562 IEEE TRANSACTIONS ON ENERGY CONVERSION VOL. 21, NO. 2, JUNE 2006Single-Phase Single-Stage Photovoltaic GenerationSystem Based on a Ripple Correlation ControlMaximum Power Point TrackingDomenico Casadei, Senior Member, IEEE, Gabriele Grandi, Member, IEEE, and Claudio RossiAbstract—A maximum power point tracking algorithm forsingle-stage converters connecting photovoltaic (PV) panels to asingle-phase grid is presented in this paper. The algorithm is basedon the application of the “ripple correlation control” using as per-turbation signals the current and voltage low-frequency oscillationsintroduced in the PV panels by the single-phase utility grid. Theproposed control technique allows the generation of sinusoidal gridcurrents with unity power factor. The algorithm has been devel-oped to allow an array of PV modules to be connected to the grid byusing a single-stage converter. This simple structure yields higherefficiency and reliability when compared with standard solutionsbased on double-stage converter configurations.The proposed maximum power point tracking algorithm hasbeen numerically simulated and experimentally verified by meansof a converter prototype connected to a single-phase grid. Theresults are presented in the paper, showing the effectiveness of theproposed system.Index Terms—Active filters, energy management, photovoltaicpower systems, power conditioning, power quality.I. INTRODUCTIONPHOTOVOLTAIC (PV) technology is the most promisingcandidate for the large-scale adoption of a renewable en-ergy source. Photovoltaic roofs represent an important share ofnew installations of PV panels. For these applications, the ratedpower is lower than 5 kW, and the PV panels are permanentlyconnected to a single-phase grid. The power flow between thePV panels and the grid is controlled by a power conditioningsystem (PCS), which should be reliable and inexpensive. To ob-tain the maximum efficiency of the system, the PCS must keepthe power extracted from the PV panels close to the maximumpower point (MPP). Several solutions for a PCS with maximumpower point tracking (MPPT) capability have recently been pro-posed, based on both single-stage [1] and double-stage convertertopologies [2].This paper deals with a single-phase, single-stage PCS con-figuration, employing a simple and effective MPPT embeddedalgorithm. The scheme of the proposed system is shown inFig. 1. The output of the PV panels is directly connected to thedc link of the single-phase voltage source inverter (VSI), andthe output of the inverter is connected to the grid through the aclink inductor Lac.Manuscript received October 7, 2003; revised July 2, 2004. Paper no. TEC-00286-2003.The authors are with the Department of Electrical Engineering, Alma MaterStudiorum-Universit`a di Bologna, Bologna, Italy (e-mail: [email protected]; [email protected]; [email protected]).Digital Object Identifier 10.1109/TEC.2005.853784Fig. 1. Schematic diagram of the PV generation system.II. OPERATING PRINCIPLEThe VSI output voltage vFis controlled in order to forcethe current injected into the mains iSto follow a sinusoidalreference waveform, synchronized and in phase with the funda-mental component of the source voltage vS. As a consequence,a sinusoidal current is obtained even in the presence of volt-age perturbations coming from the mains [3]. The amplitudeof the reference source current I∗Sis generated by the dc linkvoltage regulator on the basis of the error between the dc linkvoltage Vdcand the reference dc voltage V∗dcof the PV panels.The MPPT algorithm varies V∗dcaccording to the environ-mental conditions in order to keep the operating point of the PVpanels close to the maximum power point.The basic principle of the MPPT algorithm is to exploitcurrent and voltage oscillations caused by the pulsations ofthe instantaneous power, which are inherent in single-phasepower systems. Analyzing these oscillations allows us to obtaininformation about the power gradient and evaluate if the PVsystem operates close to the maximum power point.It is known that for a non-null value of active power injectedinto a single-phase grid PS, the instantaneous power pS(t) pul-sates at a frequency twice than that of the grid. If the currentiS(t) is in phase with the source voltage vS(t), the instantaneousvalue of power injected into the grid ispS(t)=vS(t)iS(t)=√2VScos ωt√2IScos ωt= VSIS(1 + cos 2ωt). (1)The average quantity VSIScorresponds to the active powerPS. The power pulsation at the angular frequency 2ω in (1) isreflected on the dc link bus of the VSI as a voltage pulsationsuperimposed to the average of the dc link voltage Vdc.Thevariation of Vdccan be related to active power PS, grid an-gular frequency ω, and dc link capacitor Cdcby the following0885-8969/$20.00 © 2005 IEEECASADEI et al.: SINGLE-PHASE SINGLE-STAGE PHOTOVOLTAIC GENERATION SYSTEM 563relationship:PSω= CdcV2dcMAX− V2dcMIN. (2)The dc link voltage excursion (VdcMAX− VdcMIN) can belimited by choosing a proper value for Cdc, according to (2),ensuring the correct operation of the inverter. The residual oscil-lation of Vdcdetermines a small pulsation of the power suppliedby the PV panels. On the basis of the phase relationship betweenpower and voltage oscillations, the MPPT algorithm moves theoperating point of the PV panels by varying V∗dcuntil the MPPis reached. Voltage and current oscillations must be as small aspossible in order to minimize the oscillation of power extractedfrom the panel. On the other hand, these oscillations must belarge enough to be sensed and distinguished from current andvoltage ripple due to the VSI switching effects. It has been ob-served that keeping voltage and current oscillation around 1%of their rated values leads to a good behavior of the whole PVgeneration system.The capability of the proposed system to keep under controlthe power injected into the grid for any operating condition isensured by the use of a reliable synchronizing device and aneffective current regulator applied to the current injected intothe grid. In particular, the synchronization of the inverter outputvoltages with the fundamental component of the source voltagesis carried out by a phase-locked loop (PLL) control circuit. As aconsequence, the proposed algorithm operates correctly even inpresence of nonsinusoidal source voltages. The current regulatorcould be r ealized in different ways. In this paper, a predictivePWM current regulator
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