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Cascaded H-Bridge Multilevel Converter for Grid Connected Photovoltaic Generators with Independent Maximum Power Point Tracking of each Solar Array 0.Alonso, P. Sanchis, E. Gubia and L. Marroyo Department of Electrical and Electronic Engineering Universidad Publica de Navarra 3 1006 Campus Arrosadia - Pamplona - Spain email: oscar.alonso@unavarra.es Abstract - This paper introduces a new control method and by means of a half-bridge or full-bridge converter. Fig. 1 proportional PWM modulation of the cascaded H-bridge shows an example of one of these structures [I]. There are multilevel converter for grid-connected photovoltaic systems. other structures that use the multilevel modular configuration This control makes each H-bridge module supply different and whose circuits and basic characteristics are described in power levels, allowing therefore for each module an independent [3]. Among these structures, one that has similar features in maximum power point tracking of the corresponding photovoltaic array. modularity and control with respect to the previous string converter is the cascaded H-bridge multilevel converter, 1. INTRODUCTION whose generalised power circuit is shown in Fig. 2. This converter takes advantages of one of the most important Nowadays, grid-connected photovoltaic systems are the feature of the multilevel converters, that is, an output voltage higher developing solar energy applications. In these systems, synthesis with a higher number of levels. This number of the use of all the available energy depends on the static levels grows according to the number of series-connected converter topology that is used. It has been demonstrated that modules in the converter. This characteristic has some topologies that use a lower amount of panels for each array advantages. In one hand, a reduction in the common-mode improve the global efficiency of the photovoltaic generator. perturbations is verified, that is originated by the leakage This is a result of a reduction in mismatch losses, partial photovoltaic array earth capacitance. In the other hand, lower shadows of the array, etc. Obviously, the efficient use of the amplitude harmonics are injected to the grid and with higher available solar energy of each group of panels requires the frequencies, simplifying their filtering. Although these implementation of an independent maximum power point multilevel converters are initially prepared for applications tracking technique (MPPT) for each one of these groups. where each one of the modules delivers the same power, their There are several modular structures recently developed that optimal use in photovoltaic generators require an independent work under the previous criteria. Many of these structures, delivering of each one of these modules. Therefore, in this called string converters, use a common DC bus where each paper a new control methodology and proportional PWM one of the solar receiving modules transfers the power [l] [2]. T on modulator are presented, that allow an independent MPPT Usually, these modules consist of a solar array and a DC-to- implementation for each solar array. Simulation results have DC converter controlled by a MPPT algorithm. Afterwards, been carried out to validate the proposed control technique. the available energy on the DC bus is transferred to the grid ,.. ,. ............ ............. 3 c j ibu : A : r P Y h IJ J 1 o- "*?run : 3 : e*-K!%z--.j Fig. 1. Generic single-phase String Converter 0-7803-7754-0/03/$17.00 02003 IEEE 73 1 a result of these controls, the new medium current references 11. BRIEF SYSTEM INTRODUCTION AND DESCRIPTION (IDck) of each module are obtained. The next stage calculates the available power of each module; therefore, the sum of all Fig. 2 shows a H-bridge multilevel converter circuit of them (PT) will be the total available power to be generalised for n series-connected modules. By means of transferred to the grid. This is done by means of a power modulation techniques and keeping the same DC voltage for control in the next stage, in which the output voltage VHTthat each module, the converter synthesises an output voltage V, can be synthesised by the converter is calculated. with a number of levels equal to 1+2n. As a consequence, In the last stage, the proportional PWM modulator shares common-mode perturbations and voltage harmonic the voltage Vm among the different H-bridge modules. As it amplitudes are reduced, and therefore, the filtering processes will be demonstrated, the voltage fraction of a module is results to be easier. Here, a bipolar PWM modulation method directly proportional to the power fraction that the module is for each H-bridge module has been used. Under this transferring. modulation, the multilevel voltage synthesis is achieved by means of a phase shift displacement of the carrier waves of the different modules. This angle is calculated as follows: IMPLEMENTATION 1 1 MPPTALGORITHM 1. 3 60 ATc =- n There are several techniques to fulfil the detection and tracking of the maximum power point in a photovoltaic With this modulation, a complete cancellation of output generator. In this paper a Perturbation and Observation voltage harmonic groups is achieved. In fact, in ideal (P&O) algorithm has been used, whose operating principle is conditions where each module delivers the same power from 4. exposed in [ ] This algorithm is implemented in digital the same DC voltage, the central frequency of the first processor and uses the average values of voltage and current harmonic group of the output voltage is: of the solar array. Due to the active power delivering of each module to the grid, in the DC side of each converter there is a 100 Hz ripple in all the magnitudes. Therefore, the solar array where fc is the switching frequency of any H-bridge module. current and voltage measures are filtered by means of a When the previous conditions are not kept, the harmonic digital 100 Hz window filter. The different MPPT algorithms cancellation is not complete, and the output voltage has a provide the average voltage reference of the corresponding worse harmonic spectrum. The simulation results show both photovoltaic generator. Every one of these references is operation possibilities and their effects on the output voltage processed by means of an independent average voltage and current. control of the DC link capacitor for each H-bridge module. Fig. 3 shows a simplified block diagram of the whole Fig. 4 shows the block diagram of a generalised voltage control methodology that it is proposed. The first stage control for any k-th module. From each one of these stages corresponds with the independent MPPT algorithms and the average reference current IDCk are obtained. These capacitor voltage controls for each photovoltaic array. By magnitudes and the corresponding digital filtered voltages means of using the averages values of their voltages and VDCk are sent to the next stage where the power control is currents, the capacitor voltages are controlled with the aim of camed out. getting the maximum available power of each solar array. As 732 - k~-PVarray vDck.REF h4PPT - cs IPVI VDCk IPVk Converrer ; ~, I IOOHz Window I Digital Filter 7 ipw - , IOOHz Window Fig. 4. Capacitor voltage control loop of each module Fig. 5. Simplified output circuit and unitary factor power operation I vgrid (nns value) IV. POWER CONTROL v. PROPORTIONAL PWM MODULATOR The total available power is the sum of the calculated The main task of the modulator is to synthesise the output power of each solar array. That is: reference voltage V Due to the series-connection of the ., PT = 4 + P2+ ..+ P" = VDCIIDCl VDC21,, + ..+ VD,IDC" + (3) modules it is verified that: Due to the fact that the power is transferred to a more or Vm = VNI+ v,, + ...+ v,, (5) less constant voltage grid, the power control can be done in The proportional relationship between each individual an indirect way by means of the output current (IL) control. voltage and the total voltage Vm defines what has been called As it can be observed in Fig. 5 , the power factor will be 1, proportionality factors: and for that the rms-value of the output reference current is calculated as follows: (4) In this converter, each module drives the same output current I,. Therefore, each voltage module is proportional to Fig. 6 shows the block diagram of the power control by its power, verifying the following relationship: means of the output current control, whose output is the output voltage V, that must be synthesised by the converter. For this control, the modulator and the converter stages are considered a block with a unitary gain. 733 Then, the proportionality factors of the voltages can be factors that have been obtained. As it can be observed, during calculated by means of the correspondentpowers: the initial stage the power delivered for each module is 680 W, what implies an output current with a maximum value of 13.2 A, and a proportional factors equal to a k= 113 . After the change of irradiance, and in steady-state conditions, it can By means of using a linear scalar PWM modulation, the be demonstrated how the proportional factors carry each duty cycle of any k-th module is calculated as follows: generator to its correspondent maximum power point. In these conditions, the total available power is PT = 1636 W, (9) which corresponds to an output current with a maximum value of 10.5 A. In those applications where all the modules deliver the During the first operation stage the three output voltage of same power, the proportional factors have the same value each h-bridge module have the same amplitude. However, equal to lln. Therefore, the duty cycles are: after the irradiance changes, the three voltages become different. This is only due to the ch&ge in the proportional dk .-%d= l ...n ) - (k factors, because, as it can be observed in Fig 8, with the ‘DCk * change of irradiation the MPP DC voltage barely changes. However, in this application each module k-th has to This causes an output voltage with a higher distortion, as it is deliver its own available power Pk. Therefore, the duty cycle clearly reflected in a higher output current ripple. calculation is: Fig. 7 shows a block diagram of the modulator corresponding to the k-th module. VI. SIMULATION RESULTS vHT x The main features of the simulated power circuit are the following ones: - Number of modules in series: 3 - Coupling inductance: 0,5 mH - Grid characteristics: 220V I 5 0 Hz - DC link capacitors: 2mF - Switching frequency: 2000 Hz Switching ..., s 4 k Each one of the photovoltaic arrays is a series connection of 8 panels of the model BP-585F (BP Solar). The operation Fig. 7. PWM modulator for k-th module features of each generator under standard conditions (1000 W/m2 irradiance and 25 “C ambient temperature) are the following ones: 200 , I *. ‘ ‘t. ‘ 1 - Total open circuit voltage: 178 V - Short-circuit current: 5A - MPP Voltage: 144 V - MPP Current: 4,72 A - MPP Power: 680 W A simulation experiment that shows the behaviour of the system under irradiance steps has been carried out. From the beginning of the experiment to the moment F2s, the irradiance of the three solar arrays is the same with a value of 1000 W/m2. At that moment, the irradiance changes in the second array to 800 W/m2 and in the third one to 600 Wlm2. In these new work conditions, the maximum available power and their correspondents V-I points are shown in Fig. 8. This stage continues until t=3s, where the system resumes to initial 0 1 2 3 4 5 PV current (A) conditions. Fig. 9 shows the results of the output voltage and current, power of each module, duty cycles and proportional Fig. 8. V-I characteristic under different irradiance conditions 734 ””“W , I -600,O ‘ I 15,O 10.0 5.0 0.0 -5.0 -10.0 700,O .................. 650 0 . . . . . . . . . . . . . . . . . . ........................................................................................................................... ................. 600 0 .................. ........................................................................................................................... ................. 550 0 ................. ................... .................... .... ...................................... ,... ...................................... ................. 500 0 . . . . . . . . . . . . . . . . . . ........................................................................................................................... 450 0 .................................... ....................................... I................................................................................... 4oo 0 .............................. ..................................................................................................... ................. 350 0 ................. ................... .................... ,................................................................................... 300.0 , 0,50 , I :I 0 45 .................. .................. .................. 0 40 .................. 0 35 ................................... 0 30 ................................... ................... .................. ,..................................................................................... ........................................................................................................................... .............. .............. ...................................... ..................................... ............ ............ ............................................................................................................................... ................. 0 25 .................. ............................................................................................................................... 0.15 1 I 0.20 0.25 0,30 0,35 0.40 0,45 0.50 0,55 0,60 0,65 0,70 Time (9) Fig. 9. Simulation results of an irradiance step test experiment VI. CONCLUSIONS connected increases. This is an important advantage with respect to others structures due to the reduction of common It is demonstrated an improvement in the global efficiency mode perturbations, harmonic voltage and current of the photovoltaic generator when several arrays with lower amplitudes, etc. amount of panels are used instead of a unique array with the total amount of panels. Moreover, each array will have its REFERENCES specific maximum point power. To take advantage of all the G.R. Walker, P.C. Semia, “Cascaded DC-DC Converter Connection of available power, it is necessary to use modular conversion U1 Photovoltaic Modules”. PESC 02 Conference Proceedings. structures with independent MPPT controls. In this paper a [2] M. Calais, V.G. Agelidis, M. 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