Transient Analysis of Z-Source Inverter Fed Three-Phase Induction Motor Drive by Using PWM Technique

Transcription

1 Transient Analysis of Z-Source Inverter Fed Three-Phase Induction Motor Drive by Using PWM Technique Jaswant Singh Dept. of Electrical Engineering, Shri Ram Group of Colleges (SRGC), Muzaffarnagar (U.P.), India. Abstract- This paper presents Z-Source inverters which have recently been proposed as an alternative power conversion concept for adjustable speed AC drives (ASD). It has both voltages buck and boost capabilities as they allow inverters to be operated in the shoot through state. It utilizes an exclusive Z-Source network (LC component) to -link the main inverter circuit to the power source (rectifier). By controlling the shoot-through duty cycle, the inverter system using IGBTS, reduces the line harmonics, improves power factor, increases reliability and extends output voltage range. When this proposed strategy considers like the inverter as a single unit, it greatly reduces the complexity and cost when compared with traditional systems. It has reduced harmonics, low switching stress power and low common mode noise. Keywords: Induction Motors (IM), Input Filter, Pulse Width Modulation (PWM), Shoot- through state, Z-source inverters. I. INTRODUCTION In this paper, a functional model of Z-source inverter and PWM modulated Z-source inverter (PWM VSI) using switching function based on PWM approach concept is studied and the Simulation of the developed model is proposed with the help of MATLAB/Simulink. The Traditional Inverters are voltage source inverter (VSI) and current source inverter (CSI) which consists of a diode rectifier front end, link and Inverter Bridge. In order to improve power factor, either an ac inductor or inductor is normally used. The link voltage is roughly equal to 1.38 times the line voltage. The voltage source inverter is a buck converter that produces only an ac voltage, which is limited by the link voltage. Because of this nature, the voltage source inverter and current source inverter are characterized by relatively low efficiency because of switching losses and considerable EMI generation. Inverter presents negligible switching losses and EMI generation at the line frequency. The voltage source inverter requires an output RLC filter to provide sinusoidal voltage compared with current source inverter. The RLC output filters causes additional power loss and control complexity. The voltage source converter is widely used. The switching function concept is a powerful tool in understanding and optimizing the performance of the static power converter/inverters. With the developed functional model, the simplification of the static power circuits can be achieved so that the convergence and long run-time problems. II. Z-SOURCE ASD SYSTEM Z-Source inverter based induction motor drives provides a low cost and highly efficient two stage structure for reliable operation. It consists of voltage source for the supply of rectifier section, impedance network, which consist of two equal inductors and two equal capacitors, three phase inverter and three phase induction motor. The rectification of ac voltage is done by rectifier section to obtain voltage for further supply. The rectifier output voltage is now fed to the impedance network. The network inductors are connected in series arms and capacitors are connected in diagonal arms as shown in fig.1. Depending upon the boosting factor capability of impedance network the rectified voltage is buck or boost upto the voltage level of the inverter section (not exceed to the bus voltage) [7]. This network also act as a second order filter and it should required less inductance and less capacitance. This paper addressed an efficient PWM based z-source inverter approach for the control of adjustable speed drive polyphase Induction motor. The Z-source inverter advantageously utilizes the shoot through states to boost the bus voltage by gating on both the upper and lower switches of the same phase leg [6]. Shoot through mode allows simultaneous conduction of devices in same phase leg. Therefore, On behalf of boost factor of -link, a Z-Source inverter can boost or buck to the voltage to a desired output voltage that is greater / lesser than the bus voltage[7][12]. *Corresponding Author: Jaswant Singh are Asst. Prof. & Head in Dept. of Electrical Engineering, Shri Ram Group of Colleges (SRGC), Muzaffarnagar (U.P.), India sinjaswant@gmail.com ). 856

2 Fig. 1. Main circuit configuration of proposed Z-source inverter ASD system. The voltage buck and boost capability cannot be achieved by the conventional converters, but it is easily achieved by the proposed model. As shown in fig. (1), the inverter main circuit consists of six switches. These inverters use a unique impedance network (LC), coupled between the rectifier and inverter circuit, to provide both voltage buck and voltage boost properties [17]. The unique feature of the Z-Source inverter is that the output ac voltage can be any value between zero and infinity regardless of voltage. However, three phase Z-Source Inverter Bridge has one extra zero state when the load terminals are shorted through both the upper and lower devices of any one phase leg, any two phase legs, or all three phase legs. This shoot-through zero State is forbidden in the traditional voltage source inverter, because it would cause a shoot-through. The Z-Source network makes the shoot-through zero state efficiently utilized throughout the operation. The Z-Source inverter Adjustable Speed Drive (ASD) system has many ASD applications such as: The upper and lower devices of each phase leg cannot be switched on simultaneously either by purpose or by EMI noise. Otherwise, a shoot through would occur and destroy the devices. Dead-time to block both upper and lower devices has to be provided in the voltage source converter, which causes waveform distortion, etc. An output RLC filter is needed for providing a sinusoidal voltage compared with the current source inverter, which causes additional power loss and control complexity. A current source feeds the main converter circuit, a threephase bridge. The current source can be a relatively large inductor fed by a voltage source such as a battery or diode rectifier. Six switches are used in the main circuit; each is composed traditionally by a semiconductor switching device with reverse block capability. However, the current source converter has the following conceptual and theoretical barriers and limitations. Steel mills machines, Paper machines (winder, tension reels, mill stands) Cement mills, rubber mills, mixers, crushers Conveyors Cranes and elevators cars Variable Torque applications: Centrifugal pumps Centrifugal fans A. Voltage Source Converter: Barriers and Limitations The ac output voltage is limited below and cannot exceed the bus voltage or the bus voltage has to be greater than the ac input voltage. Therefore, the voltage source converter is a boost rectifier for ac to- power conversion and the voltage source inverter is a buck inverter for -toac power conversion. For applications where over drive is desirable and the available voltage is limited, an additional - boost converter is needed to obtain a desired ac output. The additional power converter stage increases system cost and lower the efficiency. Fig. 2. Voltage Fed Z-Source inverter for ASD. B. Current Source Converter: Barriers and Limitations The ac output voltage has to be greater than the original voltage that feeds the inductor or the voltage produced is always smaller than the ac input voltage. Therefore, the current source inverter is a boost inverter for to-ac power conversion and the current source converter is a buck rectifier for ac-to- power conversion. For applications where a wide voltage range is desirable, an additional - buck converter is needed. At least one of the upper devices and one of the lower devices have to be gated on and maintained on at any time. Otherwise, an open circuit of the inductor would occur and destroy the devices. Overlap time for safe current commutation is needed in the current source converter, which also causes waveform distortion, etc. In addition, both the voltage source converter and the current source converter have the following common problems. 857

3 They are either a boost or a buck converter and cannot be a buck-boost converter. That is, the output voltage range is limited to either greater or smaller than the input voltage. Fig. 5. Equivalent circuit when ZSI in non shoot through state. Fig. 3. Current Fed Z-Source inverter for ASD. III. MATHEMATICAL ANALYSIS OF IMPEDANCE NETWORK The impacts of the phase leg shoot through on the inverter performance can be analyzed using the equivalent circuit shown in Fig. 4 and Fig. 5. Assume the inductors (L1 and L2) and capacitors (C1 and C2) have the same inductance and capacitance values respectively; the Z-source network becomes symmetrical. Fig. 4. Equivalent circuit when ZSI in shoot through state. V V V V V V c1 c2 c L1 L2 L V V V 2V d L c c V 0 i Alternatively, when in non shoot through active or null state current flows from Z-Source network through the inverter topology to connect ac load during time interval T1.The inverter side of the Z- source network can now be represented by an equivalent circuit as shown in Fig.5. The following equations can be written. V V V V L c d V V V V 2V V i c L c (1) (2) Averaging the voltage across a Z-source inductor over a switching period (0 to T), T1 Vc V (3) ( T T ) 1 0 Using equations (1) and (2) The peak DC-link voltage across the inverter bridge is 1 Vi 2V C V V (4) 2T0 1 T Vi BV. (5) where, T B i. e. 1 (6) T1 T0 B is a boost factor, T-Switching period The peak ac output phase voltage, For Z- source M. Vi B. MV Vac (7) 2 2 In the traditional sources MV. Vac (8) 2 where M is modulation index. The output voltage can be stepped up and down by choosing an appropriate buck Boost factor B B. M(it varies from 0 to ) (9) B The Buck - Boost factor BB is determined by the modulation index M and the Boost factor B. The boost factor B can be controlled by duty cycle of the shoot through zero state over the non-shoot through states of the PWM inverter. The shoot through zero state does not affect PWM control of the inverter, because it equivalently produce the same zero voltage to the load terminal. The available shoot through period is limited by the zero state periods that are determined by the modulation index. IV. MODULATION METHOD 858

4 PWM inverters can be of single phase as well as three phase types. Their principle of operation remains similar and hence in this paper the emphasis has been put on the more general, 3-phase type PWM inverter. These inverters are capable of producing ac voltages of variable magnitude as well as variable frequency. The PWM inverters are very commonly used in adjustable speed ac motor drive loads where one needs to feed the motor with variable voltage, variable frequency supply. For wide variation in drive speed, the frequency of the applied ac voltage needs to be varied over a wide range. The applied voltage also needs to vary almost linearly with the frequency. Carrier-based PWM methods are preferred in implementing modulators for inverters as they are simple and easy to realize in shown in fig. 6 & 7 To date, there are three types of carrier-based modulation schemes proposed to modulate single-stage Z-source inverters [4], [5], [16]. In simple boost modulation method, the shoot-through period is fully inserted within the traditional null period, and this is achieved simply by comparing a constant reference value with a carrier signal. With the second method, the total null period is occupied by shoot-through period and is known as maximum boost controlling. Although this method does not increase the total number of switching s per half cycle, it is found to be producing poor dynamic performance under transient conditions [17], [19]. The modulation method proposed in [15], has shoot-through period carefully inserted between the state changes from active to active and active to null. This minimizes the number of switching s per half carrier cycle and achieves improved spectral characteristics. In this topology, two inverters are connected to a single source through a common Z-source impedance network. These techniques are commonly used for the control of ac induction, Brushless Direct Current (BLDC) and Switched Reluctance (SR) motors. As a result, PWM converter powered motor drives offer better efficiency and higher performance compared to fixed frequency motor drives [2]. Fig. 7. Generation of switching signals with interleaved carrier-based PWM. Hence, modulation schemes may need to be modified to suit the proposed topology. There are two possibilities in deriving the modulation signals. The first and obvious method is to modulate the two inverters from a common carrier signal with careful insertion of shoot-through time with simple boost or minimum switching [4], [15] methods proposed for a single Z-source inverter. This pulse is used to switch ON or OFF the power switches. The width of the pulse or duty cycle can be varied by varying the frequency of the reference wave. Since the Z-source inverter bridge can boost the capacitor (C 1 and C 2 ) voltage to any value that is above the average value of the rectifier, a desired output voltage is always obtainable regardless of the line voltage. Here inverter bridge switching is provided by pulse width modulation generator. In order to show clearly the output voltage obtains from inverter an RLC filter is placed between the Inverter Bridge and induction motor. Simulation parameters for z- source are given as follows: L 1=L 2= (100e-9) H C 1=C 2= (1000e-6) F Fig. 6. PWM Pulse Generation Circuit V. SIMULATION RESULTS AND DISCUSSION Fig. 8 shows the main circuit configuration of the z-source fed pwm induction motor drive, similar to that of the traditional ASD system. The z-source ASD system s main circuit consists of three parts: a diode rectifier, link circuit and inverter bridge. 859

5 that speed reaches at steady state value that is 1718 rpm with in 1.09 second when motor is subjected to constant load 11.9N-m. So when the motor is fed by Z-source inverter then its speed increases and setling time decreases.and it is due to voltage after inverter circuit which boosted to 218V by Z-source inverter. Electromagnetic torque waveform is shown in fig. 18. Fig.8. Simulink Model for Z-source fed PWM - IM Drive. To confirm the operating principle of the new ASD system, simulations have been carried out on simulink modeling. In order to show clearly the output voltage obtained from the inverter, an output RLC filter is placed in between the inverter bridge and the motor. Different cases are considered for showing the parameter variation in the value of load. Fig. 10. Waveforms of link voltage of Z Source. Case 1: Case 2: Case 3: full load, (T fl = 11.9N-m) under load condition, (T ul = 8 N-m) Free acceleration condition, (T fa =0 N-m) Fig.11. Inverter output voltage before filter is V peak rms = V. Fig.12. Inverter output voltage after filter is V rms=220 V. Fig. 9. Input Voltage waveform. Case-1 Response of Induction motor for full load (T fl = 11.9N-m) Fig.10 shows the waveforms of -link voltage and current of Z- source fed pwm induction motor drive. Here -link voltage is boosted to 309 V due to z-source. The link voltage is roughly equal to 1.38 times the line voltage (220 V). This shows that the Z- source inverter is a can only produce an ac voltage which is not limited by the link voltage. Fig.11, 18 shows inverter voltage before output filter and load voltage after output filter circuit. After output filter load voltage is V rms =220 V and before output filter is V rms =220 V and V peakrms =307.8 V Transients in stator and rotor currents are there for short span of time that is it settles quickly as shown in fig 15, 18. The starting current is high but within 1.16 second, it reaches to steady state value. Steady state value of stator current is19.06 A. Steady state value of rotor current is A. The result for the speed estimation are shown in figure 17. It can be observed Fig.13. Inverter Load voltage after output filter V peak rms=304.5 V Fig. 14. Inverter Load voltage after output filter is V rms = 220 V. 860

6 Fig. 15. Rotor current/phase i r under full load condition i r=19.06a. Fig. 19. Rotor current/phase i r for under load condition. Fig. 16. Stator current/phase i s under full load condition i s =17.09A. Fig. 20. Stator Current per phase i s for under load condition. Fig. 17. Rotor Speed N r under full load condition ns=1714, ts=0.89. Fig. 21. Rotor Speed in rpm (N r) for under load condition. Fig. 18. Electromagnetic Torque T e under full load condition. Case-2) Response of Induction motor for under load condition (T under load = 8 N-m) The result for the speed estimation are shown in figure It can be observed that speed reaches at steady state value that is 1745 rpm with in second when motor is subjected to constant load 8 N-m. So when the motor is fed by z-source inverter then its speed increases and setling time decreases. Also the waveform for Rotor speed (rpm), input voltage, per phase rotor current, per phase stator current and also the waveform of electomagnetic torque is shown in fig Fig. 22. Electromagnetic torque T em for under load condition. Case-3) Response of Induction motor at No load condition (T nl =0 N-m) The result for the speed estimation are shown in fig It can be observed that speed reaches at steady state value that is rpm with in second when motor is subjected to constant load 0 N- m.so when the motor is fed by z-source inverter then its speed increases and setling time decreases. Also the waveform for Rotor speed (rpm), input voltage, per phase rotor current, per phase stator current and also the waveform of electomagnetic torque is shown in fig

7 Fig. 23. Rotor current i r at No load condition. PWM allows the operation of inverter in over modulation region. This proposed strategy considers the inverter as a single unit and greatly reduces the complexity and cost when compared with traditional systems. It has reduced harmonics, low switching stress power and low common mode noise. Simulation has been performed for 3 HP, 220 V, 60 Hz, 1725 rpm, induction motor with PWM z-source inverter and the results are show that to verify these new features. By comparison we conclude that z-source fed pwm induction motor drive is more efficient over Traditional variable speed drive system. Because of inverter output voltage is more boost up than that of Traditional variable speed drive system. VII. REFERENCES Fig. 24. Fig. 25. Fig. 26. Stator current/phase i s at No load condition. Rotor speed in rpm N r at No load condition. Electromagnetic torque T em at No load condition. VI. CONCLUSIONS In this paper, Induction motor with Z-source inverter are proposed and simulated in SIMULINK/MATLAB. This paper presents a new PWM adjustable speed drive system based on the Z-source inverter topology. The performance of three phase induction motor is analyzed by using this technique, Simulation results are analyzed by the output waveforms in term of induction motor outputs (performance parameters). A Pulse Width modulation technique is used for generating a desired value of pulses by using an appropriate value of switching frequencies. Performance of 3-phase Induction motor is investigated for the different load conditions and their comparison is also presented in this work. [1] Amitava Das, S.Chowdhury, S.P.Chowdhury, Prof. A. Domijan Performance Analysis of Z source Inverter Based ASD System with Reduced Harmonics IEEE Trans. Ind. Appl., pp. 1 7, [2] Poh Chiang Loh, Feng Gao, Pee-Chin Tan, and Frede Blaabjerg Three-Level AC DC AC Z-Source Converter Using Reduced Passive Component Count IEEE, Transactions On Power Electronics VOL. 24, NO. 7, JULY [3] Kuo-Kai Shyu and Hsin-Jang Shieh Variable Structure Current Control for Induction Motor Drive By Space Voltage Vector PWM IEEE Trans. on Indu. Electronics, vol. 42, no. 6, pp , DECEMBER [4] Poh Chiang Loh, Feng Gao and Frede Blaabjerg Topological and Modulation Design of Three-Level Z-Source Inverters IEEE Transactions On Power Electronics, VOL. 23, NO. 5, SEPTEMBER [5] D. G. Holmes and B. P. Mcgrath, Opportunities for harmonic cancellation with carrier-based PWM for a two-level and multilevel cascaded inverters, IEEE Trans. Ind. Appl., vol. 37, no. 2, pp , Mar./Apr [6] Fang Zheng Peng, Alan Joseph, JinWang, Miaosen Shen, Lihua Chen, Zhiguo Pan, Yi Huan, Z-Source Inverter for Motor Drives IEEE transactions on power electronics, vol. 20, no. 4, july2005. [7] M. Shen, J.Wang, A. Joseph, F. Z. Peng, L. M. Tolbert, and D. J. Adams, Maximum constant boost control of the Z-source inverter, presented at the IEEE Industry Applications Soc. Annu. Meeting, [8] M. H. Rashid, Power Electronics, Circuits Devices and Applications, Prentice-Hall International Editions, N. Mohan, T. M. Undeland, and W. P. Robbins, Power Electronics Converters, Applications, and Design, John Wiley and Sons, [9] S. Thangaprakash, Dr. A. Krishnan, Z-source Inverter Fed Induction Motor Drives a Space Vector Pulse Width Modulation Based Approach Journal of Applied Sciences Research, 5(5): , [10] F. Z. Peng, Z-source inverter, IEEE Trans. Ind. Applicat., vol. 39, no.2, pp , Mar./Apr

8 [11] E. P.Wiechmann, P. D. Ziogas, and V. R. Stefanovic, Generalized functional model for three phase PWM inverter/rectifier converters, in Conf. Rrec. IEEE-IAS Annu. Meeting, 1985, pp [12] S. R. Bowes, New sinusoidal pulse width modulated inverter, Proc. Inst. Elect. Eng., vol. 122, pp , [13] J. Holtz, W. Lotzkat, and A. Khambadkone, On continuous control of PWM inverters in the over modulation range including the six-step mode, in Proc. IEEE IECON 92, 1992, pp [14] Poh Chiang Loh, D. Mahinda Vilathgamuwa, Chandana Jayampathi Gajanayake, Yih Rong Lim, and Chern Wern Teo, Transient Modeling and Analysis of Pulse-Width Modulated Z-Source Inverter IEEE Transactions On Power Electronics, VOL. 22, NO. 2, pp , MARCH [15] W. Leonhard, Control of electrical drives, 2nd Ed, Springer, [16] Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao,Electrical Machines II, National Programme on Technology Enhanced Learning. W. Leonhard, Control of electrical drives, 2nd Ed, Springer, [17] Jin Li1, Jinjun Liu1 and Liu Zeng1, Comparison of Z-Source Inverter and Traditional Two-Stage Boost-Buck Inverter in Grid-tied Renewable Energy Generation IEEE Transactions On Power Electronics,VOL., NO., pp , dec [18] Yu Tang, Shaojun Xie, Chaohua Zhang, and Zegang Xu, Improved Z- Source Inverter With Reduced Z-Source Capacitor Voltage Stress and Soft-Start Capability IEEE, Transactions On Power Electronics,VOL. 24, NO. 2, FEBRUARY [19] Poh Chiang Loh, Feng Gao, Frede Blaabjerg, Shi Yun Charmaine Feng, and Kong Ngai Jamies Soon, Pulsewidth-Modulated Z-Source Neutral-Point-Clamped Inverter IEEE Transactions On Industry Applications, VOL.43,NO.5, pp , SEPTEMBER/OCTOBER [20] Seyed Mohammad Dehghan, Mustafa Mohamadian, Ali Yazdian, and Farhad Ashrafzadeh, A Dual-Input Dual-Output Z-Source Inverter IEEE Transactions On Power Electronics, VOL. 25, NO. 2, pp , FEBRUARY [21] Fang Zheng Peng, Miaosen Shen, and Zhaoming Qian, Maximum Boost Control of the Z-Source Inverter IEEE Transactions On Power Electronics, VOL. 20, NO. 4, pp , JULY Head in He is currently an Asst. Professor & Head in Department of electrical engineering from Shri Ram Group of Colleges (SRGC), Muzaffarnagar (U.P.), India, where he has been since August He has authored or coauthored 20 publications on power electronics, control and simulation of electrical machines and drives. His areas of interest in research are power electronics & drives and power quality problems. BIBLIOGRAPHIES Jaswant singh was born in Firozabad, (U.P), India in He received the B.Tech. degree in Electrical Engineering in 2009 from RGEC, Meerut, India, and M. Tech. in Electrical engineering (Power electronics & drive) from the Kamla Nehru Institute of Technology (KNIT), Sultanpur, (U.P.), 2011, India. In 2011, he joined the Department of Electrical & Electronics Engineering, P.K. Institute of Technology & Management, (PKITM), Mathura, U.P., India, as an Asst. Prof. & 863