CONVERSION EFFICIENCY
IMPROVEMENT FOR ENERGY
CONSERVATION USING MATRIX
CONVERTER
G.N. SURYA*
M. Tech. (EMS, final), Rajiv Gandhi College of Engineering Research. & Technology, Department of Electrical Engineering, Chandrapur. (Maharashtra), India - 442402
Mobile: - +919552454858, email: - gfsurya@rediffmail.com http://www.rcert.ac.in
SUBROTO DUTT
Asst. Professor, Rajiv Gandhi College of Engineering Research. & Technology, Department of Electrical Engineering, Chandrapur, Maharashtra, India – 442402
Abstract :
Efficient conversion of electrical energy from one form to another has always been a fascinating domain for researchers. Right from the era of mercury arc rectifiers to present AC-DC-AC two stage converters and cycloconverters, efforts are consistently being laid to minimize the losses involved in conversion process. In the present study the AC-AC conversion topology known as Matrix Converter (MC), is modeled using MATLAB. Venturini’s simplified algorithm is used to determine the switch modulation duty cycle. The model is designed for conversion of 50 Hz input frequency to 100 Hz and 25 Hz, using a simplified switch modulation strategy. The waveforms generated by MATLAB/simulink confirm the reduction of conversion losses by 12% to 15% with use of proposed model as compared to conventional (PWM) converters. It is not intended in this study just to list out various works done so for but to establish the brevity of fundamental concept representing the simplified design along with its application areas mainly the VFDs and Wind Energy Conversion Systems (WECS). The model has also been tested for control of induction motor with 100 HZ and 25 Hz output frequencies. The waveforms of changed frequencies and reduced losses confirm the results anticipated at design stage.
Keywords –matrix converter (MC); pulse width modulated (PWM) converter; simplified algorithm; straight forward control of induction motor; wind energy conversion system (WECS);
1 INTRODUCTION
recent past the advances in large power integrated circuits have triggered wide interest in single stage direct ac – ac power conversion, as an efficient all silicon and effective solution to convert electric power for motor drives, uninterruptible power supply, variable frequency generators, and reactive energy control. Hence the characteristics desired in ac – ac single stage converter are: -
• Eliminating the use of passive components such as large AC boost inductors. • Elimination of the bulky and limited life time DC-link electrolytic capacitors.
• Sinusoidal input and sinusoidal output waveforms, with minimal higher order harmonics.
“Matrix Converters” were first mentioned in the early 1980's by Alesina and Venturini [1]. They proposed a general model and a relative mathematical theory for high-frequency synthesis converters. They stated that the maximum input-output transformation ratio possible for the new AC-AC converter is √3/2 and also, they suggested a specific modulation and a feed-back-based control implementation of the proposed converter. The AC-AC Matrix Converter is optimal in terms of minimum switch numbers and minimum filtering requirements. The conventional SVM algorithm [2]-[3] usually generates both even and odd order harmonic voltages. The AC-AC Matrix Converter is optimal in terms of minimum switch number and minimum filtering requirements. A three-phase AC-AC conversion requirement can also be met through a back to back cycloconverter, but it uses 36 thyristers thus making the system quite bulky as shown in fig. 1.1. A three-phase AC-AC Matrix Converter consists basically of nine bidirectional voltage-blocking current-conducting switches. These switches are arranged in a matrix and by using this arrangement any input phase can be connected to any output phase at any time. Figure 1.2 shows such arrangement.
Almost for two and half decades no substantial work was witnessed carrying forward the work of Venturini and Alesina. Then at University of Nottingham again the research work related to converter topologies picked up in the year 2004, and useful studies were brought out through IEEE. A few amongst them are mentioned below.
In recent years, the hard switching three-phase to three phase matrix converter has received great attention as an alternative to the dc-voltage link converter with active front end dealing with modulation schemes [4]. Some studies have dealt with operation at unbalanced input voltages [5], gate-drive concepts [6], and the commutation procedure for bidirectional switches [7]. Another possibility of implementing Direct Power Conversion (DPC) providing similar input and output performance as a standard single stage MC is the two-stage DPC, referred as “Dual Bridge MC” [8].
Design and implementation of single leg matrix converter was proposed by Malcom Tabone, Cyril Spiteri Stains and Joseph Cilia [9]. A low cost micro controller supported operation of matrix converter was proposed by them. A new indirect matrix converter topology with a three level phase-neutral output voltage compatibility and reduced number of devices was proposed by Christian Klumpner et al [10]. A new and simple current
Figure 1.1: Schematic of AC-AC converter Figure 1.2: Simplified representation of MC Source- MC for frequency changing power supply applications, S. Lopez, Ph.D. Dissertation, University of
output currents are sinusoidal and balanced, was proposed by Milton E. De oliveira Filho et al [11]. Problems in commutation of matrix converter were dealt by L. Empringham [12]. A new modulation strategy was proposed for matrix converter by G. Clos [13].
Response of MC to variations in input voltage was examined by Hulusi K. and Ramazan Akkaya [14]. They proposed a PI controller based compensation scheme to prevent the negative effect of input voltage variations. Z source buck and boost MC was proposed by B. Vaikundaselvan, R. Dhanasekaran, G. Jegan and N. Prakash [15]. The proposed single phase Z source MC is capable of providing a wide range of output ac voltages in buck-boost mode with step-up/step-down frequencies. WECS consisting of wind turbines with permanent magnet synchronous generators (PMSG) and full power converters focusing on models integrating the dynamic of the system was studied by R. Melicio, Victor M.F. Mendes and Joao P.S. Catalao [16].
The characteristics desired in ac–ac single stage converter are: Eliminating the use of passive components such as large AC boost inductors and bulky and limited life time DC-link electrolytic capacitors. A three-phase AC-AC MC is shown in Figure 1.2:
1.1 STATEMENT OF THE PROBLEM
Minimizing the loss of energy involved in two stage conversion i.e. AC-DC-AC attracted the attention of many researchers. Hence, it is desired through this incorporated small study to apprise about the conversion efficiency and present the exciting contribution of modeled Converter in minimizing the conversion losses and propose the relative topology of AC-AC single stage conversion with the required output frequency.
2 METHODOLOGY
Testing of the model has been done for change in frequency by connecting a resistive load. The frequency has been changed first from 50 Hz to 100 Hz and then the energy conservation by way of conversion topology is examined by simulating the proposed model through MATLAB simulink. But prior to that the MC structure proposed by Alesina and Venturini [1] has been modeled and implemented using MATLAB, which is shown in figure 2.1, and then simplified switch modulation scheme is proposed for the MATLAB model.
2.1 SWITCH MODULATION STRATEGY
The concept of switching functions is used to derive a mathematical model of the matrix converter. To understand the modulation problem and its solution, consider the arrangement shown in Figure 1.2. The modulation scheme proposed here in this study is straight forward and improvised over schemes proposed earlier. A sequential and logical pattern is set which generates the required frequency without a serious attempt for designing of input and output filter circuits. As this study is of basic nature, the filter circuit design is ignored in this study. Hence, this study provides the opportunities to take up the further work related with designing of filter circuits at the input and output in this model.
Analysis of this arrangement shows that there are no freewheeling diodes. This restriction means that the short circuit in the capacitive input as well as the open circuit in the inductive output must be avoided. Because ideal switches are used, the commutation between switches is instantaneous. A typical switching pattern for matrix converter is shown in fig. 2.1.1
If conventional PWM is employed the switching sequence Tseq has a fixed period as shown in equation 1.
(1)
A modulation duty cycle should be defined for each switch in order to determine the average behavior of the matrix converter output voltage waveform. The modulation duty cycle is defined by equation 1, where tAa
represents the time when switch SAa is ON and Tseq represents the time of the complete sequence in PWM pattern. For calculating the tAa and mAa, Venturni’s simplified algorithm [17] is used in this study. The simplified algorithm is reproduced below: -
Vim and ωit are calculated as
(2)
ω
arctan
√
(3)
Where, VAB and VBC are the instantaneous input line voltages. The target output peak voltage and the output position are calculated as
(4)
ω
arctan
√
(5)
where Va, Vb, Vc are the target phase output voltages. Alternatively, in a closed loop system (for example a field-oriented controlled drive), the voltage magnitude and angle may be direct outputs of the control loop. Then, the voltage ratio is calculated
(6)
Where, q is the desired voltage ratio, and Vim is the peak input voltage. Triple harmonic terms are found:
sin
ω
sin 3
ω
(7)
sin
ω
sin 3
ω
(8)
cos 3
ω
cos 3
ω
(9)
Where, qm is the maximum voltage ratio (0.866). Then, the three modulation functions for output phase a are given as
(10)
–
(11)
1
(12)
3 FINAL DESIGN OF THE PROPOSED MODEL
4 RESULSTS
For obtaining the results the model was tested firstly with and then without MC with three phase load at the output. The results tabulated in table 4.1.1 clearly depicts that when MC is used the RMS value of the current reduces from 42 Amps to 35.5 Amps so also the Active Power reduces from 16600 W to 16150 W. The results have been confirmed by generating the waveforms through MATLAB/simulink and the same are presented below. Secondly, the model has also been tested for obtaining the speed variations of the induction motor by varying the frequency. Thus the envisaged model may be used for the purpose of enhancing the overall efficiency of the conversion system and thus contributing to energy conservation.
4.1 IMPROVEMENT IN CONVERSION EFFICIENCY
Table 4.1.1
Comparison of Results of Energy Conservation obtained with MC
Basis of comparison With Matrix Converter Without Matrix Converter
RMS Value of output current 35.5 Amp 42 Amp
Active Power requirement 16150 W 16600 W
Reactive Power requirement 16150 VAR 16600 VAR
Power Factor 0.71 0.71
4.2 WAVEFORMS GENERATED BY MATLAB/SIMULINK SHOWING COMPARATIVE
ANALYSIS
Figure 4.2.3: Line Voltage with filter with MC (100 Hz)
Figure 4.2.4: RMS value of Output Current with MC Figure 4.2.5: RMS value of Output Current without MC
Figure 4.2.8: Triple Harmonic Factor FFT Display with MC Figure 4.2.9: Triple Harmonic Factor FFT Display without MC
4.3 SPEED VARIATION WITH CHANGE IN FREQUENCY (SQUIRREL CAGE I.M.)
Figure 4.3.1: Speed of motor obtained at 100 Hz (800 RPM) Figure 4.3.2: Speed of motor obtained at 25 Hz (200 RPM)
5 DISCUSSIONS AND CONCLUSIONS
5.1 FUTURE SCOPE
The wave forms generated for various frequencies exhibit distortions which can be smoothened by way of appropriate design of input and output filter circuits thus providing the scope for future course of work. Generating fixed frequency output from variable frequency input using the same algorithm and thus integrating such a converter with wind power generation can also be explored.
REFERENCES
[1] Alberto Alesina and Marco G. B. Venturini, \Solid-state conversion: A Fourier analysis approach to generalized transformer synthesis," IEEE Transactions on Circuits and Systems, vol. CAS-28, No. 4, pp. 319, April 1981.
[2] M. Venturini, \A new sine wave in sine wave out, conversion technique which eliminates reactive elements," Proceeding Powercon 7, vol. E3, pp. 1 { 15 }, 1980.
[3] Alberto Alesina and Marco G. B. Venturini, “Analysis and design of optimum-amplitude nine-switch direct AC-AC converters,” IEEE Transactions on Power Electronics, vol. 4.
[4] J. Oyama, X. Xia, T. Higuchi, and E. Vamada, “Displacement angle control of matrix converter”, in Proc. IEEE PESC’97, 1997, pp. 1033-1039.
[5] P. Nielson, D Casadei, G. Serra, and A. Tani, “Evaluation of the input current quality by three different modulation strategies for SVM controlled matrix converter with input voltage unbalance”, in Proc. IEEE PEDES’96, vol. 2, 1996, pp. 794-800.
[6] C. Klumpner, F Blaabjerg, and P. Nielsen, “Speeding-up the maturation process of the matrix converter technology”, in Proc. IEEE PESC, vol 2,2001, pp. 1083-1088.
[7] M. Zeigler and W. Hoffman, “Semi-natural two steps commutation strategy”, in Proc. IEEE PESC’98, 1998, pp. 727-731. [8] L. Wei, T. A. Lipo, “ A novel matrix converter topology with simple commutation”, Proc. of IAS, vol. 3, pp 1749-1754, 2001. [9] Malcom Tabone, Cyril Spiteri Stains and Joseph Cilia (2004),”Low cost three phase to single phase matrix converter”, IEEE, Int.
Conf. on Industrial Tech
[10] Christian Klumpner et al (2007), “A new three level indirect matrix converter with reduced number of devices”, IEEE, 0197-2618/07. [11] Milton E. De Oliveira Filho et al (2006), “A simple current control for matrix converter”, IEEE, 1-4244-0365-0/06.
[12] L. Empringham (1998), “Matrix Converter bi-directional switches commutation using intelligent gate drives”, IEEE, Sept-1998. [13] G. Clos (2008), “Straight forward control of the matrix converter”, ISBN: 90-75815-08-05, IEEE.
[14] H. Karaca, R. Akkaya (2009), “Control of Venturini Method Based Matrix Converter in Input Voltage Variations”, IMECS 2009, March 18-20, Hong Kong.
[15] B. Vaikundaselvan, Dr. R. Dhanasekaran, G. Jegan, N. Prakash, “Conservation of Energy using Z-Source Matrix Converter”, IJEIT, Feb. 2012.
[16] Rui Melicio, Victor M.F. Mendes and J. P.S. Catalao, “Wind Turbines with Permanent Magnet Synchronous Generator and Full-Power Converters : Modeling, Control and Simulation”, IREE, Vol. 5, No. 2, March-April 2010.
