design and implementation of micro-inverter for photovoltaic ...voltage. this output is then given...

Design and Implementation ofMicro-inverter for Photovoltaic

Application

Seeranga Nandhini.S, Dr.R.Seyezhai,Sowmya.V, Ms.D.Umarani

Department of Electrical and ElectronicsSSN College of Engineering

Chennai, [email protected],

[email protected],[email protected],

[email protected]

May 10, 2018

Abstract

The objective of this work is to implement a two stagemicro-inverter with reduced Total Harmonic Distortion(THD)and increased efficiency. The first stage consists of a DC-DC boost converter , followed by the DC-AC inverter stage.A typical solar panel converts only 30 to 40 percent of theincident solar irradiation into electrical energy. To increasethe conversion efficiency , Maximum Power Point Track-ing(MPPT) algorithms (Perturb & Observe, IncrementalConductance) have been simulated in PSIM software andbetter tracking efficiency was observed with the Incremen-tal Conductance algorithm. To reduce the harmonics of theoutput voltage of the inverter in the second stage, differentinverter topologies namely H5, HERIC and push-pull havebeen simulated and H5 is found to have higher efficiency

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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/

and lesser distortion. In order to control the switching pat-tern of the inverter, the pulse width modulation strategiessuch as Unipolar , Bipolar, Sawtooth and Inverted Sine havebeen analysed and the micro-inverter with lesser harmonicdistortion have been suggested for power quality improve-ment. Simulation studies are carried out in PSIM softwareand the results are validated.

Key Words:Boost converter, H5, HERIC, Push-pull,P&O, InC, micro-inverter

1 Introduction

The generation of electric power from solar PV has gained momen-tum and is emerging as the most useful form of renewable energysources. It is fast growing due to lesser complexity in implemen-tation, relatively high efficiency and less maintenance. Among thevarious photovoltaic inverters, the micro-inverter[1] is a low powerinverter with a rating up to 350W. In the micro-inverter system,every panel has its own inverter and the output of each can be di-rectly connected to the grid. The individual units are termed asmicro-inverter. Each panel can operate at its MPP, thereby increas-ing the power output of the entire system. The micro-inverter isgaining popularity in residential application with advantages suchas higher efficiency, reliability and safety.

The work focuses on implementing a two-stage microinverterwith lesser harmonics and increased efficiency. Two-stage microin-verter comprises of DC-DC converter in the first stage and a DC-ACconverter in the second stage. The output of the solar panel is fedas input to the boost converter to raise the level of the outputvoltage. This output is then given to the inverter and its outputdrives the load. In this work, a 40Wp panel is taken as the source.Various analysis have been done to determine the best topology forimplementation.

2 MICRO-INVERTER

The block diagram of the micro-inverter system is shown in Fig.1.

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International Journal of Pure and Applied Mathematics Special Issue

Fig 1. Block diagram

The dual stage topology[2] of the micro-inverter is performedusing a DC-DC stage and a DC-AC stage connected in cascade ob-taining a conversion chain of the type DC-DC-AC. In this case, theDC-DC converter is connected to the PV module and the DC-ACconverter is connected to the load. To ensure maximum power istracked from the panel under varying irradiance and temperaturelevels, the maximum power point tracking(MPPT) [3] algorithmshave been incorporated to control the switching pattern of the con-verter. The inverter is controlled using the pulse width modulationtechniques[4]. These techniques have the advantages of low THDoutputs and effectiveness.

3 PV MODELLING

The modeling of a 40Wp solar panel has been carried out using thesingle diode model .The schematic of the modelling is given in Fig2.

Fig 2. Schematic of PV modelling

The specifications of the 40Wp panel taken from the datasheetare shown in Table I.

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TABLE I : PANEL SPECIFICATIONS

The various cell parameters such as Iph , Is and ideality fac-tor(n) have been calculated using the appropriate formulas givenin [5].The I-V and P-V characteristics obtained for varying irradi-ation levels is shown in Figs.3& 4 respectively.

Fig 3. I-V Characteristics of PV Panel

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Fig 4. P-V Characteristics of PV Panel

As it can be seen from the above figures, the power varies corre-spondingly to the irradiance. Tthe terminal voltage of the modulevaries between 0-Voc while the current value changes between Isc-0 when the module‘s operation point moves between short-circuitand open-circuit conditions, respectively. The panel provides themaximum power of 40W at the standard conditions of 1000W/m2.Parameters like cell‘s working temperature, irradiance, diode ide-ality factor, series and shunt resistances have all significant effectson cell‘s I-V and P-V characteristics.

4 MAXIMUMPOWER PEAK TRACK-

ING METHODS

The irradiance of the sun incident on the PV panel tends to changethroughout the day. But there occurs peak in the PV graph. Inorder to obtain maximum efficiency, the panel has to be operated atthis peak value. By employing suitable algorithms, the converteris given gate pulses to operate with maximum power. The con-verter used in this circuit is a boost converter. The switch of thisconverter has to be controlled using the MPPT [6] algorithm. Theconverters input is from the PV panel and it powers the inverter.

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The converter has an inductor with a value of 674H, a capacitor onthe input side with a value 50F and on the output side with a value500uF. This converter is made to operate with a duty cycle of 50%.The schematic of boost converter[7] with PV panel is shown in Fig5.

Fig 5. Boost Converter

MPPT algorithms are used to operate the PV system in thepeak of the PV graph. By operating it that way, the problems dueto load mismatch can also be avoided apart from increasing the effi-ciency.. The two types of MPPT algorithms taken for investigationin this paper are, Perturb & Observe and Incremental Conductance[8].

A. Perturb and Observe MPPT algorithmThis algorithm perturbs the voltage at two consecutive points

and the observes the power at that voltage. This algorithm is sim-ilar to climbing a mountain. It compares the power first. Thenthe comparison of voltage is done. Accordingly, the duty cycle isaltered in ordered to move towards the maximum power peak. Theflowchart of the algorithm is illustrated in fig 6.

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Fig 6. Flowchart of P&O algorithm

This algorithm has been implemented using blocks in PSim.The voltage and current from the PV panel have been taken usingthe voltage sensor and current sensor and given as inputs. Theouput of the algorithm is gate pulses which is given to the switch ofthe boost converter.The irradiance to the PV panel is differed andthe power across both input and output sides of the converter arenoted along with the voltage .

Fig 7. Schematic diagram of P&O Algorithm

Fig 8. Waveform of a) Power of PV panel and Power tracked dueto MPPT and b) Input and Output Voltage of the converter

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TABLE II : PARAMETERS OBSERVED DUE TO P&O MPPT

B. Incremental Conductance MPPT AlgorithmThis algorithm reaches the peak using the slope of the PV graph

as the left side slope is negative and the right side one is positive.The slope at the peak is zero. This method compares the slope ofeach point and reaches the peak. It found to be efficient than P&Omethod. Unlike P&O it does not oscillate on reaching the maximumpower peak. So, the losses due to oscillations are reduced. The fig9 depicts the flowchart of InC and the fig 10 shown below is theschematic subsystem of InC.

Fig 9. Flowchart of Incremental Conductance

According to the algorithm, the slope is checked first and laterthe current change is also checked . Based upon the results of theanalysis of the slope of the PV graph , accordingly the duty cycleis altered to reach the maximum power peak.

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Fig 10. Schematic diagram of Incremental Conductance

Fig 11. Waveform of a) Power of PV panel and Power tracked dueto MPPT and b) Input and Output Voltage of the converter

TABLE III : PARAMETERS OBSERVED DUE TO INC MPPT

As there are few drawbacks and losses due to oscillation in P&O,the tracking efficiency is found to be lesser than the InC algorithm.Since our aim is to track maximum power possible, we propose theInC algorithm for triggering the switch of the converter.

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5 INVERTER TOPOLOGIES

In order to reduce the THD, various inverter topologies [9] such asH5, HERIC and Push-pull have been simulated in PSIM software.

A. H5 InverterThe H5 inverter [10] comprises of five active switches. The basic

circuit diagram of the H5 is shown in Fig. 12

Fig 12. H5 inverter

Fig.12 shows the inverter configuration with five switches, LCLfilter at the output side and R load. The switches S2, S3, S4 andS5 operate in unipolar switching pattern. The switch S1 operateswhen on pulse is generated by comparing rectified sinusoidal ref-erence wave with triangular signal. The output of the inverter isa sinusoidal wave varying between +Vdc and -Vdc. The outputof the boost converter is fed as input to the inverter and acts asthe second stage of the micro-inverter. The various parameters ob-tained from the entire system simulation has been tabulated belowin Table. IV

TABLE IV : MICRO-INVERTER WITH H5 TOPOLOGY

B. HERIC InverterThe six active switch HERIC [11] topology is a H-bridge inverter

with two MOSFETS connected in series in between the two legs ofthe bridge. The schematic of the HERIC is shown in Fig 13.

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Fig 13. HERIC inverter

The unipolar modulation is given as the gating pattern for theswitches in the H-bridge. The on-off square pulse is given to theswitches in between the legs in complementary manner. This in-verter acts as the second stage of the micro-inverter and the valuesobtained from simulation are shown in Table V

TABLE V : MICRO-INVERTER WITH HERIC TOPOLOGY

C. Push-pull InverterThe push-pull inverter [12] is a transformer based isolated six

switch inverter topology. The circuit is shown in fig 14. Six switchesare employed and also requires a transformer with a center tappedprimary and secondary. The two switches are connected in anti-parallel in the upper and lower legs of the secondary of the trans-former. Two more switches are connected to the primary side. Theload is connected between the upper leg and the centre tap of thesecondary side of the transformer.

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Fig 14. Push-pull inverter

The Table VI shows the various values obtained from simulation.

TABLE VI : MICRO-INVERTER WITH PUSH-PULLTOPOLOGY

D. Comparison of the inverter topologies.The three inverter topologies are compared on the parameters

like Efficiency, THD and switching losses. The inverter with bestresults is chosen for the implementation.

The Table VII shows the comparison of the complete micro-inverter with three topologies of the inverter.

TABLE VII : COMPARISON OF INVERTER TOPOLOGIES

From the above table, it can be inferred that the H5 topologyhas reduced harmonic content and increased efficiency. It can alsobe seen that the switching losses for H5 is lesser than those ofHERIC and Push-pull. Thus, the H5 inverter topology is chosen asthe second stage in the micro-inverter system.

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6 MODULATION STRATEGIES

The main objective behind adopting control strategies is to generategood quality controllable AC voltage and to minimize the harmonicdistortion, switching losses and the filtering requirements. Variousmodulation techniques for VSI are reported in the literature. Themodulation strategy discussed in this paper is Pulse Width Modu-lation (PWM). Pulse width modulation is the process of modifyingthe width of the pulses in the pulse train in direct proportion to thecontrol voltage. The different techniques are discussed in this paperby varying the high frequency carrier signal, keeping the referencesignal as the sinusoidal wave. The different modifications includeunipolar, bipolar, sawtooth and inverted sine signals. The differ-ent PWM techniques are analyzed on their THD and efficiency todetermine the technique for power quality improvement. Since thechosen topology of inverter is H5, the gating patterns for the fiveswitches of the inverter will be illustrated in the upcoming sections.

A. Unipolar ModulationThe unipolar modulation[11] normally requires two sinusoidal

modulating waves Vref1 and Vref2 of same magnitude and fre-quency but 180 degree out of phase. The two modulating wave arecompared through a common triangular carrier wave Vcarrier gen-erating two gating signals Vg2 and Vg3 for the upper two switchesS2 and S3. The gate pulse generation is shown in Fig 15.

Fig 15. Comparison of reference and carrier signal for unipolarstrategy

B. Bipolar ModulationThe sampling of SPWM bipolar switching is that the reference

sinusoidal waveform having magnitude Vref is compared with tri-angular carrier signal having amplitude Vcarrier. The upper andthe lower switches in the same inverter leg work in a complimentary

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manner which means the gating signals are generated for only oneof the switches in each leg and the compliment of the same is givento the other switch belonging to the same leg. The gating patternis shown in fig 16.

Fig 16. Comparison of reference and carrier signal for bipolarstrategy

C. Sawtooth ModulationThe switches work in the unipolar switching pattern with the

high frequency carrier wave replaced by the sawtooth signal. Theon pulse is generated when the amplitude of sine is greater thanthat of the sawtooth wave. The graph is shown in Fig 17.

Fig 17. Comparison of reference and carrier signal for sawtoothstrategy

D. Inverted sine ModulationThe rectified sine wave with the diodes reversed to obtain neg-

ative polarity is given as the carrier signal to the comparator. Thismodulation is also unipolar. Fig 18. depicts the pattern.

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Fig 18. Comparison of reference and carrier signal for invertedsine strategy

E. ComparisonThe above mentioned PWM strategies were simulated and their

THD and efficiency was obtained. The tabulation of the results isshown in Table VIII

TABLE VIII : ANALYSIS OF PWM STRATEGIES

Figure 1 Subdivision of System Reliability

From the results of simulation, it can be inferred that the in-verted sine PWM technique has lesser THD. Since, harmonics playa vital role in the power quality, a compromise is made with the1% lesser efficiency of inverted sine compared to the higher unipolarefficiency. Therefore, the inverted sine PWM technique is taken forimplementation. The output voltage waveform is of H5 inverter isshown in fig.19

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Fig 19 Output voltage of H5 inverter

7 SIMULATION RESULTS

The final simulation of the proposed micro-inverter is done with thePSIM software. The source is the 40Wp PV module modelled us-ing the single diode model. Incremental conductance is the MPPTalgorithm to track the maximum power from the panel and con-trols the switching of the converter switch. The H5 inverter withthe inverted sine PWM technique completes the second stage of themicro-inverter system. The inverted sine PWM strategy is imple-mented.

The complete schematic is shown in Fig 20.

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Fig 20. Schematic of the complete simulation

The simulation parameters are tabulated below.

TABLE IX : SIMULATION PARAMETERS

The gating pattern for the five inverters of H5 inverter is shownin fig 21.

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Fig 21. Gating pattern of the H5 inverter

The simulation waveform of the complete micro-inverter systemis shown in fig 22.

Fig 22. Waveform of a) Input of converter and output of converterb)Output of H5 inverter.

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The simulation results of the micro-inverter system is tabulatedin Table X.

TABLE X : RESULTS FROM SIMULATION

8 CONCLUSION

This paper has analyzed a two-stage micro inverter for photovoltaicapplications. For obtaining maximum power from PV, this paperhas explored incremental conductance algorithm whose tracking ef-ficiency is higher compared to the conventional P&O method. Var-ious inverter topologies were analysed for the micro inverter andfrom the simulation results, it is found that H5 inverter resulted inreduced total harmonic. To control the operation of the inverter,different modulation techniques were investigated and the techniqueusing inverted sine wave as carrier was efficient than the other tech-niques. Hence, finally H5 was interfaced with PV based DC-DCboost converter with INC MPPT algorithm and overall, this two-stage micro-inverter resulted in reduced total harmonic distortionthereby improving the power quality. Therefore, the proposed mi-cro inverter is an appropriate topology for PV applications.

9 ACKNOWLEDGMENT

The project is internally funded by the management of SSN Col-lege of Engineering. We thank the management for the continuousencouragement and the support they have given us in pursuing thisproject.

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