Research On Transformerless Grid-connected Photovoltaic Inverters

ABSTRACT:With the development of the distributed generation, low power single-phase photovoltaic (pv) grid-connected inverter has been applicated broadly. The traditional grid-connected inverter is always equipped with power frequency or high frequency transformer for playing the role of electrical isolation and voltage conversion. However, the power frequency transformer increases volume and power losses of the system, and add the installation difficulties also. The high frequency transformer reduce the efficiency of power generation. These shortcomings can be made up by removing the transformer. In consequence, the current research hot spot focus on the transformerless grid-connected photovoltaic inverters. However,the leakage current will be generated in the transformerless grid-connected photovoltaic inverter system due to the existence of the stray capacitance between the pv panels and ground, which can endanger safety of people. Therefore, it has important significance to research that how to suppress or even eliminate the leakage current for the development and implication of the transformerless grid-connected photovoltaic inverters.In this thesis, the research of transformerless grid-connected photovoltaic inverters mainly contains five aspects as follow.(1) The operation principle of the transformerless grid-connected photovoltaic system is introduced and the mechanism of leakage current is analyzed.(2) Various transformerless grid-connected photovoltaic topologies which can suppress the leakage current are compared from the aspects of device number, filtering effect and system loss. Simulation models are built for verification, and the results indicate that H5topology is more suitable for developing in practice.(3) For the neutral voltage ripple and leakage current of the traditional two-stage transformerless NPC topology, a new two-stage transformerless NPC topology is proposed in this thesis. The leakage current can be eliminated completely, the neutral voltage ripple can be restrained and capacitor value of the DC side can be reduced, and the correctness of results are verified by the simulation.(4) The control strategies of the new topology are studied which use the current loop control in front stage to achieve the maximum power point’s tracking of the PV, and the double closed-loop control is used in backward stage. The advantages and disadvantages of PI, PR, quasi-PR controllers are compared by the simulation results which verified that the PR controller is the best.(5) A5kW prototype of the new topology is built to compare with the traditional topology, and the feasibility and advantages of the new topology is verified by the experimental comparison.