Boost-Buck Inverter Variable Structure Controlfor Grid-Connected Photovoltaic Systemswith Sensorless MPPTCarlos Meza*, Domingo Biel*, Juan Negroni**, Francesc Guinjoan*** UPC-Institute of Industrial and Control Engineering, Barcelona, Spain** UPC- Electronic Engineering Department, Barcelona, SpainAbstract—The present work describes the analysis,modeling and control of a transformerless Boost-Buckpower inverter used as a DC-AC power conditioning stagefor grid-connected photovoltaic (PV) systems. The power conditioning system’s control scheme includes a variable structure controller to assure output unity power factor anda sensorless Maximum Power Point Tracking (MPPT)algorithm to optimize the PV energy extraction. Tomaximize the steady-state input-output energy transferratio a discrete linear controller is designed from a large-signal sampled data model of the system. The achievementof the DC-AC conversion at unity power factor and the efficient PV’s energy extraction are validated withsimulation results.I. INTRODUCTIONPhotovoltaic (PV) systems that supply power directly to the grid are becoming more popular due to the cost reductionachieved from the lack of a battery subsystem. Moreover, astring inverter topology allows further cost savings because of the more efficient energy extraction than the widely usedcentralized topology. In string inverter topology PV modules are connected in series from a string up to 2 kW producing PV array voltages around 150-450V. As reported in [1] this topology can be used in high power ranges providing high system flexibility. A power conditioning system linking the solar array and the utility grid is needed to facilitate an efficient energy transferbetween them; this implies that the power stage has to be able to extract the maximum amount of energy from the PV and to assure that the output current presents both low harmonicdistortion and robustness in front of system’s perturbationsIn order to extract the maximum amount of energy the PV system must be capable of tracking the solar array’s maximum power point (MPP) that varies with the solar radiation value and temperature. Several MPPT algorithms have been proposed,namely, Perturbation and Observation (P&O), incrementalconductance, fuzzy based algorithms, etc. They differ from its complexity and tracking accuracy but they all required sensing the PV current and/or the PV voltage. In [10] a sensorless MPPT has been presented preserving acceptable results. In the present document a MPPT algorithm based in the one presented in [10] is proposed. Then, regarding the proper DC-AC conversion, atransformerless Boost-Buck DC-AC voltage converter using sliding mode control described in [2] shows good performance in front of input voltage and load perturbations.Taking into account the mentioned antecedents, this paper presents the control design of a Boos-Buck inverter for PV-gridconnected systems, gathering all the advantages of a stringinverter topology , a transformerless converter and a sensorless MPPT control. Sections II and III are devoted to the dynamic modeling and to derive the control scheme of the power stage. Section IV is focused on the control design: first, a variable structure control is designed to ensure an output current injection to the grid at a unity power factor; secondly, the maximization of the steady-state input-output power transfer ratio is performed by the design of a discrete linear controller based on a linear sampled data model of the system derived from the dynamic power balanced equation. Afterwards, Section V focuses on the design of a sensorless MPPT algorithm based on a modified version of the proposal presented in [10]. Finally as aconclusion, the last section presents the simulation resultsvalidating the proposed design.II. GRID-CONNECTED BOOST -BUCK INVERTERFigure 1.a shows the power converter structure used tointerface the photovoltaic array with the power grid. A more suitable representation for analysis purposes is depicted in figure 1.b, where S1 is a conventional power switch and S2 corresponds to a full bridge switch.PV-arrayL1L2Ci1i2v2v1+vC+S1S2PowerGridFig. 1.a. Power conditioning system’s circuitPV-arrayL1L2Ci1i2v2v1+vC+S1S2u2=1u1=1u2=-1u1=0PowerGridBuck converterDC -ACBoost converterDC-DCFig. 1.b. Power conditioning system’s scheme Noticing that u1 and u2 stand for the control signals of S1 and S2 respectively, the system can be represented by differential equations (1), (2) and (3).()()111111cdivv udt L=−− (1)()()112211cdviuiudt C=−−(2)()22221cdivu vdt L=− (3)Where{}10,1u ∈ and {}21,1u ∈− .This work has been partially sponsored by the Ministerio de Ciencia y Tecnología, España, DPI2003-08887-C03-01, DPI2002-03279 and by the European Union (FEDER)IEEE ISIE 2005, June 20-23, 2005, Dubrovnik, Croatia0-7803-8738-4/05/$20.00 ©2005 IEEE 657III. CONTROL SCHEMETwo main objectives have to be fulfilled in order to transfer efficiently the photovoltaic generated energy into the utility grid:1. To track the PV’s maximum power point (MPP).2. To obtain unity power factor and low harmonicdistortion at the output.Figure 2 shows the control scheme used to accomplish the previous objectives. Switch S1 of the boost converter is governed by control signal u1 generated by a controller that dealsexclusively with the photovoltaic array’s maximum power point tracking (MPPT). The algorithm that achieves the PV’s MPP is based on a power sensorless scheme proposed by Kitano et. al [10]. Besides the sensorless advantage the proposed controlscheme rejects the input power’s oscillations due to the 100Hz output power’s ripple. The operation principle of this algorithm is presented in section V.The unity power factor controller outputs signal u2 that controls switch S2 of the buck inverter. This controller consists of an inner current loop and an outer voltage loop. The inner current loop is responsible of obtaining a high power factor and output current low harmonic distortion. The outer voltage loop assures a steady-state maximum input-output energy transfer ratio and a desired steady-state averaged DC-link voltageguarantying proper Buck inverter dynamics. In section IV the mentioned controllers will be described in more detail.Boost converterDC-DCBuck converterDC-ACCurrentsensorInner current loopMPPTcontrollerOutervoltage loop++i1v1vCi2v2u2u1KPV-arrayPowerGridvC*DC-linkv2i2Fig. 2 Control
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