2007 IEEE Canada Electrical Power Conference

The Application of the Cascaded MultilevelConverters in Grid Connected Photovoltaic Systems

S. Ali Khajehoddin*, Alireza Bakhshai+, and Praveen Jain§*khajeddin@ieee.org, talireza.bakhshai@queensu.ca, §praveen.jain@queensu.ca

Energy and Power Electronics Applied Research Laboratory (ePEARL)ECE Department, Queen's University, Kingston, Canada

Abstract- Unique features of multi-level converters have re-cently nominated them as significant alternatives for solid-state PVpower converting units, even in the low and medium power range. cl IThe fact that Multilevel converters need several DC sources in theDC side, makes them attractive for Photovoltaic(PV) applications.This paper presents a new control strategy to control Cascaded ceniralMultilevel converters in a multi-string configuration for single 2-leN7el convertersphase grid connected systems. Eventually, simulation results are striLgsprovided to validate the control system under various insolation Multilevel convertersconditions.

I. INTRODUCTION With DC-DC Converter

Nowadays, the main energy supplier of the worldwideeconomy is fossil fuel. This, however has led to many prob- Without DC-DC Coinveerlems such as global warming and air pollution. Therefore,with regard to the worldwide trend of green energy, solarpower technology has become one of the most promising Withisolationenergy resources. The number of PV installations has had anexponential growth [1], mainly due to the governments and Witlio-Lt isolatonutility companies who support the idea of the green energy.One of the most important types of PV installation is the

grid connected inverter configurations. These grid connectedPV systems can be categorized from two viewpoints: PVcell and inverter configurations, see Fig. 1. The PV cell ar- On the HFsiderangements fall into four broad groups: centralized technology,string technology, multi-string technology and AC-module andAC-cell technologies [2]. Fig. 1. PV systems categorized by different PV cell configurations andinverter types.

All approaches have advantages and disadvantages [2], [3];and will compromise various attributes such as harmonicgeneration, complexity, efficiency, flexibility, reliability, safety,modularity and cost. However, for residential PV installations, topologies which are able to generate better output quality,the most suitable configuration seems to be the string or multi- while operating at lower switching frequency. This impliesstring technologies where one or more strings of PV cells are lower switching dissipation and higher efficiency. Moreover,connected to a single inverter. Using this type of configuration, this topology utilizes switches with lower breakdown voltage;there will be no losses associated with the string diodes therefore, it can be used in higher power applications atcompared to centralized technology. Moreover, independent lower cost. It is worth mentioning that although the numberMaximum Power Point Tracking (MPPT) is possible for of switches in this approach is higher than other two levelall strings which might be installed in different sizes and topologies, for a sufficient high number of levels, the outputorientations. This also increases the overall efficiency under filter can be avoided which means less weight, cost and space.special circumstances like partial shadowing. On the other hand, even with the same size of filter at the

There are different approaches to implement string and output, the switching frequency can be decreased which meansmulti-string topologies. Usually, these modules consist of a higher efficiency. In general, a greater number of switches insolar array and a DC to DC converter controlled by a MPPT multilevel converters can be justified since the semiconductoralgorithm. Afterwards, the output of the DC/DC converters cost decreases at a much greater rate than the filter componentsbuild up a DC voltage which is then converted to AC by means cost. This projects the total cost of multilevel converters to beof an li426ell T35o^sX0tt use multilevel296omparable or even lower than that of two-level converters.

2007 IEEE Canada Electrical Power Conference2

Among various multilevel topologies, the most important Leg a Leg bones are [5]: Diode-Clamped Multilevel Converter(DCMC) VI

[6] and Flying Capacitors Multilevel converters (FCMC) and C l+ LsCascaded Multilevel Converters (CMC). The first, simplest String Vdl +and the most modular topology is CMC. However, the main lSi Vollproblem associated with the CMC topology is the need for

IL+

isolated DC sources which are not usually available without 'Vg+dthe use of transformers. In some specific applications suchas photovoltaic systems, separate dc sources exist and can beused in the CMC topology [7], [8]. A diversity of multilevel V2 +converter topologies have been used in photovoltaic applica- C2 + Vo2tions [9], [10], [11] and a comparison of some topologies ispresented in [12].

This paper presents a new control strategy to control Cas-caded Multilevel converters in a multi-string configuration for (a)single phase grid connected systems. This topology generates mfi!reXe Leggb,,wce ChrilerbridgICarierbrid04high quality output current under any circumstances specif-ically in partial shading, while tracking the MPP of each Q5(string independently. The topology does not consist of anyextra DC-DC converter stage which causes some limitationin the performance but definitely reduces the overall cost -land efficiency. Simulation results are provided to validate theproposed control system. -'

II. CASCADED MULTILEVEL CONVERTERSWUI--------

A. Basic Principle of Operation -|MThe Cascaded Multilevel Converters (CMC) are simply a 0,0 - -

number of conventional two-level bridges, whose AC terminalsare simply connected in series to synthesize the output wave-forms. Fig. 2(a) shows the power circuit for a five-level inverterwith two cascaded cells. The CMC needs several independentDC sources which may be obtained from batteries, fuel cellsor solar cells. 100MI

Through different combinations of the four switches of "A)each cell, each converter level can generate three different . 001l

voltage outputs, +VdC, 0, -Vd, The AC output is the sum _20_ _ _ _' . l~~~~ ~ ~~~0.iSt00 I(llojiwJ( 2iQ11of the individual converter outputs. The number of output- Tis

phase voltage levels is defined by n = 2N+1, where N is the (b)number of DC sources. For instance the output range of theFig. 2(a) swings from -2Vd, to +2Vd, with five levels. If the Fig. 2. (a) Cascaded multilevel converter with separate dc sources, (b) Phasestraightforward fundamental frequency modulation technique shifted modulation strategy and the associated outputs.is chosen it can be shown that, the charge and discharge ofthe cells in different levels will not be equal which resultsin capacitor voltage unbalance or unequal loading of input . The modularity of this topology is an important feature,sources. It is possible to utilize CMCs without input sources and because of that some redundancy is possible by usingas a reactive power compensator. But, if the output load in more cells per phase than is actually required.a CMC is resistive, these capacitors should be connected to . Because of its modular structure, control is more easily"isolated DC sources" to supply the real power. applied.

* Compared to other multilevel topologies, CMC requiresB. Features least number of components, because there is no need for

clamping diodes and flying capacitors.In summary, the advantages and disadvantages of the CMC capn idsadfyn aaiosare as folloWS: 2 os

1) Pros. . Each cell needs an isolated DC supply and normally this* Device voltage sharing is automatic and there is no requires some sort of complicated transformer arrange-

restriction on switching patterns. ment.* CMC has smaller dv0/dt compared to series connected The aforementioned disadvantage is not an issue in Photo-

2-level. 29voltaic applications, because discrete strings of PV modules

2007 IEEE Canada Electrical Power Conference3

I ~~~~~+ V L

ID IPV 'i-0

Rh~ ~~~~~~~~~~~,VLS2

ip Rs xiV".VLgrid

Fig. 4. Vector diagram of the output power circuit shown in Fig. 2(a).Fig. 3. Equivalent circuit for a PV cell.

Using this PV array model it is possible to simulate theprovide isolated input voltage sources. As shown in Fig. 1, dynamic performance of the power and control systems andin photovoltaic applications the inverter stage can be imple- MPPT strategy in response to the radiation and temperaturemented with or without a DC-DC converter. However, in this step changes.paper the topology without a DC-DC stage is examined. Thereare different options for modulation of multilevel converters IV. POWER CONTROL ALGORITHM[13]. One of the simplest strategies is the phase shifted carrier A. Basic Principle of Operationmodulation technique where the n carriers of the full bridge The basic structure of the grid connected multilevel multi-cascaded converters are phase shifted by 180/n degrees, asshown in Fig. 2(b). This modulation technique is utilized in volstring PV system is shown in Fig. 2(a). Since the outputthis paper due to its simplicity. However, it can be shown

and the temperature of the cells the operating point of the PVthat under partial shading the harmonic cancellation is not as and he te rte of th ce he on the PVperfect as the ideal case but still much better than a two level arrays has to be controlled to be held on the Maximum Powerconerfect as the ideal case but still much better.than a twolevelPoint. This objective can be satisfied simply by setting the DC

voltage of the capacitors to a reference voltage provided by aMPPT algorithm. Examination of different MPPT algorithms

III. SIMULATION MODEL FOR PV CELL seems to be out of place in this paper and thus it is assumed

The building block of the PV array is the solar cell, which is that a reference DC voltage is provided for each string.basically a p - n semiconductor junction that directly converts Since the power delivered by each full bridge converter canlight energy into electricity. The equivalent circuit is shown in be controlled by switches, the power provided by the DCFig. 3. capacitors is controllable. On the other hand, at any specific

To simulate a PV array, a PV simulation model which was time the power supplied to the DC capacitors is known becauseobtained using PSIM(Power SIMulator) software, was used the voltage of the PV arrays is fixed by the DC voltage of thebased on the following equation: capacitors. Therefore, whenever the capacitor voltage needs to

be increased/decreased, it is possible to decrease/increase the

power delivered to the power circuit and the difference of theIPV = nplph - npIrs exP ( A

V-p i (1) active power controls the voltages of the DC capacitors.kTAlPl nhs Fig. 4 shows the vector diagram of the voltages and thewhere Ipv is the PV array output current (A); Vpv is current of the output power circuit. It is desired to have the

the PV array output voltage (V); ns is the number of cells output current in phase with the grid voltage in order toconnected in series; np is the number of strings connected have zero reactive power delivered to the grid. By simplyin parallel; q is the charge of an electron; k is Boltzmanns controlling the angle of this current and the grid voltage,constant; A is the pn junction ideality factor; T is the cell reactive power compensation is feasible which is not examinedtemperature (K); and Irs is the cell reverse saturation current. here. It can be shown that the power delivered to the grid is:The factor A in Eq. (1) determines the cell deviation from V Vithe ideal p - n junction characteristics. The ideal value ranges P XL sin ,o (3)between 1 and 5 and in our case, A equals 2.15. The cell XLreverse saturation current Ir varies with temperature and the Therefore, by means of changing the output voltage of thephotocurrent Iph depends on the solar radiation and the cell multilevel inverter the power delivered to the grid can betemperature as shown in the following equation: controlled. If the power supplied by each string is Pi, the

total power sent to the grid is Pto =ZPi. The power'ph =[sr+ i(- S) (2) supplied by each string is Pi V0i.IL cos f and the angle

[Iserk~(T Tr)1100 is cosy = Vgrid/ZV0i. Therefore, Pi V0i Pt0t/ZV0i whichwhere Isc is the cell short-circuit current at reference means that if the input power provided by a string is changed

temperature and radiation, ki is the short-circuit current tem- the output voltage of that cell has to be adjusted accordingly.perature coefficient, and s is the solar radiation in m/2.298Howvr as shown by the dotted line in Fig. 4, if the output

2007 IEEE Canada Electrical Power Conference4

CompensatorDe voltage regulator

I M / ,, X ~~~~~~Stabilizer Vdcl

P;n:~~~~~~~~~~~~~~~~L _v 20 cl ( i

Contributionfactor

Fig. 5. Control system block diagram of cell #1 of the circuit shown in Fig. 2(a)

voltage of one cell is reduced, the output voltage of other cells of the PV voltage leads to a reduction in power generation,have to be increased to keep the output current in phase with which may even descend to zero value. When the inputthe grid voltage. It can be shown that for a given input power, power becomes zero the contribution factor also becomesgrid voltage and inductance, the output voltage of the cells zero. This is multiplied by the feedback signal and then,will be set to: causes the current reference signal to become negative, which

rleads to instability. Once this occurs, the stabilizer switchesPi grid (4) the reference signal to be a constant positive number, which

°tPtot Vjri discharges the DC capacitors and causes the control system toexit from the unstable state.

B. Control Strategy If the fundamental components of the grid voltage andAs shown in Fig. 5, the control system consists of different current are in phase, the instantaneous power injected into the

sections. The main objective is to generate output voltages grid equals to:according to Eqn. (4) so that each string contributes its maxi- Po ttlcs2vrd) 5mum available power to the grid. This task is accomplished by - COS(2wgridt)) (5)the main control loop. Based on the calculated input power, This power has to be drawn from the input sources, whichthe reference output current is found which will be multiplied leads to an oscillation in the voltage and power of the PVby a sinusoidal waveform in phase with the grid voltage. The arrays. By increasing the size of the capacitors, this oscillationdifference of this reference current and the measured output can be limited to a desired value. To reduce the output currentcurrent is used for the feedback loop which in turn can be harmonics, a compensation factor is multiplied by the outputused as a reference for the output voltage. Since there is more reference voltage. In the end this compensation factor isthan one cell that builds the output voltage each cell generates divided by the average capacitor voltage to limit the controlan output voltage equal to V0o, =ZV0i.Pil/Pt0. Therefore, the signal to [-1, 11. Nevertheless, this oscillation is not desiredmeasured feedback current is scaled down by a contribution because it causes the PV arrays to operate slightly off of thefactor as shown in the block diagram in Fig. 5. MPP.By increasing the reference current signal, the output power

will be increased. Therefore, to regulate the DC voltage of V. SIMULATION RESULTSthe capacitors it is sufficient to feedback the error of the In order to demonstrate the impact of the shading andcapacitor voltage to the current loop as demonstrated in the irradiance level on the performance of the proposed system, ablock diagram. Because of the contribution factor which is simulation is setup as shown in Table I.a feed-forward compensation, and because of the existence It is worth mentioning that the output voltage of the PVof two control loops for each cell, the control system can string arrays should be chosen based on the grid nominalbecome unstable. Specifically this happens in the transient voltage and the minimum desired operating power of eachstate when the dc capacitor voltage overshoots. Escalation299cell. If the power generated by all strings are equal, the output

2007 IEEE Canada Electrical Power Conference5

Vo Vo

200.00 ----| t I 11 Q H 1 | | 200.00 4------L-L-=-L-=--------=-L-L-L----L-L-=-L-=-L-L-L-----L-0-L -=LL---L-L1-L=----100.00 [ I-Ill 1II[300000 IVv

-200.00 PP 1t ri| -10000

20.00 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ -00 L--l_ ___ '_ __-t l_ __--}X_ __-t- ________

1_ _l _'__ _i__ _._iL-2 0 0 --X--X---- --l ------tV 8-A - - 17 7X -V --0

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20.00 100.00<-100.0 0 /1 5000 _V500

-120.00

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.OI - --- -- - - - - - - - -- - - - - - - - - - - -

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Vol LOOK ---

100.00 0--50K50.00 ---

0.00.0

-100.00 ||| 1nl 211lSl b 11" 2 1: 1 :Vcapl Vcap2 Vgrid100.00 4

Veontrol for cell 1 50M01.00 00L ~ K71K~~K0O -50.00 L 2 4 - X 1 ' Z

0~~~~~~~0 ~~~~~~-100 00-0.50-1.00 A------v 0.0 0.20 0.40 0 60 0.80 1.00

0.20 0.25 030 0.35 Time (s)Time (s)

Fig. 7. Simulation results when the grid voltage is smaller than the voltageFig. 6. Simulation results for a step change in the input power when the grid of the individual PV strings.voltage (180V) is larger than the voltage of the individual PV strings(120V).

the stabilizer section in the controller, the system becomesvoltage of all cells will be equal. In this case, by referring to unstable after the irradiance decrease at t=0.7s.Eq. (3) it can be observed that the inverter output voltage hasto be slightly larger than the grid voltage as shown in Fig. 6and the DC voltage of an individual string is at least Vg--d/N VI. CONCLUSIONwhere N is the number of strings. However, as discussedbefore, in case of partial shading the output voltage of the Afe- re nrdcino dfeetpsil hie oshadedo stingwillcdecreaseof iacdingly, dthe output voltae minverters in Photovoltaic applications, it is shown that the

toaedtrigwlsincore yB thi nt .ge Cascaded Multilevel Converter is a suitable choice for PVof the other cells have toices.Btt1 Sntposstble

becase otherDcell hvo e to inreahstring tthiis notdesig tosver systems. Then after describing the basic principle of operation,because the DC voltage As sh ing is will creas the paper presents a new control strategy for cascaded multi-the whole output voltage. As shown in Fig. 6 this will increase lee cnetrs Tocos h etsz o h Vsrnsthe harmonic components of the output current. In order to elevelconverters. To choose the best size for the PV strings aavoid this problem, the minimum power of PV strings should gui ibe selected and then the DC output voltage of the PV strings the system under all environmental conditions. The simulation

results show that for partial shading and different input powers,can be calculated.the system is able to inject the maximum available power to

Fig. 7 demonstrates the dynamic performance of the PV the grid.system for a step changes in irradiance. At the time t=O.5sthe insolation of the second PV string is decreased by 50percent, and then at t=O.Ss the normal irradiance is applied. At ACKNOWLEDGMENTt=0.75s the whole insolation is decreased by 70 percent and itcan be observed that the harmonic components of the output The authors would like to thank the contribution of the Solarcurrent are not increased in case of partial shading. Without300Buildings Research Network for their support.

2007 IEEE Canada Electrical Power Conference6

TABLE I[4] G. Walker and P. Sernia, "Cascaded DC/DC converter connection ofSIMULATION PARAMETERS photovoltaic modules," IEEE Transactions on Power Electronics, vol. 19,

Parameters Values 1 pp. 1130-1139, 2004.[5] J. Rodriguez, J. Lai, and F. Peng, "Multilevel inverters: A survey of

DC Capacitors 5000 ,u topologies, controls, and applications," IEEE Transactions on IndustrialCoupling inductor 500 , Electronics, vol. 49, pp. 724-738, 2002.

[6] A. Nabae, I. Takahashi, and H. Akagi, "A new neutral-point clampedSwitching frequency 2 KHz PWM inverter," IEEE Trans. Ind. App., vol. 17, pp. 518-523, 1981.

Number of Cascaded Cells 2 [7] M. Calais, V. G. Agelidis, L. J. Borle, and M. S. Dymond, "A trans-Grid voltage 110 V formerless five level cascaded inverter based single phase photovoltaicsystem," Power Electronics Specialists Conference, PESC, vol. 3, pp.

Grid frequency 60 Hz 1173-1178, June 2000.MPP current for each string 15 Amp [8] 0. Alonso, P. Sanchis, E. Gubia, and L. Marroyo, "Cascaded h-

bridge multilevel converter for grid connected photovoltaic generatorsMPP voltage for each string 120 V with independent maximum power point tracking of each solar array,"

MPP for each string 1800 W PESC'03. IEEE 34th Annual Power Electronics Specialist Conference,vol. 2, pp. 731-735, June 2003.

[9] S. Alepuz, S. Busquets-Monge, J. Bordonau, J. Gago, D. Gonzalez, andJ. Balcells, "Interfacing renewable energy sources to the utility grid usinga three-level inverter," IEEE Transactions on Industrial Electronics,

REFERENCES vol. 53, no. 5, pp. 1504-1511, Oct. 2006.[10] H. Ertl, J. W. Kolar, and F. C. Zach, "A novel multicell DC-AC converter

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[3] Y. Xue, L. Chang, S. B. Kjaer, J. Bordonau, and T. Shimizu, "Topologies single-phase grid connected photovoltaic systems: an overview," Proc.of single-phase inverters for small distributed power generators: an IEEE International Symp. Industrial Electronics, pp. 224-229, 1998.overview," IEEE Transactions on Power Electronics, vol. 19, no. 5, pp. [13] D. G. Holmes and T. A. Lipo, "Pulse width modulation for power1305-1314, Sept. 2004. converters," IEEE Press Series on Power Engineering, 2003.

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