Digital Control of Grid-Tied Photovoltaic Micro-Inverters

Abstract

This research paper presents stand-alone and grid-tied micro-inverters for Photo Voltaic (PV) applications. It also addresses several key technical control related issues in the domain of micro-inverter technology for PVs. One portion of this research deals with the digital control strategies for micro-inverters. One Sampling Ahead Preview (OSAP) control has been used to track sinusoidal ac voltage/current of micro-inverters. Microinverter is controlled and analyzed in stand-alone and grid-tied states of operation. In each state, initially the OSAP controller is applied and inverter performance is analyzed. An improved OSAP controller (conventional OSAP + digital resonant controller) is also then applied to compensate the drawbacks of OSAP alone and better track the reference signal. Simulation results are provided which confirm the effectiveness of the proposed control for micro-inverter based solar systems. Another contribution is performance analysis of the proposed control scheme with a Maximum Power Point Tracking (MPPT) strategy. Atmospheric conditions being one of the most influential and uncontrollable factor in solar generation, vary the output of the solar panel nonlinearly. MPPT techniques enable PV panels to operate at the optimal power point and hence generate maximum power. Therefore, various MPPT strategies were considered, and finally an improved variable step size P&O MPPT strategy is used in this research work. Simulation results are given to demonstrate the validity of P&O MPPT strategy along with the proposed control scheme in the events of step changes in irradiation and ac current.