Cascaded h-bridge multilevel converter for grid connected photovoltaic by odq14517

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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
                                                                                                       ,..
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                                                                                                             .............

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          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
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                 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 .................                                        ...................
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               300.0      ,




                0,50      ,                                                                                                                                                                                           I


                  :I
                0 45 ..................
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                0 40 ..................
                0 35 ...................................
                 0 30 ...................................
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                                                                                ..................                       ,.....................................................................................
                                                                               ...........................................................................................................................
                                                                           ..............
                                                                            ..............                 ......................................
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                 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. Meinhardt, “Multilevel Converters For
new control technique for the cascaded H-Bridge multilevel                                                                              Single-phase Grid Connected Photovoltaic Systems: An Overview”.
                                                                                                                                        Solar Energy. Vol66, No5, pp 325-335. 1999.
converter is introduced. Its main feature is to allow each
module to deliver its maximum available power calculated                                                                     PI         H. Valderrama-Blavi, L. Martinez-Salamero, C. Alonso, J. Maix6-
                                                                                                                                        Altks, “Nuevas estructuras de Onduladores Fotovoltaicos”, SAAEI 0 1
for an independent MPPT control. Moreover, due to its                                                                                   proceedings. 2001
multilevel nature, the output voltages are synthesised by                                                                    [4l        D. B. Snyman and J. H. R. Enslin, “An experimental evaluation of
means of several voltage levels, whose number increases                                                                                 MPPT converter topologies for PV installations”, Renewable Energy,
                                                                                                                                        vol. 3, no. 8, pp. 841-848, 1993.
when the number of single-phase H-bridge modules series


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