Research On Improved Variable Step-size MPPT For Photovoltaic System

With the increasing problem of environmental pollution and approaching depletion of conventional fossil-fuel energy source, solar energy, as the clean, the environmental friendly,and the abundant energy source, is attracting more attentions around the world. An effective way of using solar energy is photovoltaic(PV) generation; however, the output characteristics of PV array exhibit strong nonlinear characteristics, the output power varies with the connected load, as well as the environment condition(irradiation and atmospheric temperature). The maximum power point tracking(MPPT) techniques are thus employed to enhance the efficiency of PV generation system.In recent years, many MPPT strategies have been proposed with the differences in complexity, hardware requirements, cost, application, control accuracy and dynamic response speed. To research on the PV system MPPT technology, a PV cell model is established first and its output characteristic is analyzed. Then the paper analysis the realization mechanism of MPPT, and introduce series of MPPT approach. At last, the problem of MPPT under rapidly changing atmospheric conditions is analyzed in detail, an improve variable step-size incremental conductance(INC) MPPT algorithm is proposed based on the analysis of existing MPPT methods.The proposed algorithm automatically regulates the step size to track the MPP through a novel step size adjustment coefficient, the step size is increased when the operating point is far from the MPP, and it is decreased gradually when the operating point gets close to the MPP. Besides, user predefined constant is not required to ensure the convergence of MPPT method, thus simplifies the design of PV system greatly. A tuning method of initial step sizes is also presented, which is derived by the approximate linear relationship between the open-circuit voltage and MPP voltage. Therefore, the PV system keeps a larger step size even under sudden change irradiance, which improves the start and dynamic response speed when compared with the conventional variable step size method. Furthermore, a voltage close-loop control is used to ensure the stability of system. A Matlab/Simulink model and a 5kW PV system prototype controlled by a digital signal controller(TMS320F28035) were established, simulation and experimentalresults indicate that the proposed strategy can enhance the response speed, as well as the tracking accuracy.