simulation and analysis of modulation strategies for pv

Simulation and Analysis of ModulationStrategies for PV Based T-Type Inverter

S.Devi, II Year M.E. (PED),Dr.R.Seyezhai,Department of Electrical and Electronics Engineering

SSN College EngineeringChennai, India

sdevi [email protected] ,[email protected]

May 21, 2018

Abstract

This paper reports the design and simulation of T Typeinverter for photovoltaic applications. A 100W single phaseT Type inverter is modeled using MATLAB/SIMULINK.It involves implementation of various modulation strategiesnamely sinusoidal pulse width modulation, phase disposi-tion modulation and phase opposition disposition modula-tion for T Type inverter and also brings out the comparisonamong them in terms of reduction in total harmonic distor-tion, variation of switching frequency and modulation index.The results are verified.

Keywords:inverter; t- type inverter ; photovoltaic in-verter; pulse width modulation;

1 INTRODUCTION

Photovoltaic inverters are used to convert DC voltage to AC volt-age. Depending on the application, the photovoltaic inverter canbe broadly classified as standalone, grid- tie and battery backupinverter. In the case of conventional two-level inverter, the inputDC is converted into AC of desired frequency and voltage with the

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International Journal of Pure and Applied MathematicsVolume 118 No. 24 2018ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/

help of semiconductor power switches. The series and parallel com-bination of power switches are used in order to obtain the requiredvoltages and currents [1][2]. As the required number of switches andsources are high, they contribute to significant power loss. Also, athigher switching frequencies, the switching losses are even higher.Hence, there is requirement for an inverter topology with reducednumber of switches in order to minimize the losses associated withit [3]. Therefore, this paper discusses about T-type inverter whichovercomes the above-mentioned drawbacks.

T Type inverter requires only a single switch to connect to itsouter upper and lower dc buses. Thus, it allows to increase theefficiency by reducing the switching losses incurred [4]. With theincreased switching frequency, the filter size reduces, thus reduc-ing the size of the inverter and making it compact for low voltageapplications.

In this paper, solar based T -Type inverter [5] is modelled inMATLAB/ SIMULINK. Various modulation strategies such as si-nusoidal pulse width modulation, phase disposition pulse widthmodulation and phase opposition disposition pulse width modu-lation are implemented and a comparison is brought in terms totalharmonic distortion with respect to change in modulation indexand switching frequencies.

The paper is organized as follows: Section II briefs about theoperation of T- type inverter, Section III discusses the various mod-ulation strategies implemented for T -type inverter, Section IV givesthe simulation results obtained in MATLAB/SIMULINK and Sec-tion V concludes the work discussed in this paper.

2 T TYPE INVERTER

The basic circuit of one phase leg of a three level T-type inverter[6] is shown in Fig. 1. The inverter switches are operated as thecomplementary pairs S1/S3 and S2/S4 in accordance with TABLEI to obtain the required output voltages of +VDC/2, 0 and VDC/2.Thus, it produces a three-level AC output voltage.

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Fig. 1 Circuit diagram of the single-phase T Type inverter

There exist four modes of operation as represented in Figs. 2 - 5.Mode 1: Zero Output Voltage and Positive Output Current:

During this mode, the positive output current flows from the mid-point M through the diode D2 and switch S2 to the load as shownin Fig. 2, and both the outer switches S1 and S4 block a voltage ofVDC/2.

Mode 2: Positive Output Voltage and Positive Output Cur-rent: When the switch S1 is turned on, the output current flowsthrough S1against an off-state blocking voltage of VDC/2 andD2/S2

are commutated as shown in Fig. 3. The outer switch S4now blocksa voltage of VDC . Thus, this cycle follows throughout the fundamen-tal positive half cycle.

TABLE I. SWITCHING STATES FOR T TYPE INVERTER

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Fig. 2 Mode 1: Zero output voltage and positive output current

Fig. 3 Mode 2: Positive output voltage and positive outputcurrent

Mode 3: Zero Output Voltage and Negative Output Current:When switch, S3 is turned on during the negative half cycle, zerooutput voltage and negative output current is obtained as shown in

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Fig. 4. During this mode both the outer switches block a voltageof VDC/2.

Mode 4: Negative Output Voltage and Negative Output Cur-rent:

Similar to the mode 2 during the positive half cycle, whenswitch, S4 is turned on, the negative current flows through S4 com-mutating D3/S3, against an off-state blocking voltage of VDC/2,and now the switch S1 blocks a voltage of VDC as shown in Fig. 5.

Fig. 4 Mode 3: Zero output voltage and negative output current

B.POWER SPECTRUM OF DWT BASED OFDMSYSTEM

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Fig. 5 Mode 4: Negative output voltage and negative outputcurrent

3 ANALYSIS OFMODULATION STRATE-

GIES

Pulse width modulation, (PWM) is a method of modulating thewidth of the pulses. The width of the pulse is directly proportionalto the control voltage [7]. The higher the control voltage, the widerare the resulting pulses. The pulse width modulation techniquecompares two waveforms; one is called the reference waveform whilethe other is called as the carrier waveform. The ratio of the am-plitude of the reference wave to the amplitude of the carrier waveis called as the modulation index. Depending upon the referencewave and the carrier wave chosen, there exist different types of pulsewidth modulation techniques based on the phase relationships be-tween the carriers [8]. Three among them namely; sinusoidal pulsewidth modulation, phase disposition pulse width modulation andphase opposition disposition pulse width modulation are discussedin this section.

A. Sinusoidal Pulse Width ModulationIn sinusoidal pulse width modulation (SPWM), the reference

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waveform is a sine wave. The reference and carrier waveform ar-rangement for sinusoidal pulse width modulation is shown in Fig.6.

Fig. 6 Carrier arrangement for SPWM strategy

Using the SPWM technique the gate pulses for the switch S1, S2,S3 and S4 are generated as shown in Fig. 7. The gate pulses forthe switch S1 and S4 are generated by comparing a sine wave andan inveretd sinewave with a single triangular carrier wave respec-tively while the gate pulses for the switch S2 and S3 are generatedby comparing the sine and two triangular waves where, both thetriangular waves are out of phase with each other, one above andother below the axis.

Fig. 7 Gate pulse generation using SPWM strategy

The gating pattern for the T type inverter obtained using SPWMstrategy is shown in Fig. 8.

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Fig. 8 Gating pattern for T- type inverter obtained using SPWMstrategy

B. Phase Disposition Pulse Width ModulationIn phase disposition pulse width modulation (PDPWM) strat-

egy, has all the carrier waveforms are in phase with each other. Thereference and carrier waveform arrangement for phase dispositionpulse width modulation is shown in Fig. 9

Fig. 9 Carrier arrangement for PDPWM strategy

The gate pulses for the switch S1, S2, S3 and S4 are generated usingthe PDPWM technique as shown in Fig. 10. The gate pulses forthe switch S1 and S4 are generated by comparing a single sine wavewith two triangular carrier waves that are in phase with each other,one above the axis and the other below the axis respectively whilethe gate pulses for the switch S2 and S3 are generated by comparingthe sine and two triangular waves where, both the triangular wavesin phase with each other, one above and other below the axis.

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Fig. 10 Gate pulse generation using PDPWM strategy

The gating pattern for the T type inverter obtained using PDPWMstrategy is shown in Fig. 11.

Fig. 11 Gating pattern for T- type inverter obtained usingPDPWM strategy

C. Phase Opposition Disposition ModulationIn phase opposition disposition pulse width modulation (POD-

PWM) strategy, has all the carrier waveforms above zero referenceto be in phase with each other and those below the zero referenceto be 180◦ out of phase with the other. The reference and carrierwaveform arrangement for phase opposition disposition pulse widthmodulation is shown in Fig. 12

Fig. 12 Carrier arrangement for PODPWM strategy

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The gate pulses for the switch S1, S2, S3 andS4 are generated usingthe PODPWM technique as shown in Fig. 13. The gate pulsesfor the switch S1 and S4 are generated by comparing a single sinewave with two triangular carrier waves that are in phase oppositionwith each other, one above the axis and the other below the axisrespectively while the gate pulses for the switchS2and S3 are gen-erated by comparing the sine and two triangular waves, with boththe triangular waves out of phase with each other, one above andother below the axis.

Fig. 13 Gate pulse generation using PODPWM strategy

The gating pattern for the T type inverter obtained using PDPWMstrategy is shown in Fig. 14.

Fig. 14 Gating pattern for T- type inverter obtained usingPODPWM strategy

4 SIMULATION RESULTS

The various parameters that were considered for the simulation aretabulated in TABLE II.

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TABLE II. VARIOUS PARAMETERS CONSIDERED FORSIMULATION

The output voltage waveform for the inverter employing sinusoidalpulse width modulation with switching frequency of 1kHz and filtervalues: L= 15mH and C = 300µF is shown in Fig. 15.

Fig. 15 Output voltage waveform for SPWM

The output voltage waveform shown in Fig. 15 is sinusoidal and itsFFT analysis spectrum is shown in Fig. 16

Fig. 16 FFT analysis of output voltage waveform for SPWM

From the FFT analysis spectrum shown in Fig. 16, the total har-monic distortion of the output voltage was found to be 5.27%.

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The output voltage waveform for the inverter employing in phasedisposition pulse width modulation with switching frequency of 1kHz and filter values: L= 19mH and C = 330µF is shown in Fig.17.

Fig. 17 Output voltage waveform for PDPWM

The output voltage waveform shown in Fig. 17 is sinusoidal innature and its FFT analysis spectrum is shown in Fig. 18.

Fig. 18 FFT analysis of output voltage waveform for PDPWM

From the FFT analysis spectrum shown in Fig. 18, the total har-monic distortion of the output voltage was found to be 5.89%

The output voltage waveform for in phase opposition dispositionpulse width modulation with switching frequency of 1 kHz and filtervalues: L= 17mH and C = 380uF is shown in Fig. 19.

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Fig. 19 Output voltage waveform for PODPWM

The output voltage waveform shown in Fig. 19 is sinusoidal innature and its FFT analysis spectrum is shown in Fig. 20

Fig. 20 FFT analysis of output voltage waveform for PODPWM

From the FFT analysis spectrum shown in Fig. 20, the total har-monic distortion of the output voltage was found to be 5.46%

The comparison of various PWM Strategies for T-Type inverterwith respect to variation in modulation index is shown in Fig. 21.

Fig. 21 Comparison of various PWM techniques for T Typeinverter with respect to variation in modulation index

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Fig. 21 shows that the sinusoidal pulse width modulation giveslower THD value in comparison with the phase disposition pulsewidth modulation as well as phase opposition disposition pulsewidth modulation with increase in modulation index. Also, withthe increase in the modulation index the value of total harmonicdistortion reduces.

The comparison of various PWM strategies for T-Type inverterwith respect to variation in switching frequency is given in Fig. 22.

Fig. 22 Comparison of various PWM techniques for T Typeinverter with respect to variation in switching frequency

The graph shown in Fig. 22 shows that the sinusoidal pulse widthmodulation gives lower total harmonic distortion in comparisonwith the other two pulse width modulation strategies namely, phasedisposition pulse width modulation and phase opposition disposi-tion pulse width modulation for the same switching frequency. Also,with the increase in the switching frequency there is a decrease inthe vales of THD.

The comparison between THD and filter size for different PWMstrategies for the switching frequency of 1 kHz is given in TABLEIII.

TABLE III. COMPARISON BETWEEN VARIOUS PWMSTRATEGIES WITH RESPECT TO FILTER SIZE

From the TABLE III, it is seen that the sinusoidal pulse widthmodulation has the reduced filter values in comparison with the

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phase disposition pulse width modulation as well as phase oppo-sition pulse width modulation thereby reducing the overall size offilter required.

5 PV MODELING

A. PV Modeling and CharacteristicsThe MATLAB/SIMULINK PV model is obtained as given in

[9] and the output characteristics are obtained as shown in the Figs.23 and 24.

Fig. 23 I - V characteristics of PV

The open circuit voltage obtained from the I V characteristicsshown in Fig. 23 is 22V.

Fig. 24 P -V characteristics of PV

From the P- V characteristics shown in Fig. 24, the power obtainedis 100W for an open circuit voltage of 22V.

B. Interfacing PV Module with T-Type Inverter and Conven-tional Inverter

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The interfacing of PV module with T Type inverter in MATLABSIMULINK is shown in Fig. 25.

Fig. 25 T Type inverter interfaced with PV

The output voltage waveform of single phase T- type inverter isshown in Fig. 26

Fig. 26 Output voltage waveform of single phase T- Type inverterinterfaced with PV

The output voltage waveform of T Type inverter as shown in Fig.26 is non-sinusoidal and symmetrical.

C. Comparison between T Type and PWM invertersThe TABLE IV gives the comparison between the conventional

VSI and T Type inverter with respect to THD.

TABLE IV. COMPARISON BETWEEN CONVENTIONAL VSIAND T TYPE INVERTER WITH RESPECT TO THD

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From TABLE IV, it is found that the T- type inverter have lowerTHD as well as filter values in comparison with the conventionalPWM inverters.

6 CONCLUSION

In this paper, design and simulation of photovoltaic T- Type in-verter is presented. The total harmonic distortion values of theT type inverter is found to be less than that of the conventionalvoltage source inverter. The comparative analysis of various pulsewidth modulation strategies for t- type inverter was studied and si-nusoidal pulse width modulation was better in comparison with theother two techniques namely phase disposition pulse width modu-lation and phase opposition disposition pulse width modulation.Hence, the proposed T-type inverter is a suitable topology for PVapplications.

ACKNOWLEDGMENTThe authors thank the management of SSN institution for pro-

viding the required facilities and financial support to carry out thiswork.

References

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[3] K.Ratnamadhuri, A.Arjunrao, Switching Losses and HarmonicInvestigations in Multilevel Inverters, International Journal ofIndustrial Electronics and Electrical Engineering, vol. -4, Iss.-7,pp. 45 52, Jul.-2016

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simulation and analysis of modulation strategies for pv

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