Control of power electronic interfaces for photovoltaic power systems

This thesis deals with the control of power electronic interfaces for photovoltaic applications. The main objective is to develop and employ effective control algorithms and topologies that are optimally suited for photovoltaic power conversion systems. The studies, comprising of analysis, modeling, control and experimental evaluation, clearly enhance overall system performance. The major items of investigation in this thesis are power electronic converter design and modeling, current mode control, maximum power point tracking techniques, and control for energy storage systems. The nonlinear and time varying characteristics of both the photovoltaic module and switch-mode power converters cause problems in photovoltaic systems control. The weather dependency of photovoltaic system makes these problems even more severe. As the photovoltaic source usually acts as a current source, this thesis proposes current mode control methods as a suitable control solution in photovoltaic power systems. The thesis proposes an improved algorithm, based on current mode control, to ensure an accurate tracking of the maximum power point of the photovoltaic module. In addition, four current mode control methods are investigated for the dc-dc converter controller to obtain a stable dc bus voltage. The results obtained are compared with the voltage mode control. Finally, a new control algorithm is proposed for charging and discharging of the photovoltaic battery storage system, which allows versatile power transfer among the photovoltaic source, battery and load. Extensive simulation studies were performed based on MATLAB/Simulink/ Simpower to verify the control methods of the maximum power point tracking, dc-dc converter control and energy storage interface for a 3 KW photovoltaic power system. The current mode control methods have shown significant improvement in different aspects of the power quality, especially in case of input changes, load variations, transient responses and harmonics. A dSPACE DS1104 DSP (digital signal processor) hardware and software system was used in a prototype experimental evaluation to validate the improved performances of the current mode control of dc-dc converter for photovoltaic systems.