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Buck-Boost Interleaved Inverter for Grid Connected Photovoltaic System

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  Buck-Boost Interleaved Inverter for Grid Connected Photovoltaic System Omar Abdel-Rahim,  IEEE Student Member  , Mohamed Orabi,  IEEE Senior Member   and Mahrous E. Ahmed,  IEEE Member    APEARC, South Valley University, Aswan City 81542, Egypt orabi@ieee.org   Abstract   —in this paper, a single stage buck-boost inverter is proposed for grid connected PV system with a very high voltage gain. The proposed inverter not only  boosts DC output voltage of the PV module but also converts it into AC voltage which is required for grid connection. Discontinuous conduction mode (DCM) is employed to achieve unity power factor with the grid voltage and a maximum power point tracking (MPPT) control. The proposed topology has several desirable features such as high gain, low cost, compact size and simple control. Only two switches operate at high switching frequency and so switching losses are minimized. Simulation and experimental results are given to prove the proposed system.  Keywords —   Single Stage, Buck-Boost Inverter, Low-Cost, Grid-Connected, PV system, Simple-Control, DCM, MPPT. I.   I  NTRODUCTION  Renewable energy has become an important source of energy; Photovoltaic system (PV) is an example of renewable energy. PV modules convert sunlight into electrical power, so they provide an important source of energy. PV modules can't be connected to the grid directly, but this could be done by using power conditioning system, that is for example H-bridge inverter, to convert dc output power of PV modules into ac output  power. Output voltage of PV modules is not very high, so we may be in need to connect more than one module in series to get the required dc voltage. H-Bridge Inverter, shown in Fig.1, is a buck inverter that has the requirement of input voltage greater than output voltage. In case of input voltage is smaller than output voltage, a boost converter is used before the inverter stage to provide the required voltage for the inverter as shown in Fig. 2. Inverters may be classified as single stage or two stages according to how is the input voltage will be  boosted. In the two stage inverters, the first stage is boost converters which boost input voltage to become greater than output voltage and the second stage is a buck converter used to convert dc input voltage into an ac output voltage. In the single stage inverters, the inverter does two functions boosting the input voltage and converting DC power into AC power. Single stage inverters have some advantages over two stage inverters such as low cost and compact size but suffer from low voltage gain and low efficiency compared to two stage inverters. In [1], the proposed topology suffers from low gain and complex control. Flyback inverter proposed in [2] suffers from high stress on switches and also the efficiency decreases as the turns ratio increases. A single stage inverter which was proposed in [3] has the following advantages such as simple, compact, low cost and simple control, but suffers from low voltage gain and low efficiency. In [4], the proposed inverter could buck or  boost input voltage, but suffer from complex control and the gain is not very high. In this paper a single stage inverter is proposed it has a very high gain, low switches stresses and simple control, as it will be explained in the next section. Figure 3 shows the schematic of the proposed inverter. The proposed inverter consists of two buck boost converters, one converter operates during positive half cycle and the other operates during the negative half cycle. Figure 1: Traditional voltage source inverter.  The proposed system is suitable for AC module technology, where each PV module can be attached to the grid directly. This application is good as electrical characteristics of the PV module are greatly affected by shading condition. Shadow causes the output power of the PV module to be reduced. The proposed inverter helps in solving this problem by reducing number of connected modules per inverter. A maximum power point tracking control is applied for better utilization of the PV module. As the module power is low and to simplify the control for unity power factor DCM mode is applied. The  paper is organized in the following way. 2010 IEEE International Conference on Power and Energy (PECon2010), Nov 29 - Dec 1, 2010, Kuala Lumpur, Malaysia 978-1-4244-8946-6/10/$26.00 ©2010 IEEE63    Figure 2: Two stage inverter Figure 3: Schematic of the pro  Section II presents switched induconverter. Section III presents analysithe proposed single stage inverter. Sethe MPPT control techniques used for t power from PV. Section V summarizeof the proposed system. Section experimental results of the proposed sy II.   S WITCHED I  NDUCTOR B UCK  -BTraditional buck-boost converter c buck or boost converter according to cycle. To increase the gain of the conhas replaced by the switched inductowith this replacement the gain of improved. Figure 4 shows the propconverter. The converter operates in Dthat's inductor current ripple is highevalue, When the converter operates in modes of operation as shown in Fig.4. in Fig. 4 (b) takes place when SW1 aD3 are ON and the steady state equatiois given by:         (1)    Mode 2 occurs when diodes D2 and D4is OFF as shown in Fig.4 (c); configuration.  posed inverter. ctor buck boost and operation of tion IV describes racking maximum simulation result VI summarizes tem. OST C ONVERTER   uld operate as a the adjusted duty erter, its inductor r proposed in [5] the converter is sed buck- boost M when    , than its average DCM, it has three Mode 1 as shown d diodes D1 and n of the converter      (2)   are ON and SW1        (3) Mode 3: when SW1 and all dicurrent becomes zero as shown    0  (5) Fig. 5 shows inductor currencurrent D4, from steady state athat:   0  (7) Where     means avera     means average of dio(8) we could obtain the gfollowing:     √ 2  Where        , D: Ts: switching period, L: convresistance. Equation (9) shoconverter is higher than traditi by  √ 2 . Figure 7 shows a compthe switched inductor and tconverter. The figure shows inductor is approximately oneconventional converter. III.   P ROPOSED S INGLE SThe proposed system conconverters; each converter opshown in Fig.8. Operation of follow, Switches SW1 and frequency equal to grid fundaoperates for one half cycle. Shalf cycle while SW1 operates Switches SW3 and SW2 ofrequency. To provide a higreduce the size of the output filfor only one half cycle. SW3 ocycle while SW2 operates iFigure 9 shows the operation  positive half cycle. During tinverter has three modes of opeMode1: when switch SW3 isMode2: when diode D4 is OMode3: when both SW3 anFig.9(c). In order to inject a sinusoidal unity power factor, the operati           (4) odes are OFF and inductor in Fig. 4 (d);        (6) and Fig. 6 shows diode nalysis , it can be conclude        (8) e of inductor voltage and de current. Using (7) and ain of the converter as (9) the converter dusty cycle, erter inductor and R: load s that the gain of the onal buck boost converter arison between the gain of he traditional buck-boost the gain of the switched and half higher than the AGE DC-AC   I  NVERTER   sists of two buck boost erates for a half cycle as he proposed inverter is as W4 operate at switching ental frequency, each one 4 operates in the positive in the negative half cycle. erate at high switching quality grid current and ter each converter operates  perates in the positive half the negative half cycle. odes of the inverter during e positive half cycle the ration: ON as shown in Fig. 9 (a). Fig. 9 (b). D4 are OFF as shown in ac current into the grid at n of converters must be in 64  the DCM mode. The proposed inver switches so that it has lower cost anlosses as compared to two stage inver  buck boost converter helps in reducimodules connected in series which heleffect of environmental condition, such  performance of the PV module anoperation. IV.   MPPT   C ONTROL A LG PV module which is connected to deliver its maximum power to the grid. environmental conditions will shift the the PV module to a lower power p power point tracking (MPPT) controllemodules to ensure that the PV modulesits maximum power. Maximum poalgorithm proposed in [7] will be usFigure 10 is the flowchart of the ustechnique.   Where    , and     are the and current of the PV array.    And  voltage and current, respectively. Threplaced by  I  V , making the calcmajor check of this algorithm is achiev   V , and then D (duty) will be admove the operating point into the dire power point of the PV array. The algochecking if dV0  or not. If   dV checked. For   dI0 , D is held unchanmust be incremented, while if dI decremented. On the other hand, if     V should be checked. For   I held unchanged. But if I  V decreased and if I  V0, D Then, the algorithm continues until theits maximum value. Figure 4: (a) schematic of the proposed Switchconverter; (b) mode 1; (c) mode2; (d) mod ter has only four lower switching er. The high gain g number of PV s in reducing the as shadow, on the d MPPT control ORITHM  grid must always Any change in the operating point of oint, a maximum r is used with PV always operate at er point tracking ed in this paper. ed MPPT control omentary voltage are the previous e P  term can be lation easier. The d by detecting  I usted in order to tion of maximum rithm begins with 0 , then dI  is ged. For   I0  , D 0  , D must be V0 , then  I  V0, D is 0, then D must be ust be increased.  power reaches to d inductor buck-boost 3. V.   S IMULAT The proposed system wasoftware. Two PV modules module were used [8]. Electused PV module are shown in were connected in series to g170 W. Circuit parameters ainput capacitor   Cp  =10 mF, switched inductors L1 = L2 =1and output filter inductor Lf and frequency are 311V and 50  Figure 5: inductor current.   Figure 7: Comparison between converter and traditional b   Figure 8: Proposed single sION R  ESULT     s simulated using PSIM of the BP485 85W PV ical characteristics of the Table 1. Two PV modules ive output power equal to e as follow fs = 10 kHz, filter capacitor Cf   = 1uF, 0 µH for each converter 3.5 mH   and grid voltage Hz, respectively. Figure 6: diode D4 current. switched inductor buck boost uck boost converter  . age inverter. 65  As switches SW2 and SW3 are modulated using sine wave this cause a large oscillation in the PV output voltage, this large oscillation make MPPT controller doesn't work well so that a large capacitor is used in the input, The value of the switched inductors is very small as the converter operates in the DCM mode and output filter is very small due to sine modulation of the converter switches SW2 and SW3. Figure 11 shows simulation results of the grid current which is in phase with a grid voltage. Unity power factor is achieved without any feedback from grid current this is due to the converter operating in DCM mode. Figure 12 shows output power of the PV module, the figure indicates that the MPPT control is operating well and able to extract maximum power from PV modules. Figure 13 shows one inductor current as shown each conductor carry current for only one half cycle, the current is discontinuous and the ripple is high due to DCM operation. (a) (b) (c) Figure 9: operation modes of the inverter during positive half cycle (a) mode 1 when SW3 is on while D4 is off (b)mode 2 when SW3is off while D4 is on (c) mode 3 when SW3 and D4 are off Figure 14 and 15 show the switches’ pulses. Figure 14 (b) and Fig. 15 (b) show that switches SW4 and SW1 operate at grid fundamental frequency, while Fig. 14 (a) and Fig. 15 (a) show that Switches SW3 and SW2 operate at high switching frequency for only one half cycles. Inverter Efficiency was found 87% at full load. This is a good efficiency for DCM operation with such high voltage gain. Figure 10: Flowchart of the MPPT control. Table .1 Electrical characteristics of Bp 485 PV module Electrical Characteristics BP 485 Maximum power (Pmax)85W Voltage at Pmax (Vmp)17.8V Current at Pmax (Imp)4.9A Short-circuit current (Isc) 5.4A  Open-circuit voltage (Voc) 22.0V  Temperature coefficient of Isc (0.065±0.015)%/ °C  Temperature coefficient of Voc -(80±10)mV/°C  Temperature coefficient of power  -(0.5±0.05)%/ °C  66

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