Abstract— Non-linear behavior of power semiconductor

switch and non-linear load creates power quality problem in an ac to ac converter. These non-linear switches and load injects significant harmonic on Load side as well as on source side. In this paper an attempt has been made to reduce these harmonics using staircase PWM technique and passive filter. For load side harmonic mitigation, staircase PWM technique and for source side harmonic mitigation shunt passive power filters have been used. The simulation has been done using MATLAB software and results are used to evaluate the signal THDs.

Index Terms-- Ac to Ac converter, Power Quality, Staircase-

PWM, THD.

I. INTRODUCTION N ac to ac converter is a static frequency changer device that converts the input supply power at fixed frequency to

output load power at variable frequency with the use of single stage conversion i.e. there is no use of dc link capacitance storage. This converter can be used in two modes: 1) step-up mode and 2) step-down mode. In step-up mode the output frequency is greater than the input frequency so in this mode ac to ac converter is called a cyclo-inverter. Similarly for step-down mode the output frequency is lower than the input supply frequency so in this mode ac to ac converter is called a cyclo-converter [1]. These ac to ac converters are used in various application like lighting control (to control the r.m.s value of voltage and current), industrial and domestic heating, speed control of hoist drives, soft starting of induction motor, aircraft etc [2]-[5]. The power semiconductor devices used in the converter operation have non-linear behavior in voltage and current. Also the load used for its operation is non-linear. Because of the presence of non-linear switch and non-linear load, the power quality problem occurs in the converter [6]-[7]. Due to this, the ac-ac converter will inject significant harmonics in load side as well as in source side i.e. both output and input is rich in harmonics [11]. The main task in ac to ac converter is to remove the harmonics in load side as well as in source side. The use of conventional filtering techniques in load side for harmonic mitigation is not preferable [2] because the output frequency is variable and hence PWM technique is used in load side harmonic mitigation. In this

Jagtar Singh Bhatti, Ayush Vardhan Goyal and Vineeta Agarwal are with the Department of Electrical Engineering, Motilal Nehru. National Institute of Technology,Allahabad-211004, India (email: bhatti.jagtarsingh@gmail.com, mistyayush@gmail.com, vineeta@mnnit.ac.in)

978-1-4799-4939-7/14/$31.00 ©2014 IEEE

paper staircase-PWM has been used for harmonic mitigation in load side. The input supply frequency is fixed and hence Filter can be used for source side harmonic mitigation [12]. The performance of the converter is evaluated using T.H.D as the performance criteria.

II. PRINCIPLE OF OPERATION Fig. 1 shows the power circuit of single phase static

frequency converter. It mainly requires four bi-directional semiconductor switches for its operation. The main function of these semiconductor switches is to block the voltage and conduct current in both directions [3]. If the bi-directional semiconductor switch is not present in a power circuit, in that condition anti-parallel IGBT switches with diode pair can be used. The main purpose of diode is to provide reverse blocking capability. For high power application generally IGBT switches are used which have high current carrying capacities and high switching capability.

Let the output frequency is reduced to half to that of input supply frequency. Then in the positive half of input cycle (Fig.2 (a)) if switches T1a and T4a conduct, the output is positive and if switches T2a and T3a will conduct the output will be negative(fig.2(b)). Similarly for negative half of input supply if switches T2b and T3b will conduct the output is positive (Fig.2 (a)) and if switches T1b and T4b will conduct, the output is negative (Fig.2 (b)).

For cyclo-inverter operation, if the output frequency is say, double to the input supply frequency i.e.in the positive half of input cycle only two switches will conduct. For example in Fig.3.(c) if switches T1a and T4a will conduct the output is positive and if switches T2a and T3a will conduct, the output will be negative. Similarly for negative half of input supply in Fig. 3 (d), if switches T2b and T3b will conduct the output is positive and if switches T1b and T4b will conduct the output will be negative.

. Fig.1. Power circuit of ac to ac converter

Harmonic Mitigation in AC-AC Converter Jagtar Singh Bhatti, Ayush Vardhan Goyal and Vineeta Agarwal, senior member, IEEE

A

Fig. (a)

Fig . (b) Fig.2. cyclo-converter operation.(a) For positive output (b)

For negative output

Fig. (c)

Fig. (d) Fig.3. cyclo-inverter operation.(c) For positive input cycle

(d) For negative input cycle Fig. 4 illustrates the logic diagram to generate IGBT gate

pulses. In order to improve the output of ac to ac converter and mitigation of Load side harmonic component staircase PWM technique has been used and the gate pulses obtained from this PWM technique is connected to different gates of IGBTs and the output is compared on the basis of fundamental voltage [9].

Fig.4. Logic diagram to generate IGBT gate pulses

III. STAIRCASE PWM TECHNIQUE Staircase PWM technique works on the principle of

comparing high frequency carrier wave with a modulating wave (staircase waveform). In order to obtain the acceptable output the PWM pattern is optimised with the use of less hardware and also there is no software requirement. Basically it is not a sampled representation of sine wave. To obtain the desired output voltage we have to change the no of steps or change the frequency ratio [4].

Fig. 5 shows the logic diagram for the generation of modulated pulses. In this method the carrier wave is a triangular wave with high frequency (in kHz) and modulating wave is a staircase waveform. The staircase is not considered as a pure sine wave because it is a stepped wave and we can control this step size according to our requirements. It is also called as optimised staircase PWM technique [8]. Fig. 6 shows the method to obtain the desired output voltage waveform. The pattern of the triangular wave is retained but the sine wave is replaced by a stair case waveform. The stair case is not meant to represent a sampled approximation to the sine wave. The modulation frequency ratio m = fc / fa and the number of steps are chosen to obtain the desired quality of output voltage.

Fig.5. Logic diagram to generate modulated pulses

Fig.6. Staircase PWM Technique

IV. PASSIVE FILTER For source side harmonic mitigation passive filters are used

because this is a simplest conventional solution for converter application. The passive elements generally used in passive filter are resistance, inductance and capacitance. The main purpose of these elements in passive filter is to control the harmonics. The passive filter is connected in two ways: either in series or in shunt on the source side. The main purpose of shunt connection of passive filter is to provide low impedance path for harmonic current at tunning frequency and series connection of passive filter is designed to carry full load current. The series connected passive filter requires additional over current protection relay for its operation and hence for harmonic mitigation in source side generally shunt passive filter is used. Single tuned filter is commonly used as a passive filter. The designing of this filter is very simple and least expensive [13]. The most common type of filter is LC Single tuned filter. This filter is connected in shunt to the ac supply source and for a particular harmonic frequency it will provide a low impedance path. Therefore harmonic component is provided with a low impedance path through this filter. The assumption for designing of single tuned filter is to select appropriate capacitor value that provides good power factor at signal frequency. In this paper passive LCL Filter has been used for source side harmonic mitigation.

V. SIMULATION RESULTS MATLAB SIMULINK SOFTWARE and its facilities are used to test the model of the ac- ac frequency converter with both resistive as well as RL Load. The resistive Load R=100 ohm and RL Load (R=10ohm, L=7mH) have been taken for all cases. The results have been shown for different output frequency. The supply voltage 220Vrms and supply frequency 50Hz have been taken for all cases. The variation of carrier frequency and variation in level in staircase has been taken according to the output frequency. The modulation index for all converter operation has been taken unity. Fig.7, 9 and 11 shows the output voltage of converter at output frequency of 150 Hz, 400Hz and 750Hz respectively for pure resistive Load without any filter or modulation technique. It is shows that THD is approximately 61.5%, 65.59% and 63.84% respectively which is very high. Figure 8, 10 and 12 shows the output waveform of the converter along with THD for f0= 150 Hz, 400Hz and 750Hz with modulation technique. The carrier frequency for f0= 150 Hz is 2 kHz and for f0= 400Hz and 750Hz is 10 kHz has been taken. THD for this case has been reduced to 23.37%, 23.88% and 24.77%. The number of level is kept nine for all cases. The simulation was carried out for 3,5,7,9 and 11level. The THD of output voltage decreased with increase in number of levels. For levels above nine the decreased in THD was not significant hence the analysis was carried out for nine levels. Figure 13 shows the Output voltage, input source current and THD of input source current without compensation for f0=150Hz with RL Load. It is shown that THD is approximately 34.61% in this case which is high. The 5th and 7th harmonic component was found to be the dominant harmonic component in source current in this case. Figure 14 shows the input source current and THD of input source current with compensation for f0= 150Hz with RL Load. THD for this case has been reduced to 4.59%.

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05

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(b) Fig.7. (a) Output voltage (b) THD of output voltage without compensation for f0=150Hz with R Load

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(b) Fig.8. (a) Output voltage (b) THD of output voltage with compensation for f0= 150Hz with R Load

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(b) Fig.9.(a) Output voltage (b)THD of output voltage without compensation for f0=400Hz with R Load

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Fig.10. (a) Output voltage (b) THD of output voltage with compensation for f0=400Hz with R Load

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(b) Fig.11.(a) Output voltage (b) THD of output voltage without compensation for f0=750Hz with R Load

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Fig.12. (a) Output voltage (b) THD of output voltage with compensation for f0=750Hz with R Load

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Fig.13. (a)Output voltage (b) input source current and (c)THD of input source current without compensation for f0=150Hz with RL Load

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Fig.14. (a) Input source current (b) THD of input source current with compensation for f0=150Hz with RL Load

VI. CONCLUSION In this paper the harmonic mitigation techniques for single

phase AC-AC frequency converter has been purposed. For source side harmonic mitigation Passive filter designing technique and for load side harmonic mitigation staircase PWM technique has been used. Staircase PWM technique was carried out for various output frequencies like 150Hz, 400Hz and 750Hz. The lowest THD in output side of 23.37% was achieved for 150Hz output for a resistive Load. The THD levels for higher output frequencies were found to be lying between 23-25% for the same staircase PWM technique. The output voltage and THD plots for various output frequencies have also been plotted. For RL Load it is seen that the input source current gets distorted. The 5th and 7th harmonic components were dominant in input source current. Peaks were observed in output voltage waveform for RL Load. The lowest THD in source side of 4.59% was achieved for 150 Hz output for a RL Load. The THD levels for higher output frequencies were found to be lying between 4-6% for the same passive LCL filter.

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converter," International Conference on Power, Control and Embedded Systems (ICPCES), Dec. 2010, pp. 1-5.

[2] V. Agarwal, A. Kumar, R. Singh and T.J Robin, "Modified PWM schemes for Cyclo-inverters, "International Conference on Power Engineering (IPEC), Dec. 2007, pp .655-660.

[3] Vineeta Agarwal and Peeuesh Agarwal, “IGBT Based Cyclo-Inverter” Asian Power Electronics Journal, vol. 2, pp. 58-62, Apr. 2008.

[4] K Thorborg, and A Nystrom, "Staircase PWM: an uncomplicated and efficient modulation technique for AC motor drives," IEEE Transactions on Power Electronics, vol.3, pp. 391-398, Oct.1988.

[5] A Maamoun, "Development of cycloconverters," Canadian Conference on Electrical and Computer Engineering, IEEE CCECE, May.2003, pp. 521-524.

[6] Liu Yazhou, G.T Heydt, and R.F Chu, "The power quality impact of cycloconverter control strategies," IEEE Transactions on Power Delivery, vol.20, no.2, pp.1711-1718, Apr.2005.

[7] M Basirifar, and A Shoulaie, "Impact of different control strategies on cycloconverter harmonic behavior," 2nd Conference on Power Electronics, Drive Systems and Technologies(PEDSTC), Feb .2011, pp. 385-391.

[8] B Diong, and K Corzine, "WTHD-optimal staircase modulation of single-phase multilevel inverters," IEEE International Conference on Electric Machines and Drives, May. 2005, pp. 1341-1344.

[9] J.R. Wells, Geng Xin, P.L Chapman, P.T Krein, and B.M Nee, "Modulation-Based Harmonic Elimination," IEEE Transactions on Power Electronics, vol. 22, pp. 336-340, Jan. 2007.

[10] M.A Rahman, John E. Quaicoe, and M. A. Choudhury, "Performance Analysis of Delta Modulated PWM Inverters,"IEEE Transactions on Power Electronics, vol. PE-2, pp. 227-233, Jul. 1987.

[11] P. Syam, P.K Nandi, and A.K. Chattopadhyay, "Effect of output current ripple on the input supply current and the power quality for a cycloconverter-fed drive,"IEE Proceedings on Electric Power Applications, vol.151, Jul. 2004, pp. 425-433.

[12] Wang Mingyu, Li Yang, Tan Bingbing, and Wei Bingjuan, "Harmonic Analysis and Suppression Methods Study of Cycloconverter-feed Synchronous Motor Drive System," International Conference on Power System Technology, Oct. 2006, pp. 1-6.

[13] H.A Kazem, "A comparative evaluation of active and passive input

current waveshaping methods for single-phase rectifier," 2nd International Symposium on Systems and Control in Aerospace and Astronautics(ISSCAA),pp.1-4, Dec.2008.