Modeling and control of grid-connected DC/AC converters for single-phase micro-inverter application

This paper presents modeling and control for a single-phase grid-connected two-stage micro-inverter system. An average signal model has been developed for a single-phase bridge inverter. The proposed controller has three cascaded control loops. The inner current loop consists of a variable frequency hysteric current mode control which regulates output filter inductor current thereby improved system response. The small signal modeling of the proposed control method is discussed. Again, as the dc-link capacitor plays a crucial role in two-stage micro inverter applications, a DC-link voltage controller is employed in outer control loop to keep the bus voltage constant. The grid current loop regulates the injected current to the grid at a reduced THD level. The stability of the above control loops are also verified by using Bode-diagram. Finally the proposed method is then applied to 500 Watts, 110V, 50Hz, two- stage single-phase grid-connected Photo-voltaic (PV) system. The simulation and the experimental results are presented to validate the theoretical analysis, effectiveness and feasibility of the proposed control strategy.