Novel Multi-Input Solar PV Topologies for 1-𝜙 and3-𝜙 Stand alone Applications to Mitigate the Effects

of Partial ShadingRupesh G. Wandhare, Student Member, IEEE

and Vivek Agarwal, Senior Member, IEEEDept. of Electrical Engg.,

Indian Institute of Technology- BombayMumbai, INDIA- 400076, Ph: (+91) 22 2576 4440Email: rupesh@ee.iitb.ac.in, agarwal@ee.iitb.ac.in

Sachin JainDept. of Electrical Engg.,

National Institute of Technology- WarangalAndhra Pradesh, INDIA- 506004

Email: jsachin@nitw.ac.in

Abstract— This paper proposes novel boost type invertertopologies and their control scheme for solar Photovoltaic (PV)applications. The proposed topologies provide a single powerstage, low part count, highly efficient solution for 1-𝜙 and 3-𝜙 stand alone applications. A notable feature of the proposedtopologies is that they facilitate splitting of a given PV sourceinto two or more sub-parts by offering as many input ports. Thus,they offer excellent features with respect to optimal arrangementand configuration of the PV panels and maximum power extrac-tion, which are especially beneficial under partially shaded condi-tions. For analysis of the control strategy, a model is derived forone of the sections of the proposed system, which is then extendedfor the controller design of the overall system. The plant polecancellation and reference signal frequency information approachis used for the compensator design to track the reference signalthat includes both AC and DC components. The compensatorsare designed for a fast dynamic response and accurate referencetracking. The topology modification for the three phase loads hasalso been analyzed and evaluated. The system is simulated andresults show the ability of the proposed configuration to producedesired high quality supply, fed from split sub-sections of a PVsource. A laboratory prototype is developed with available PVpanels for 1-𝜙 topology. Experimental results obtained from thisprototype validate the analysis and simulated results.

I. INTRODUCTION

Solar PV converts solar energy directly into electricitywithout any intermediate conversion stage. As this process iseco-friendly and the solar radiation (energy) is cost free, solarenergy harvesting through PV is a leading area of researchin the renewable energy field today. Unfortunately, the DCelectricity generated by the solar PV rarely has the form and/ormagnitude that can be directly used to drive an electrical load.A power conditioning unit (PCU) is almost always requiredand constitute a vital component of any solar PV system. Thus,energy engineers have always been looking for compact, highperformance power converter topologies.

In recent times, there has been an unprecedented interest intapping solar energy through PV, which has led to tremendousresearch efforts in all aspects of solar PV technology. As aresult of this and continuously increasing demand, the PVpanel prices have declined sharply - so much so that the

PCU cost now constitutes a significant fraction of the overallcost of the solar PV system [1]. With the growing marketcompetition, cost has become an important factor in the designand development of the PCU’s for solar PV systems. Nowonder, a major research component today is focussed onways to decrease the cost of the PCU.

Due to their low cost and simple control, the single powerstage H-bridge converter and its derivatives with single inputport have been extensively exploited for PV power condi-tioning applications [2]. However, wide variation in the PVoperating voltage and step down nature of the H-bridge resultsin the use of long strings of PV modules to meet the high inputvoltage demand of these topologies. However, use of long PVstrings results in the following drawbacks:

1) Partial shading issues are more likely and more pro-nounced with long strings of PV modules,

2) Long strings are more likely to have module mismatchproblems resulting in reduced power yield and

3) Long PV strings suffer from drawbacks such as hot-spots, reduced safety and increased probability of leak-age current through the parasitic capacitance betweenpanel and system ground [3].

To overcome the above issues, one of the alternativestried out by the researchers uses a front end transformer forstepping-up the output voltage, to meet the high load voltagerating [1], [4]. Adding a transformer, however, adds to the bulkof the system, increases the cost and reduces the efficiency.Thus, there is a requirement of another single power stagetopology with boosting and MPPT capability.

Some of the latest work reported in the literature describestopologies with voltage boosting capability by their invertersections to interface PV panels with the load or grid. Some ofthese single power stage topologies with single port input havebeen discussed by Xue et al. [5]. These are good solutions;however most of these topologies are based on buck-boostconverter operation and have higher part count. Another goodproposal given by Peng [6] is based on a Z-source converter.

978-1-4673-4355-8/13/$31.00 ©2013 IEEE 76

Fig. 1. (a) PV installation with single output, modules in series; (b) PVinstallation with single output, strings in parallel; (c) PV installation withmultiple output, PV arrays are split into sub-sections and independentlyplaced.

This is an interesting solution but requires more number ofdevices and have a single input port for the PV source. Otheralternate solutions using buck-boost operation with singleinput port for PV source have also been given by Jain et al. [3]and Patel et al. [7]. These topologies have a lower part count.Though these are low part count, highly efficient topologiesand promising as they can transform low input voltages,perform MPPT and do inversion, there is scope for furtherimprovement. As the given system uses single input port forthe PV source, it suffers with problem of partial shading. Also,probability of partial shading further increases with the usageof single input port. Apart from increase probability of shadingthere is also an issue with MPPT in such systems.

In the context of partial shading of PV source, a majorresearch effort is underway on distributed power conditioningand MPPT of individual PV module to enhance the poweryield and to attain optimal MPPT [8], [9]. The distributedschemes overcome the partial shading effect with compen-sating converters connected across each PV module. Thephilosophy behind this development is to avoid the operationof bypass and anti-blocking diodes without incurring reversevoltage across the shaded panels. Distributed MPPT schemesare still evolving and currently seem to suffer with drawbackssuch as high part count and control complexity.

In view of above and current trend, the multi-port typephotovoltaic sources with low voltage PV strings and theirpower conditioning is a pioneering area of research. Theyreduce the number of modules in series to form an array andalso reduce the number of PV strings connected in parallel,for the same power capacity, as shown in Fig. 1. Single panelis shown in figures for each part for more clarity, but the

installation may include string or array of PV modules for eachset. The PV configuration with independent outputs shown inFig. 1(c) is always beneficial compared to the arrangementshown in cases (a) and (b) with single output. The splitPV arrays can be located at strategic locations, so that theindividual PV sets get uniform radiation through out the dayor session. These PV arrays form independent inputs to thesingle power conditioning unit.

In this paper, topologies are proposed based on singlestage power conversion for the photovoltaic generation withmulti-port input. The PV installation can be divided into two(or more) sections for a single phase and three (or more)sections for three phase topologies. Additional advantage ofthe proposed topology is that it provides boost operation.Hence, compared to the single stage H-bridge converters (andits derivatives) which need higher input DC voltage, proposedtopology can work with comparatively much lower inputvoltage.

The battery storage can be included depending upon thecriticality of the load. In this paper, analysis is done formodelling and compensator design to achieve fast dynamicresponse and power quality of the load voltage for a singlephase load. Further, extended topology is provided for a threephase load which has same working philosophy as that ofsingle phase version, with the multi-input PV sources. Theenergy storages are not considered in the proposed system, asthe main focus is on the topological aspect for the multi-inputPV sources.

The topological description for the proposed system isprovided in section II with adequate explanation for the advan-tages of split PV sources in the proposed system. This sectionis followed by the modeling and controller designing issuesand analysis in section III, based on the equivalent circuit ofthe proposed converter. The control strategy is formulated andsystem is simulated with chosen and design parameters andevaluated further in sections IV and V. A developed prototypeis explained and results are verified by experimentation insection VI.

II. SYSTEM DESCRIPTION

The conventional photovoltaic installation with bypassdiodes and reverse power flow blocking diodes [Fig. 1(a) and(b)], are good for protection but yield less power during partialshading. The PV modules form a string or array and provideda single output to the conventional power conditioning unit,which consists of only one input port as shown in Fig. 2(a).The shaded panels from the installation get bypassed or anti-blocked by the protection diodes and system loses energy.In the proposed topology of the DC-AC inverter, the PVinstallation can be divided equally into two parts. Each ofthese parts can be located for more chances of getting uniformillumination, though these two sets can be accommodated withdifferent radiation level, individually, as shown in Fig. 2(b).PCU accept two inputs from the PV system. A power circuit

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Fig. 2. Partial shading of PV installation with: (a) one output for converter; (b) two outputs for converter

Fig. 3. The proposed power circuit for the single stage split type PV sources for a single phase load.

of the proposed system is shown in Fig. 3 which can feed withtwo individual inputs from the PV installation.

The inductor, capacitors and two power semiconductorswitches on the left side of Fig. 2 form one section ofthe converter. One set of the PV array with input capacitorbank forms the DC source to this section. Likewise, similarelements on the right side of the converter and other set of PVarray form the other section of the converter. Complementaryoperating switches and inductor provide the boost action.Though, individual section appears as a bidirectional DC-DCconverter, the combined control result in the inverter action, ifthe output capacitors generate anti-phase output voltages withequal DC offsets, explained in the section III.

The voltage at the output capacitor of each section canbe controlled by the reference derived from the load voltageregulation. The subsequent generated references can includethe anti-phase AC signals at the desired output frequency andthe DC signal of equal magnitude that would eventually getcancelled at the output terminals.

The high switching frequency elements, including inductorwith ferrite core or high flux density amorphous iron corecan make the system more compact and light weight. In thiscase, system can give more power density and is expected toprovide high efficiency. It also invites the modular design forthe dedicated PV-load system.

Each section of bi-directional DC-DC converter and itsdesign are crucial element in the desired operation of theproposed system, as it includes the AC signals. In such atype of AC reference control, controllers must have very highbandwidth and it should include converter (plant) parameters.The controllers employed should be elegant enough such thatthe output must track the AC reference without any phase lag

or/and magnitude attenuation. The widely used PI regulatorcan not perform desired task on the above requirement forthe AC operation. In this case, the lead-lag compensator canprovide a suitable response in the DC and low frequencyregion. But the high bandwidths with accurate tracking are stillnot achievable with only conventional lead-lag compensators.The resonance controllers are one of the extensively exercisedarea in the control system. The reference signal frequency canbe included in the compensator filter to track it. Alternativeand more analytical approach of compensator design based onplant pole cancellation and reference signal frequency infor-mation method, proposed in the [10] is used for developingcontroller for the proposed system.

III. SYSTEM CONTROLLER DESIGN

The performance of the inverter greatly depends on thecontroller employed to control output capacitor voltages byeach section of the converter as shown in Fig. 4. A small signallinear model is derived for one section to design compensator,later both sections are controlled using the same values of thecompensators.

Equation (1) is the linearized model of the output capacitorvoltage to duty control [11] for one of the sections. Theperformance of the converter is greatly influenced by theconverter element values; 𝐿1, 𝐶1 and 𝑟𝐶1.

Each section of the converter forms a second order system.In the controller, information about the reference input signaland poles of the transfer function derived for sectional powerconverter are incorporated by the controller transfer functiongiven by (2), where 𝜔𝑜 is the frequency of AC component ofthe reference signal.

The effect of the double poles by elements of the converteron the plant frequency response is nullified by the added

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𝑣𝐶1(𝑠)

𝑑𝑆1(𝑠)=

𝑟𝐶1

𝑅(1−𝐷1)2𝑣𝑃𝑉 (𝑠+

1𝑟𝐶1𝐶1

)[𝑅−𝑟𝐶1

𝐿1(1−𝐷1)

2 − 𝑠]

𝑠2 + ( 1𝑅𝐶1

+ 𝑟𝐶1

𝐿1(1−𝐷1))𝑠+

𝑟𝐶1

𝑅 (1−𝐷1)1

𝐿1𝐶1+ (1−𝐷1)2

1𝐿1𝐶1

(1)

𝐺1(𝑠) =𝑠2 + ( 1

𝑅𝐶1+ 𝑟𝐶1

𝐿1(1−𝐷1))𝑠+

𝑟𝐶1

𝑅 (1−𝐷1)1

𝐿1𝐶1+ (1−𝐷1)

2 1𝐿1𝐶1

𝑠2 + 𝜔2𝑜

(2)

Fig. 4. Equivalent circuit of one of the sections of the converter

zeros. Hence, further stages of the compensator can be usedfor a wide bandwidth and required phase margin to achieveacceptable response. Further, the added poles around desiredreference frequency 𝜔𝑜 in (2), provide very high gain so as toensure attenuation-free AC signal tracking of this frequencywithout significant phase lag.

The resultant phase roll off by the controller 𝐺1(𝑠) iscompensated by the lead compensator as given in (3), so as toachieve the desired phase margin, 𝜙𝑃𝑀 at the chosen gain cut-off frequency 𝜔𝐶 . The switching frequency of the convertersections and reference load frequency are important criterionfor deciding this cut-off frequency.

𝐺2(𝑠) =𝑠+ 𝜔𝐶

𝑠+ 𝜔𝐶

√𝑡𝑎𝑛−1(∅𝑃𝑀 )

(3)

The resultant gain plot of the above compensated plant canbe made to cross at the desired gain crossover frequency byadjusting the DC loop gain K.

Additionally, to enhance the gain plot in lower frequencyregion and to achieve accurate and optimal reference signaltracking, the lag compensator can be added as follows;

𝐺3(𝑠) = 𝜉

𝑠𝜔𝑔1

+ 1𝑠

𝜔𝑔2+ 1

(4)

where 𝜔𝑔1 and 𝜔𝑔2 are chosen for very low frequencythan desired output reference signal frequency 𝜔𝑜, with lagcompensator condition 𝜔𝑔1 = 𝜉 𝜔𝑔2 . This condition and thechosen region for the lag compensator ensure that, it would

not disturb the phase plot at and above the desired outputfrequency.

The parameters considered for the study of inverter sectionare presented below. The element values given for one ofthe sections of the converter are shown in Table I. The sameelemental values are used for the other section of the converter.

TABLE I

PARAMETERS OF THE SYSTEM FOR STUDY

Parameter Value𝑅𝐿 70 Ω𝑉𝑟𝑒𝑓 230 𝑉𝐴𝐶

𝑉𝑃𝑉 𝑀𝑃 230 𝑉𝐼𝑃𝑉 𝑀𝑃 5 𝐴𝐶1 47 𝜇𝐹𝑟𝐶1 0.15 Ω𝐿1 500 𝜇𝐻𝐷1 0.413𝜔𝑜 2𝜋50 rad/s𝑓𝑆 30 𝑘𝐻𝑧𝑉𝐷𝐶𝑠ℎ𝑖𝑓𝑡 375 𝑉

where 𝑓𝑠 is the switching frequency of the converter sec-tions. The band width for the system operation is chosen as𝜔𝑐=10 000 𝑟𝑎𝑑/𝑠𝑒𝑐.

The above design procedure has been followed for thedesign of controller elements and their resultant transfer func-tions are tabulated in Table II.

TABLE II

CONTROLLER ELEMENTS FOR THE DESIGNED SYSTEM

Compensator Element Transfer Function

𝐺1(𝑠)𝑠2+392.6𝑠+3.742×106

𝑠2+9.87×104

𝐺2(𝑠)𝑠+2679

𝑠+3.732×104

𝐾 0.0180

𝐺3(𝑠) 40𝑠20

+1𝑠

0.5+1

The above obtained compensator elements are used with thesectional converter transfer function to plot the Bode plot todepict the performance of the controller and it is shown inFig. 5.

The sharp peak in the gain plot at the frequency 𝜔𝑜 of theFig. 5 ensures AC signal tracking without any attenuation in

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Fig. 5. Bode plot for one section of the Propose System

the magnitude at the output capacitor voltage.

The advancement in microelectronics has led to high com-putational speed and cheaper micro-controllers which can pro-vide great flexibility and act as crucial element in the controlpart. The above compensator can be transformed and dis-cretized using a suitable approximated numerical integrationtechnique and can be used in the control strategy, explainedin the next section.

IV. CONTROL STRATEGY

Control strategy for the proposed system is shown in Fig.6. It is based on the output capacitor voltage control using thederived reference signal and output capacitor voltage feedback.The reference signal for the load output voltage is passedthrough the power availability check and the correspondingload power. If the available PV power is more than the loadrequirement, same reference signal can be passed from theblock. In case, the power availability with PV installation fallsbelow the load requirement, the reference signal is modifiedand limit the reference voltage equivalent to MPP power. Thecritical loads can also be fed by this proposed topology withthe intervention of batteries in the input stages, owing to thecriticality of the load.

In further control, the AC reference signal 𝑣∗𝑜 is divided intotwo parts and phase shifted to opposite phase before addingto DC shift. The DC shift in the reference should ensure theconverter operation with finite duty ratio i.e. above zero value.It is also necessary to ensure that the converter operation doesnot cause the saturation of the inductor and does not violatepeak current capacity of the power semiconductor devices.The DC shift value can be taken just above half the valueof the peak of required AC output voltage. Obtained referencefor the control of output capacitors are used further for thecontrol of the system output voltage. The controlled duty ratiois compared further with triangular carrier signal to generatethe duty cycle for the four switches of the converter as shownin Fig. 6.

Fig. 6. Control strategy for the operation of the proposed system for a singlephase load.

Advance micro-controller includes a dedicated PWM gener-ator with lots of flexibility for frequency of PWM, comparingwaveform shapes (saw-tooth or triangular) etc. It acceptscompare value (which hold duty cycle) and directly givesPWM pulses. It is also possible to directly generate the anti-phase PWM on two different channels or pins. The obtainedcompensator can be discretized and above task of control canbe easily performed.

Some loads demand power for long run or uninterruptedpower supply. Energy storages like batteries, ultra-capacitors,etc can be added to provide the integrated PV fed systems. Theenergy storages are not considered in the proposed system, asthe main focus is made on the topological configuration for thesplit sources. The energy storages can be added to the inputterminals through suitable interface.

V. SIMULATION RESULTS

The complete system for the single phase load is modeledin detail in the MATLAB-Simulink software for validation ofthe presented single phase power and control circuit. The PVpanels are modeled with their nonlinear equation involvingexponential term. The total PV installation capacity of the1000 𝑊𝑎𝑡𝑡 at 1000 𝑊/𝑚2 is used for the simulation withload power of 750 𝑊𝑎𝑡𝑡. The simulation results are shown inFig. 7.

Simulation results of the reference and actual voltage ofoutput capacitor show the efficient tracking by the superiorsets of compensator parameters. The duty ratio modulationfollows the output capacitor voltage reference. The simulationresults corresponding to the output capacitor voltage of boththe sections show that the DC offsets would get cancelledacross the load loop and AC components will add up forthe total load voltage, correspondingly load voltage and loadcurrent are shown in the graph. Both active and reactive powersare shown in the graph. Reactive power is zero for resistiveload and high quality output voltage can be achieved.

The proposed system is modified for the possibility of thesplit photovoltaic sources feeding three phase standalone load.The power circuit is shown in the Fig. 8.

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Fig. 7. Simulation results for reference and actual voltage at the outputcapacitor of one section, duty ratio of the same section for boost switch,output capacitor voltages for both section, output load voltage and currentand output power.

The three phase topology shown in Fig. 8 is modelled usingMATLAB-Simulink. The corresponding simulation results areshown in Fig. 9. The results show that the output voltagewaveforms are rich in quality and balanced.

VI. PROTOTYPE DESIGN AND EXPERIMENTAL

VERIFICATION

Multi-input topology, presented in this paper, has beendesigned in a prototype with the available 1.5kW solar pho-tovoltaic panels shown in Fig. 11(a). Standard modules areconfigured in series and parallel in two different sets toachieve desired PV array voltages of 220V near to MPP atfull radiation level. Both the outputs from PV installationare used as inputs to the developed hardware. The prototypeshowing I/n PV1, I/n PV2, high frequency L1 and O/p C1form one section of the proposed inverter, similarly othersection components are marked in Fig. 11(b). Texas Instru-ments Piccolo Microcontroller TMS320F28027 is used forthe control. The discretized model of the controller derivedfrom Trapezoidal Dicretization Technique is coded and load

Fig. 8. A power circuit for the single stage split type PV sources for a threephase load.

Fig. 9. Simulation results for the three phase load voltages.

in to the microcontroller using Code Composer Studio CCS.Signal conditioning for ADC and driver circuit for the PWMare designed to integrate microcontroller to the power circuit.

Turning on the system, generates output voltage across load,𝑣0 is shown in Fig. 10(a). Each section of the hardwareproduces voltages, 𝑣𝐶1 and 𝑣𝐶2 and they are in anti-phase.These anti-phase voltages add-up in the output and DC shiftgets canceled, resulted in the symmetric AC load voltage, 𝑣0.Fig. 10(b) shows the harmonic histograms with dominatingfundamental components. Inaccuracy and mismatch of theelements used in feedback path of the signal conditioningcircuit may induce DC shift in the output under steady state. Amodified DC biasing scheme is incorporated in the program,where differential average voltage between 𝑣𝐶1 and 𝑣𝐶2 isreduce by biasing to 𝑣𝐶1 or 𝑣𝐶2 over a wider time scale. Iteliminate the DC voltage at the output and produces qualitypower as it is clear from Fig. 10(b). To present running valuesof duty cycles, digital signals, 𝑑1 and 𝑑2 are fetch to the PWMpins and they are filtered by RC filter at the output of the PWMpins. It acts as DAC operation. The resultant duty cycles 𝑑1and 𝑑2 are shown in Fig. 10(c). As expected they are also

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(a) (b)

(c) (d)Fig. 10. Experimental results obtained from a prototype

Fig. 11. Prototype of the proposed single phase topology.

anti-phase to produce 𝑣𝐶1 and 𝑣𝐶2. Finally, Fig. 10(d) showsload voltage and current for the resistive load regulated at thedesired RMS voltage value.

VII. GENERALIZED MULI-PORT TOPOLOGY FOR SOLAR

PV

The topology presented in Fig. 3, can be generalized forn ports (for a PV installation split into n sub-sections) withmultiple DC-DC boost converter modules shown in Fig. 12.AC component of voltage generated by each sections add togive cumulative large voltage across the load from relativelyless voltage PV strings. The DC shifts generated across eachoutput capacitors get cancel in the loop. Similar generalizedmulti-port topological configuration can also be produced forthe three phase system. It can be the future scope of work.

VIII. CONCLUSION

The presented topology has shown encouraging resultsunder partial shading effects on photovoltaic panels. The 2

Fig. 12. Generalized multiport topology for PV.

input terminals in case of proposed single phase topology (and3 input terminals in case of three phase topology) divide thePV source and provide more probability of uniform radiationon the split PV panel sections, thereby reducing the chancesof bypassing the PV array by the protection diodes in case ofpartial shading. The power output of split PV sources can beconditioned using the presented single stage power convertersfor both single phase and three version. Additionally, the pro-posed topologies, provide the boosting feature. It is observedthat analytically derived controllers faithfully track both theAC and DC signals. The possibility of grid integration withthe proposed topologies can be further explore.

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