Research On Transformerless Grid-connected Photovoltaic System

The research object of this thesis is transformerless grid-connected photovoltaic (PV) system.The existing key issues such as control strategy, common-mode currents suppression, and DCinjection are analyzed in the paper. It summarizes the background and significance of the researchtopic, presents the state-of-the-art of both internal and international grid-connected PV systemtechnology and the development of grid-connected system, and analyzes the machanism oftransformerless grid-connected photovoltaic system. Based on the control theory of grid-connectedPV system, it also constructs equivalent models with feedforward control module, analyzes in detailseveral common control strategies and validates the realization of no-steady-state error track on thegrid-connected current with frequency fluctuation by the modified proportional resonant control.As to the common-mode currents problem in the non-isolated system, this paper builds newcommon-mode equivalent models for single-phase and three-phase transformerless grid-connectedPV system and obtains the basic regularities of common-mode currents suppression. As to fullbridge topology, it analyzes and simulates the effects of various modulation strategies oncommon-mode currents suppression. It analyzes the proposed topologies that are able to effectivelysuppress common-mode currents, and based on the single-phase full bridge topology, the paperproposes a developed H6topology and corresponding modulation strategies able to suppresscommon-mode currents and analyzes the machanism and properties of this topology by simulation.In connection with the DC injection problem in the grid-connected PV system, the cause andharm are analyzed. According to different theories, the paper classifies the solutions to DC injectionproblem and the pros and cons are given. Based on this, it proposes a control strategy on the basisof the modified proportional resonant and integral joint control able to suppress the DC currentinjection and conducts simulation. The simulation results indicate that this joint control strategy iscapable of eliminating the DC part of the grid-connected current.