CONVERSION EFFICIENCY IMPROVEMENT FOR … · conversion process. Matrix Converter forms one of such...

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: - [email protected] 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

Having witnessed the severe oil crisis in the year 1973, the attention of the developed and developing economies of the world was focused on institutionalizing the measures related with that of finding the alternate and renewable resources of energy and optimally putting these resources to use for sustaining the technological development of the world. But, as the initial cost for tapping these resources was very high the immediate focus was shifted upon conservation of energy and thus controlling energy losses by energy efficient equipments and minimizing the losses related with the systems and the processes so that the widening gap between demand and supply can be apprehended. Conversion topologies provide avenues for reduction of losses involved in conversion process. Matrix Converter forms one of such areas, has specific importance not only from the view point of the technology but also focal point of effective means for reduction of losses in conversion process. In

G.N. SURYA et al. / International Journal of Engineering Science and Technology (IJEST)

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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 control for matrix converter based on the fact that if the magnitude of output current space vector is constant, the

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

Nottingham, 2008.


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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.

Figure 2.1: MATLAB model of proposed MC


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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

Figure 2.1.1: Switch modulation scheme for proposed MC


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(6)

Where, q is the desired voltage ratio, and Vim is the peak input voltage. Triple harmonic terms are found:

sin ω sin 3ω (7)

sin ω sin 3ω (8)

cos 3ω cos 3ω (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

Figure 3.1: Compact structure of model with MC and three phase load


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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 4.2.1: THD with MC Figure 4.2.2: THD without MC


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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 4.2.6: Active and Reactive power with MC Figure 4.2.7: Active and Reactive power without MC


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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

Conversion topologies like matrix converter provides avenues for reduction of losses involved in conversion process. As the title of the study suggests, the aim of the present study was set to carry out the modeling of the single stage AC-AC conversion using the most exciting concept of Matrix Converter envisaged way back in 1980 and still holding the main area of interest for the researchers. In the present study the converter modeling is implemented in MATLAB and then a simplified switch modulation scheme is proposed for the envisaged model. The variable frequency output was generated from the designed model for 100 Hz and 25 Hz. Tabulated results and the wave forms generated through MATLAB/simulink confirms the exciting contribution of the model in improving conversion efficiency by way of reduction of conversion losses anticipated at design stage. This simple yet very effective model gives out single stage three phase AC-AC conversion. The study thus is able to pinpoint the specific contribution of the envisaged converter model in reducing the overall size of the converter by at least half (1/2) thus minimizing the space requirements and at the same time reduction in losses due to single stages of conversion. Speed variation obtained with change in frequency is also evident through the wave forms generated by simulink.


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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. [17] Lopez, S., “Matrix Converter for frequency changing power supply applications”, Ph.D. dissertation submitted to U. of Nottingham,

unpublished, 2008.


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