved by the sinusoidal PWM. Therefore, the third harmonic PWM provides better utilization of the dc supply voltage.

= +j.(8) 4.2 PRINCIPLE OF SPACE VECTOR MODULATION An inverter is now-a-days commonly used in variable speed ac motor drives to produce a variable, three phase ac output voltage from a DC voltage. Since AC voltage is defined by two characteristics, amplitude and frequency, it is essential to work out a strategy that permits control over both these quantities. PWM controls the average output voltage in a sufficiently small period, called switching period, by producing pulses of variable duty-cycles [3]. Here, sufficiently small means the switching is small compared to the desired output voltage which may be considered as equal to desired. Fig. 3.1 Third Harmonic Injected PWM with Triangular Carriers for Multilevel Inverter 4. SPACE VECTOR MODULATION (SVM) 4.1 INTRODUCTION The 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 inverter Since + + =0; = + ;..(4) = + ;.(5) = + ; (6) = ( )/3..(7) Where is the common mode voltage From 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 two rectangular components namely and as in Eqn. (7). It is customary to place the α-axis along the A-phase axis of the motor. Hence: = (9) Also, the relationship between switching variable vector [a b c] t and line-line voltage vector [ ] t can be expressed in Eqn. (10) = (10) As illustrated in Fig. 4.2 there are eight possible combinations of on and off patterns for the three upper power switches [11]. The on and off states of the lower power devices are opposite to the upper one and so are easily determined once the states of the upper power transistors are determined. According to Eqns.(4),(5),(6), the switching vectors, output line to neutral voltage, and

output line-line voltages in terms of DC link are given in table 4.1 and Fig. 4.2 shows the eight inverter voltage vectors ( to ) Table 4.1 Switching vectors, line to neutral voltages and line to line voltages Voltage Vector Switching Vectors a b c Line to neutral voltage Line to line voltage 0 0 0 0 0 0 0 0 0 1 0 0 2/3-1/3-1/3 1 0-1 1 1 0 1/3 1/3-2/3 0 1-1 0 1 0-1/3 2/3-1/3-1 1 0 0 1 1-2/3 1/3 1/3-1 0 1 0 0 1-1/3 2/3 2/3 0-1 1 1 0 1-2/3 1/3 1/3 1-1 0 1 1 1 0 0 0 0 0 0 Fig. 4.3 illustrates the basic circuit for the three-level DC3LI. The circuit employs 12 power switching devices and 6 clamping diodes (D 1 -D 6 )and the DC bus voltage is split into three-levels(+v dc /2, 0,-V dc /2). Thus, the voltage stress of the switching device is greatly reduced. The output 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. For voltage +V dc /2, S a1 and S a2 need to be turned on. We can define these states as 2, 1, and 0, respectively [12].The switching variable S a in table 4.4,is similar to three-phase two-level inverter, the switching states of each bridge leg of three-phase three-level inverter is described by using switching variables S a, S b and S c.the difference is that, in three-level inverter, each bridge leg has three different switching states. Table 4.4 Switching variables of phase A V ao S a1 S a2 S' a2 S' a1 S a +V dc /2 ON ON OFF OFF 2 0 OFF ON ON OFF 1 -V dc /2 OFF OFF ON ON 0 Using switching variable S a and DC bus voltage V 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 be expressed as follows: V ab = V ao - V bo = 1/2*V dc (S a -S b )...(12) 4.4 SPACE VECTOR PWM FOR THREE LEVEL INVERTER Fig. 4.2 Inverter voltages vectors ( to ) 4.3 OPERATION OF THREE-PHASE THREE-LEVEL INVERTER There are altogether 27 switching states in a DC3LI. They correspond to 19 voltage vectors whose positions are fixed. These space voltage vectors can be classified into four groups, where the first group corresponds 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 voltage vectors (V8-V13) and finally the fourth group consists of small voltage vectors (V1-V6). The last three groups can be distinguished into three hexagons illustrated in Fig. 4.6. Fig. 4.3 Power circuit for Three-phase three-level inverter Fig. 4.4 Space Vector hexagon

The plane can be divided into 6 major triangular sectors (1-6). Each major section represents pi/3 of the fundamental cycle. Within each major sector, there are 4 minor triangular sectors. There are totally 24 minor sectors in the plane and the vertices of these sectors represent the voltage vectors. The modulation ratio of three-phase three-level inverter is represented as follows: M= / (2/3V d ) = 3 /2V d (15) Table 4.5 The switching states (27 states) for a threelevel inverter THE SWITCHIN G STATES S a S b S c V S 1 0 0 0 V 0 S 2 1 1 1 V 7 S 3 2 2 2 V 14 S 4 1 0 0 V 1 S 5 1 1 0 V 2 Fig. 4.5 Space Vector hexagon displaying switching states S 6 0 1 0 V 3 S 7 0 1 1 V 4 S 8 0 0 1 V 5 S 9 1 0 1 V 6 S 10 2 1 1 V 1 S 11 2 2 1 V 2 Fig. 4.6 Three-level inverters hexagons (a) Small hexagon (b) Medium hexagon (c) Big hexagon In three-phase three-level inverter, when the rotating voltage vector falls into one certain sector, adjacent voltage vectors are selected to synthesize the desired rotating voltage vector based on the vector synthesis principle, resulting in three-phase PWM waveforms. By the examination of the phase angle and the magnitude of a rotating reference voltage vector V*, the sector wherein V* resides can be easily located. From table 4.5, each small voltage vector and zero voltage vector 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 following equations can be written: X+Y+Z=1... (13.1) S 12 1 2 1 V 3 S 13 1 2 2 V 4 S 14 1 1 2 V 5 S 15 2 1 2 V 6 S 16 2 1 0 V 8 S 17 1 2 0 V 9 S 18 0 2 1 V 10 S 19 0 1 2 V 11 S 20 1 0 2 V 12 S 21 2 0 1 V 13 S 22 2 0 0 V 15 S 23 2 2 0 V 16 S 24 0 2 0 V 17 S 25 0 2 2 V 18 S 26 0 0 2 V 19 V x *X +V y *Y+ V z *Z =.(14) S 27 2 0 2 V 20

Where is the magnitude of the reference voltage vector, which rotates with an angular speed of ω=2 f in d-q coordinate plane and 2/3 V dc is magnitude of the large voltage vector, e.g., V 13.states: + V dc /2, 0, and - V dc /2 5. SIMULATION RESULTS AND DISCUSSION Fig. 5.1 illustrates the Line Voltage of DC3LI with SPWM and Fig. 5.2 illustrates the THD spectrum of DC3LI with SPWM.In this modulation technique the fundamental 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-LEVEL INVERTER 5.1.1. Sinusoidal Pulse Width Modulation (SPWM) Fig. 5.3 Line Voltage of DC3LI with SVPWM Fig. 5.1 Line Voltage of DC3LI with SPWM MODULTION TECHNIQUES DC3LI THD Fundamental Component Sinusoidal PWM 29.95% 200.1 V SVPWM 20.31% 173.1.7 V Fig. 5.2 THD spectrum of DC3LI with SPWM Fig. 5.4 THD spectrum of DC3LI with SVPWM

Fig. 5.4 illustrates the Line Voltage of DC3LI with SVPWM and the THD spectrum of DC3LI with SVPWM. In this modulation technique, the fundamental voltage is 312.7 V and THD is 23.20%. 6. CONCLUSIONS AND FUTURE SCOPE 6.1 Conclusions Diode Clamped Multi-Level Inverter topologies are developed with 3-levels for various modulation techniques i.e., Sinusoidal PWM, Space Vector PWM and DC3LI topology. Space Vector PWM technique gives lesser THD compared to that of Sinusoidal PWM. 6.2 Scope For Future Work The presented simulink model can be setup experimentally using semi conductor devices like Transistor, Thyristor, MOSFET etc., and controlling strategies by using any of the following devices like microprocessor, microcontroller, digital signal processor and FPGA. The controlling strategies can be implemented using m-file programming with FPGA for better processing speed and performance. ACKNOWLEDGEMENT Authors acknowledge the support, encouragement and facilities provided by the Electrical&Electronics Engineering Department and management of Bharat Institute of Engineering & Technology (BIET),Mangalpally, ibrahimpatnam,hyderabad,ap, India in 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 new neutral-point clamped PWM inverter, IEEE Trans. Ind. Applications., vol. IA-17, pp. 518 523, Sept./Oct. 1981. [3] T. A. Meynard and H. Foch, Multi-level choppers 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 for static var compensation, in Conf. Rec. IEEE- IAS Annu. Meeting, Oct. 1994, pp. 921 928. [5] P. Hammond, A new approach to enhance power 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, A new medium voltage PWM inverter topology for 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, Electric power converter, U.S. Patent 3 867 643, Feb. 1975. [8] R. H. Baker, Switching circuit, U.S. Patent 4 210 826, July 1980. [9] Bridge converter circuit, U.S. Patent 4 270 163, May 1981. [10] P.W. Hammond, Medium voltagepwmdrive and method, U.S. Patent 5 625 545, Apr. 1997. [11] F. Z. Peng and J. S. Lai, Multilevel cascade voltage-source inverter with separate DC sources, U.S. Patent 5 642 275, June 24, 1997. [12] P.W. Hammond, Four-quadrant AC-AC drive and method, U.S. Patent 6 166 513, Dec. 2000. [13] M. F. Aiello, P. W. Hammond, and M. Rastogi, Modular multi-level adjustable supply with series connected active inputs, U.S. Patent 6 236 580, May 2001. [14] RODRÍGUEZ et al.: MULTILEVEL INVERTERS 737 Modular multi-level adjustable supply with parallel connectedactive inputs, U.S. Patent 6 301 130, Oct. 2001. AUTHOR S BIOGRAPHY Dr. JBV Subrahmanyam is a Doctorate in Electrical Engineering from JNTU-Hyderabad, India, with two decades of rich experience in teaching, training, research, industry, projects and consultancy. He published 15 research papers in reputed international journals and 20 papers in international and national conferences.his research interest is in automation of power systems. He is an expert in condition monitoring of industrial equipment through modern diagnostic techniques. He implemented

the latest GPS and GIS technologies in many power utilities in India successfully. He executed many international and national level technical projects effectively funded by Power Finance Corporation, Ministry of Power, Government of India, APDRP, DRUM, USAID and DFID-UK. Mr. Sankar is a faculty in electrical engineering department of HITS, Hyderabad, India, with many years of rich experience in teaching, training, research. His research interest is in automation of power systems.