Research On The Topologies And Control Strategies Of Transformerless Grid-connected Photovoltaic Inverters

During last decade, photovoltaic technology developed rapidly. Various types and power levels photovoltaic systems have been applied. Due to the high cost of solar cell panels as well as reduce system installation costs and to meet the convenience needs of users, how to increase the efficiency of grid connected inverter, reduce the facilities size and weight became a research focus. Traditional grid connected inverter in order to ensure the safe operation, isolation transformers were generally added. But the introduction of isolation transformer will reduce the efficiency and increase the volume and weight. Therefore, in recent years many new transformerless photovoltaic inverter topologies have been proposed. Compared with the traditional grid-connected solar inverter, these transformerless inverters have higher efficiency, smaller size and weight. But there is common mode leakage current in transformerless topologies, which would create a potential safety hazard. How to restrain the common mode leakage current in transformerless topology became a research difficult.The author used Kirchhoffs law to systematically analyze the PV system leakage current generation principle both in the time domain and complex frequency domain and proposed a new method to estimate the leakage current, which provide a reliable theoretical foundation to suppress common mode leakage current. Subsequently, the author made an analysis and comparison of the leakage current generated by different existing new transformerless PV inverters, which validated by simulation and experiments. Meanwhile the efficiency of different topologies was also analyzed and compared under the same experimental conditions, related simulation results and analysis theory was given. Compared with traditional topology, these new topologies need more additional switching devices. The author first time got the losses distribution maps of different topologies both with Simulink PLECS box simulation and experiments. During the tests of losses distribution, it was found that by using different PWM strategy with the Active NPC topology, the device switching loss distribution could be changed. Then a new PWM strategy named "Adjustable Losses Distribution", which could balance the losses distribution between different switches was proposed.During the previous control work, it was found that when using proportional integral (PI) controller to track the sinusoidal current reference, the grid current cannot follow the reference exactly. Then a new controller named proportional resonant (PR) controller was introduced in the current control. It could track the current reference without steady error. Then the author applied PR controller to the three-phase stationary frame current control. In order to further enhance the dynamic response of the controller, a positive sequence grid voltage feed-forward method was proposed, the good dynamic response and control characteristics were validated by the simulation and experiments.In summary, this thesis put forward new ideas on theory analysis, topologies research and control strategies, which were all validated by simulation and experiments.