Development of a Research Platform for Power Electronic Converter Modeling in Real Time Digital Simulation Applications using F28335 eZDSP Mandar Bhalekar, Umashankar S, Surabhi Chandra, Vijayakumar D School of Electrical Engineering, VIT University, Vellore, India E-mail:
[email protected],
[email protected],
[email protected] [email protected] ABSTRACT – This paper presents the research platform
for real time digital simulation (RTS) applications which replaces the requirement of full scale or partial scale validation of physical systems. To illustrate this, a three phase AC-DC-AC converter topology has been used with diode rectifier, dc link and IGBT inverter with inductive load. This paper presents design and realization of open loop model of AC-DC-AC PWM converter. TMS 320F28335 is used for PWM pulse generation by SPWM technique. The simulation is carried out in MATLAB/SIMULINK and results of both simulation and actual model are compared. The proposed PWM algorithm has been validated in simulation and real time DSP controller domain. Keywords – real time; PWM, induction motor, back-toback converter, DSP.
I.
INTRODUCTION
During the recent years, power electronics has undergone a fast evolution which is mainly due to two factors. One of them is the development of fast semiconductor switches with high switching frequency and high power handling capability. The other factor is the exposure of real-time computer controllers that can implement advanced and complex control algorithms. These factors together have resulted in the development of cost-effective and gridfriendly converters. Power electronic converters are now used in many Grid connected applications including STATCOMs, UPFCs and active interfaces for distributed generation systems like PV or wind. Pulse Width Modulated (PWM) converters are used to overcome the problem of dominant harmonics related with input line current rectifier circuit. PWM converter helps to eliminate the higher order frequency harmonics by employing a small passive filter [1], [2]. Also, the dynamic response of PWM rectifiers is much faster with a cut down in devices bulk and weight [3]. The PWM converters can be operated in variable frequency mode. The analog circuits used for this control are made up of voltage-controlled oscillator, logic circuits, dead-band circuits and frequency limiter circuit and overload protection circuits [4]. There are disadvantages with analog control circuits like performance degradation due to
Kothari D P Director General J B Group of Institutions Hyderabad, India E-mail:
[email protected]
aging, component tolerances and drift. There is no flexibility to adoption of different control strategies. Hence, digital circuit is used. Among all PWM techniques most effective and easy to implement method is Sine Pulse Width Modulation. By adjusting the frequency of carrier wave and modulation index, pulses of required frequency and amplitude can be obtained. The SPWM technique also helps to reduce heat loss in the stator winding [5]. The digital control using a digital signal processor (DSP) provides precision and improvement in the system performance. The DSP controller gives reliability, programmability and the flexibility to adapt to the different control strategies with the same hardware. Moreover, because of high speed central processor designer can process control algorithm in real time. Owing to these reasons the digital control of the three-phase two-level voltage source converter to control output three-phase ac voltage is realized using digital signal processor. DSP TMS320F28335 can be interfaced with MATLAB thereby reducing the efforts in pulse generation. It can be programmed as per the requirement of pulses in MATLAB. Total harmonic distortion of the output voltage of inverter can be efficiently controlled by a closed loop established between output and the pulse generation circuitry. Closed loop can be formed with the help of a voltage regulator and output voltage can be controlled [5] [7]. Most of the wind energy conversion systems consist of back to back convertors after the induction generator so that a symmetric and pure sinusoidal voltage is available at the output end. The WECS is modelled in MATLAB by using an asynchronous motor [8]. In this paper, IGBT inverter converting DC to AC is given SPWM pulses. This PWM generation can be easily implemented in DSP interfaced with MATLAB. This paper is organized in the following sections. The II section describes block diagram and circuit diagram of the proposed system. The simulation part is explained in section III. Hardware circuit description and DSP implementation of proposed system is in section IV and V. Section VI concludes the paper with references in section VII.
II.
PROPOSED SYSTEM CONFIGURATION
The basic block diagram of the proposed system is given. Basically it is a back-to-back converter model normally used WECS. It consists of an uncontrolled converter at the output of wind energy conversion system, IGBT based PWM three phase inverter and a voltage controller. Voltage controller can be implemented with the help of MATLAB-DSP interface to provide gate pulses to IGBTs.
Fig. 3 Open Loop Simulation Model
Fig1. Block Diagram of Proposed Model
CIRCUIT DIAGRAM
The symmetric three phase voltage supply is taken equivalent to induction generator in wind energy conversion system. Phase to phase RMS voltage of 265.41 V is supplied as AC voltage to a universal bridge. The 4700 µF of capacitor is provided for the constant voltage at the input of inverter. The snubber resistance and capacitor is connected in parallel to the DC link capacitor to protect the inverter switches from high current. This DC voltage of 159.04 V is converted to AC voltage by IGBT inverter. The AC voltage obtained at the output of inverter Vab is 175.49V RMS. The SPWM technique is used to trigger the switches of inverter. The reference wave is compared with a sine wave to generate pulses at 2250 Hz. The amplitude of reference wave decides the generated AC voltage amplitude and the frequency of reference wave adjusts the generated AC voltage frequency. The level required to trigger IGBT is Vge (th) =15V and Ic =50mA, can be obtained easily. Figure 4 represents the pulses generated by SPWM.
Fig.2 Open Loop Circuit Diagram
Figure 2 below shows the circuit diagram for back to back converter normally used in wind energy conversion system. In this the main concentration is given on the highlighted inverter part. The pulse generation is done with digital signal processor. IGBT switches are used to have a higher current carrying capacity and high switching frequency. The capacitor provided gives the constant DC voltage at the input side of the inverter. Fig. 4 Gate pulses by SPWM method
III.
MATLAB SIMULATION
Simulation is carried out with discrete powergui of sample time 2 µsec. The simulation model is as shown in the figure 3.
An asynchronous motor is connected as a load to the inverter. A step signal of 24.88 Nm is given as mechanical torque to the motor. It is calculated depending on power and speed relation. The waveform quality can be improved by LC filter at the output of the PWM inverter. For the convenience output without LC filter is considered in this paper. The simulation run time is fixed at 0.5 sec and results are summarized.
period. The DC link voltage and phase-phase AC output voltage, current obtained are shown in figure 6, 7 and 8. 150
IV.
HARDWARE CIRCUIT DESCRIPTION
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The circuit description can be divided into two sections: The control and the power circuit. The opto-couplers are provided as a separator between them. Control circuit design: The control circuit section is having three parts namely PC, DSP board and IGBT driver. DSP is programmed to produce gate pulses by comparing a triangular carrier wave of 2250 Hz and a sinusoidal reference wave of 50 Hz. The amplitude of reference wave decides the generated AC voltage amplitude and the frequency of reference wave adjusts the generated AC voltage frequency. Opto-coupler amplifies the DSP output signal to the level required to trigger IGBT (Vge (th) =15V and Ic =50mA) and isolated DSP from power circuit.TLP250 is used as isolator. Power circuit design: The power circuit section consists of four parts namely uncontrolled converter, full bridge inverter, DC link capacitor and load. The waveform quality can be improved by LC filter at the output of the PWM inverter.
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DSP IMPLEMENTATION
The open loop system model is implemented in real time hardware setup using DSP-MATLAB interface and the circuit description as specified. Using the target support package tool of MATLAB, the DSP TMS320F28335 is connected to PC. ePWM Module is used to generate six pulses at a time from ePWM pins of DSP. These pulses are being fed to opto-coupler which boost up the voltage level to an extent at which IGBT switches can be triggered. The system setup is shown in the fig. IGBT gate pulses are observed in digital signal oscilloscope. Figure 9 represents the block diagram of the implementation of AC-DC-AC scheme using DSP. The firing pulses for the three phase inverter are given from DSP board.
Fig. 6 DC Link Voltage
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Fig. 7 Phase-phase AC output Voltage
RTS
Fig. 9 Block Diagram representation of DSP based RTS system
Fig. 8 Ph-Ph AC output Voltage, Phase A Current for one cycle
Figure 5 is the input three phase voltage waveform showing phase-phase voltage for single cycle of 20 msec of
TMS320F28335 eZdsp is a floating point DSP so it is useful for advanced calculations. The conversion of multiple analog signals sequentially can be done with the help of inbuilt property of 12 bit ADC with a sequencer in DSP. The pulse pattern can be obtained by connecting a digital oscilloscope with a DSP kit. F28335eZdsp include Event Manager Modules which can be used for pulse generation.
Each Event Manager module contains General Purpose (GP) timers, PWM circuits and output logic. The DSP processor has ePWM output. This DSP has 6 ePWM (Enhanced PWM) modules, which can generate the desired PWM signals [13] [14].
the opto-coupler is in figure 11 (b). Figure 11 (c) reveals that output voltage between two phases is closely matching with that of the simulation results. Blue colour represents phasephase voltage output with the multiplier of 100 and red one represents the phase-phase current waveform output.
MATLAB2009b has target support package library which supports the DSP implementation. The library contains ePWM blocks which are used for generating pulse patterns. The PWM logic was developed as a simulink model. One of the models developed in MATLAB is given below:
Fig.11 (a) Hardware setup of the system
Fig. 10 Matlab/Simulink model
The above figure shows the logic generation of bipolar PWM for three phase inverter. The sine wave block is taken and is sampled with respect to time with a sample time of 64/80000. The sine wave is taken at a fundamental frequency of 50Hz. The amplitude and the bias value were calculated according to the count given for the timer period value (TxPR). The bias point is half of the TxPR value and the amplitude of sine wave is taken as 86% of the bias value. The sine wave block is converted into as per the data type and the scaling of the output [9] [10].
Fig11 (b) Gate Pulses from ePWM pins of DSP for one cycle
In ePWM block the carrier wave of required frequency was generated. The counting mode was specified as up-down counting and the compare value for the ePWMA and ePWMB pulses were given from input port i.e. sine wave. Now whenever the compare match occurs a pulse is generated. The timer period value can be calculated from the following formula: TxPR= (CPU clock frequency)/ (Desired frequency*2) Figure 11 (a) below shows the hardware setup of the system described. The PWM pulses generated at the output of
Fig11 (c) Phase-Phase output Voltage and Current Waveform
VI.
CONCLUSION
This paper presents the analysis and design of a digitally controlled three-phase PWM inverter to develop more theoretical and practical knowledge of control applications. The basics of software optimization and Hardware installations for proposed system have been presented in brief. The effectiveness of the simulation results are verified experimentally using TMS320F28335 DSP at 2.25 kHz switching frequency. The very close similarity between simulation and actual hardware results of output voltage waveforms illustrates the efficiency, accuracy of ACDC-AC PWM converter.
VII.
ACKNOWLEDGEMENTS
I would like to acknowledge the support and laboratory facilities extended by the School of Electrical Engineering at VIT University, Vellore, India with the permission and encouragement from the Director and Faculty Colleagues. I also wish to acknowledge the support extended by my guide for this project, whose relentless and optimistic approach paid dividends in completing this project with accurate and useful results. VIII.
REFERENCES
[1] Madhuri Avinash Chaudhari, “Implementation of Digital Signal Processor to Control Three-Phase Voltage-Source Inverter”, International Journal of Power System Operation and Energy Management, ISSN (PRINT): 2231–4407, Volume-1, Issue-2, 2011. [2] Mika Ikonen, Ossi Laakkonen, Marko Kettunen, “Two-Level and ThreeLevel Converter Comparison in Wind Power Application”, Department of Electrical Engineering Lappeenranta University of Technology, Finland. [3] Juan Manuel Carrasco, Leopoldo Garcia Franquelo, Jan T. Bialasiewicz, “Power-Electronic Systems for the Grid Integration of Renewable Energy Sources: A Survey”, VOL. 53, NO. 4, AUGUST 2006, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS. [4] Naik Raghuram. L., Jangamshetti Suresh.H.,“A Critical Study of Modulation Techniques for Three Level Diode Clamped Voltage Source Inverter for Grid Connection of WECS.” 2011, IEEE. [5] Manjesha; Jyothi, B.; , "Minimization of heat in the stator windings of a 3 phase induction motor using SPWM technique — an experimental study," Power Electronics Systems and Applications, 2009. PESA 2009. 3rd International Conference on , vol., no., pp.1-2, 20-22 May 2009 [6] Nima Yousefpoor, Seyyed Hamid Fathi, Naeem Farokhnia, “THD Minimization Applied Directly on the Line-to-Line Voltage of Multilevel Inverters”, IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 59, NO. 1, JANUARY 2012. [7] F. Blaabjerg, F. Iov, Z. Chen, K. Ma, “Power Electronics and Controls for Wind Turbine Systems”, IEEE International Conference, 2010. [8] H. Bludszuweit, J.A. Domínguez and M. García, “The impact of induction generator and PWM Inverter with energy storage on weak grids.” Department of Electrical Engineering University of Zaragoza. [9] Kirubakaran, K.; Jain, S.; Nema, R.K.; , "DSP-Controlled PowerElectronic Interface for Fuel-Cell-Based Distributed Generation,"
Power Electronics, IEEE Transactions on , vol.26, no.12, pp.3853-3864, Dec. 2011. [10] Pop, O.; Chindris, G.; Dulf, A.; , "Using DSP technology for true sine PWM generators for power inverters," Electronics Technology: Meeting the Challenges of Electronics Technology Progress, 2004. 27th International Spring Seminar on , vol.1, no., pp. 141- 146 vol.1, 13-16 May 2004. [11] Yun Xu; Yunping Zou; Chengzhi Wang; Wei Chen; Bangyin Liu; , "A Single-Phase High-Power-Factor Neutral-pointer Clamped Multilevel Rectifier," Power Electronics and Drive Systems, 2007. PEDS '07. 7th International Conference on , vol., no., pp.1487-1491, 27-30 Nov. 2007. [12] Guangchen Liu, Shengtie Wang, Hong Zhang and Bo Wang, “Integrated Control Strategy of Multibrid Wind Power Generation System” 2012 IEEE 7th International Power Electronics and Motion Control Conference - ECCE Asia. [13] Texas Instruments, Code Composer StudioTM IDE. Houston, TX: Texas Instruments Inc., 2009. Available: http://www.ti.com.Spec. Conf., Australia, 2002 [14] Texas Instruments, “TMS32081xDSP Event Manager (EV) Reference Guide,” Jun. 2007. Available: focus.ti.com/lit/ug/spru065e/spru065e.pdf.
AUTHORS’ INFORMATION Mandar Bhalekar received his Bachelor Degree in Electrical Engineering in the year 2010. Currently he is pursuing his Masters Degree in Power Electronics and Drives in the School of Electrical Engineering at VIT University, Vellore. He has presented papers based on PV Inverter and Digital Control of Boost Converter. He is currently focusing on the research areas like renewable energy sources and digital control. Umashankar. S (M10) received his Bachelor Degree in Electrical and Electronics Engineering and Master Degree in Power Electronics in the year 2001 and 2004 respectively. Currently he is Asst. Professor-Senior in the School of Electrical Engineering at VIT University, Vellore. He worked as Senior R&D Engineer and Senior Application Engineer in the power electronics and Drives field for more than 7 years. He has published/presented many national and international journals/conferences. He has also co-authored/edited many books/chapters on wind power/energy and allied areas. His current areas of research activities include renewable energy, real time digital simulator, HTS generator, FACTS, and power quality.
Surabhi Chandra received her Bachelor’s Degree in Electrical and Electronics Engineering in the year 2010. Currently she is pursuing her Masters Degree in Power Electronics and Drives in the School of Electrical Engineering at VIT University, Vellore. She has presented papers based on power factor correction and DSP based control of inverters. Her area of interest is in renewable energy systems.
D. Vijayakumar received his Bachelor Degree in Electrical and Electronics Engineering and Master Degree in Power Systems in the year 2002 and 2005 respectively. He worked as a Lecturer in Pallavan College of Engineering from 2005 to 2006. He received his Doctorate in 2010 at Electrical Department in Maulana Azad National Institute of Technology (MANIT), Bhopal, India. Presently, He is an Associate Professor in the School of Electrical Engineering, VIT University, Vellore. His current areas of research interest are power system protection, and Renewable energy sources.
D. P. Kothari (F’10) received the B.E. degree in electrical engineering, the M.E. degree in power systems, and the Ph.D. degree in electrical engineering from the Birla Institute of Technology and Science (BITS), Pilani, India. Currently, he is Director General, J B Group of Institutions, Hyderabad, India. He was Head, Centre for Energy Studies, lIT Delhi (1995-97), and Principal, Visvesvaraya Regional Engineering College, Nagpur (1997-98). He has been Director i/c, lIT Delhi (2005) and Deputy Director (Administration), lIT Delhi (2003-06). He has published/presented around 600 papers in national and international journals/conferences. He has also coauthored/co-edited 22 books on power systems and allied areas. His activities include optimal hydrothermal scheduling, unit commitment, maintenance scheduling, energy conservation, and power quality. He has guided 28 Ph.D. scholars and has contributed extensively in these areas as evidenced by the many research papers authored by him. He was a Visiting Professor at the Royal Melbourne Institute of Technology, Melbourne, Australia, in 1982 and 1989. He was a National Science Foundation Fellow at Purdue University, West Lafayette, IN, in 1992. He is a Fellow of the IEEE,
Indian National Academy of Engineering (INAE) and Indian National Academy of Sciences (FNASc). He has received the National Khosla award for Lifetime Achievements in Engineering for 2005 from lIT Roorkee. The University Grants Commission (UGC) has bestowed UGC National Swami Pranavananda Saraswati award for 2005 on Education for outstanding scholarly contribution.
