Performance Analysis of Modified Z- Source Inverter for Renewable Energy System Using Modified Space Vector Pulse Width Modulation K. Mahendran Department of Electrical and Electronics Engineering, Vivekananda College of Technology for Women, Thiruchengode, Namakkal, TamilNadu, India. mahae1987@gmail.com Abstract-- The peak need of alternative energy resource to satisfy the demand in energy also elicited the research in the area of power converters. The Z-Source Converter which can be used for both current and voltage sources is used for both buck and boost operation in various renewable energy applications like wind and solar energy conversion system. To improve the operation of the converter further, different topologies are being proposed. The T-source inverter is the modified form of Z-source inverter with the reduced number of passive components. The paper proposes the modified space vector pulse width modulation for the modified Z-source inverter or T-source inverter. The performance of the T- source inverter is analyzed based on the gate pulse fed by the modified space vector pulse width modulation generator. The system level simulation is performed using MATLAB/SIMULINK software. Keywords- Z-source inverter (ZSI), T-source inverter (TSI), PWM, space vector pulse width modulation. I. INTRODUCTION The increase in energy demand on the other hand triggers the research on power converters. The various renewable energy resources like wind, solar, tidal, etc can be used with maximum efficiency by using appropriate power converter. As the availability of these resources is greatly uncertain the power conversion system relies on the suitable power converter and controller unit such that they deliver constant output voltage with constant frequency. The z-source inverter designed by F.Z.Peng in the year 2003 overcomes the difficulties in conventional voltage and current source inverters. The major advantage of the Z-source inverter is that it can be used for both current and voltage sources without any change in circuit design. Besides both buck and boost operation can be accomplished using-source inverter. The power conversion stages also get reduced resulting in improved efficiency of the system in energy conversion process. To improve the performance of the inverter further different topologies of Z-source inverter has been proposed by various researchers by different possible combination of arrangement of passive components. Among different topologies of Z-source inverter, the topology that is being considered for analysis is T-source inverter. As the inverter is derived from the root of Z- source inverter and the arrangement of passive components looks like the alphabet T, the inverter is called modified Z-source inverter or T-source inverter. The boost operation of the inverter is achieved using the concept of shoot-through time period. Different PWM techniques are available in literatures to control the gating pulse to the inverter switches. The method considered throughout the paper for analysis is space vector pulse width modulation. Better DC voltage utilization is achieved using this space vector pulse width modulation than any other pulse width modulation techniques. The organization of the paper is as follows: Section II describes the modified Z-source inverter or T- source inverter. Section III inculcates modified space vector pulse width modulation. Simulation results and discussions are held in section IV. ISSN : 2249-913X Vol. 1 No. 2 Dec 2011-Feb 2012 186
II. MODIFIED Z-SOURCE INVERTER Inverters are mainly classified as the Voltage Source Inverters (VSI) and Current Source Inverters (CSI). The VSI is one in which the dc source has small or negligible impedance. In other words, a voltage source inverter has stiff dc voltage source at its input terminals. Figure 1.T Source Inverter Because of low internal impedance, the terminal voltage of a voltage source inverter remains substantially constant with variations in load. On the other hand, the current source inverter is supplied with a controlled current from a dc source of high impedance. The load current rather than the 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 current source inverter changes substantially with a change in load. The ZSI [F.Z.Peng, 2003], overcomes the limitations of VSI in the traditional WECS. It is a single stage buck-boost dc-ac converter hence the efficiency is improved over the traditional WECS [Jin Li, et. al, 2009]. The Z-source inverter employs a unique impedance network coupled between the variable dc voltage from the rectifier and the with the inverter main circuit. It consists of the rectifier, impedance network and three phase inverter. The impedance network consist of two equal inductors (L 1, L 2 ) connected in series arms and two equal capacitors (C 1, C 2 ) connected in diagonal arms (F. Z. Peng, et al., 2005). The impedance network either bucks or boosts the input voltage depending upon the boost factor. The T-source inverter is the modified form of Z-source inverter which is achieved by modification in the impedance network. The series arm inductors L 1 and L 2 and the diagonal arm capacitors C 1 and C 2 are replaced with impulse transformer with small inductive leakage and a capacitor C. the reduction in passive components reduces harmonic content in the output voltage. Fig 1 represents the circuit of T-source inverter. The impedance network design allows the VSI to be operated in a state called the shoot-through state in which the two switching devices in the same leg are simultaneously switched-on to effect short-circuit of the dc link. During this state, energy is transferred from the inductors to capacitors, thereby giving rise to the voltage boost capability of the inverter. The absence of dead time in the gate signals, improves the power quality and transient response of the system consequently desired sinusoidal output voltage is obtained with low value of LC filter (Zhi Jian Zhou, et al., 2008). The voltage and frequency of the inverter is controlled by controlling the shoot-through zero states (F. Z. Peng et. al 2004). III. MODIFIED SPACE VECTOR PULSE WIDTH MODULATION The space vector modulation is the approximation the reference voltage by averaging the inverter output voltage in the given period same as that of the reference voltage. This helps in producing synchronized and symmetric output voltage which leads to the self balancing of the dc bus voltage over every cycle of the fundamental. Due to lower current harmonics and wide range of modulation index the SVPWM finds its major participation in the industrial drives. The reference voltage is located between two arbitrary voltage vectors V i and V i+1. (Poh Chiang Loh, et al, 2005) (Quang-Vinh Tran, et al, 2005). As T- Source Inverter have shoot-through period in addition to six active states and two null states the shoot through time period is inserted into the space vector. The insertion of additional shoot through period into the normal space region differentiates this method from the conventional space vector modulation hence the name modified space vector modulation. ISSN : 2249-913X Vol. 1 No. 2 Dec 2011-Feb 2012 187
Figure 2. Voltage space vector of three phase voltage source inverter The shoot through period insertion is accomplished without disturbing the active vectors in the space region. It is evenly assigned to each phase within the null or zero state periods. The voltage space vector of three phase voltage source inverter is given in Fig 2. The distribution of shoot-through time period at sector-i can be given as in Fig 3. Figure 3. Swtiching sequence of conventional SVPWM at sector- I The reference voltage in the space vector is given by, V ref = Vi T 1 + Vi+1 T 2 (1) Figure 4. Switching sequence of MSVPWM at sector I The modulation index in the modified space vector modulation technique can be given as follows, ISSN : 2249-913X Vol. 1 No. 2 Dec 2011-Feb 2012 188
V ref M= (2/3)V dc (2) Time period of the active voltage vector is, 3T z V ref π n-1 T= 1 Sin( -α+ π) Vdc 3 3 (3) and 3T z V ref n-1 T= 2 Sin(α- π) Vdc 3 (4) Time period of the zero vectors is, o s- 1-2 T =T T T (5) The shoot through period is given by, Tsh T = 3 Where, 0 60 T s - Total switching period T sh Shoot through period (6) IV. RESULT AND DISCUSSION In this section the performance of T-source inverter is analyzed based on different parameters using the system level simulation made using MATLAB/SIMULINK software. Fig 5 represents the simulation model for the generation of PWM pulses for TSI by using MSVPWM technique. In this method the reference voltage is located between two voltage vectors V i and V i+1. Time period for all states varies with the voltage sector. The voltage vector is then compared with ramp signal. Based on the angle of the rotating reference voltage in the six voltage space vector the pulses are generated. Figure 5. Simulation model for MSVPWM The gate pulses for the switches in the inverter circuit generated by modified space vector pulse width modulation are given in Fig 6. The pulse is produced along with shoot-through time period. The output voltage obtained for a non linear load of 1Kw from the T-source inverter is given in Fig 7. For an input of 100V an output of 420V is generated at a boost factor of 4.2. Fig 7 represents the output voltage without filter and Fig 8 represents filtered output voltage for the load of 1Kw. The active and reactive power measured for the non linear load of 1Kw is given in Fig 9. ISSN : 2249-913X Vol. 1 No. 2 Dec 2011-Feb 2012 189
Figure 6. Simulated PWM pulses by modified MSVPWM. Figure 7. Output voltage for a non-linear load of 1Kw without filter. Figure 8. Output voltage for a non-linear load of 1Kw with filter Figure 9. Active and reactive power at the output for a load 1Kw. The harmonic order of the output voltage is given in Fig 10 and the harmonic order of the input current is given in Fig 11. The percentage of harmonics in output voltage is around 2% and the same in the input current is around 25%. ISSN : 2249-913X Vol. 1 No. 2 Dec 2011-Feb 2012 190
Figure 10. Harmonic order in the output voltage of T-source inverter. Figure 11. Harmonic order in the input current fed of T-source inverter. The change in boost factor with the change in dc link voltage across the impedance network in Fig 12. Similarly the variation in harmonics for different load profile is given in Fig 13. Boost factor 8 7 6 5 4 3 2 1 0 90 110 180 230 290 330 DC link voltage (V) Figure 12. DC link voltage Vs boost factor THD(%) 30 25 20 15 10 5 0 0.25 0.5 0.75 1 Load (Kw ) Input Current THD (%) Output Voltage THD (%) Figure 13. Load Vs THD REFERENCES [1] F. Z. Peng, Z-Source Inverter, IEEE Trans. Ind. Appl., Vol. 39, No. 2, March/April 2003, pp. 504 510. [2] Ryszard Strzelecki, Marek Adamowicz, Natalia Strzelecka, Wieslaw Bury New type T Source Inverter, Power Quality, Alternative Energy And Distributed Systems, pp.191-195. [3] Poh Chiang Loh, D.Mahinda Vilathgamuwa, Yue Sen Lai, Geok Tin Chua, and Yunwei Li, Pulse-Width Modulation of Z-Source Inverters, IEEE Trans. Power Electronics, Vol. 20, No. 6, November 2005, pp. 1346 1355. ISSN : 2249-913X Vol. 1 No. 2 Dec 2011-Feb 2012 191
[4] Atif Iqbal, S.K.Moin Ahmed, Mohammed Arif Khan, Haitham Abu-Rub, Generalized simulation and experimental implementation of space vector PWM technique of a three phase voltage source inverter, International Journal of Engineering and Technology, Vol. 2, No. 1, 2010, pp. 1 12. [5] Das.A, Lahiri.D, Kar.B, Space vector PWM based output voltage control of Z-source inverter, International Conference on Control, Automation, Communication and Energy Conservation INCACEC-2009, IEEE conference, pp 1-4. [6] Von Zimmermann.M, Lochler M Piepenbreier, Z-source drive inverter using modified SVPWM for low voltage output and regeneration operation 13 th European Conference on Power Electronics and Applications-2009, pp 1-10. BIOGRAPHY K.Mahendran received his B.E degree in Electrical & Electronics Engineering from Bannari Amman Institute of Technology, Anna University-Chennai, in the year 2009 and M.E degree in Power Electronics and Drives from Bannari Amman Institute of Technology, Anna University of Technology-Coimbatore in the year 2011. Currently he is working as Lecturer in Vivekananda College of Technology for Women, Namakkal. His field of interest includes Power Converters and Renewable Energy Resources. ISSN : 2249-913X Vol. 1 No. 2 Dec 2011-Feb 2012 192