Research On Single-Phase Transformerless Photovoltaic Grid-Connected Inverter

One of the most important utilize forms of solar energy is photovoltaic power generation.However,its economic constraints hinder the development of the photovoltaic industry.To reduce the cost of photovoltaic power generation system,the development of the efficiency of inverter structure is a feasible breakthrough.Transformerless grid-connected inverter structure has the advantages of high efficiency,small volume,light weight,low cost and so on,but the elimination of the transformer between the panels and grid causing electrical connections.A common mode circuit is formed between the grid side and the earth,parasitic capacitance and photovoltaic array.The leakage current caused by the common mode circuit have the potential threat to the safety of persons and equipments.This paper focuses on the research of high efficiency and low leakage current transformerless grid-connected inverter.Firstly,a transformerless grid-connected inverter common mode circuit model is established.Two ways to eliminate the common mode leakage current is summarized.It is applied to the traditional full bridge topology.The common mode characteristics of the topology under different modulation strategies are analyzed.The feasibility of this method can be proved.Secondly,the advantages and disadvantages of the common characteristics of six kinds of typical topologies are analyzed and summarized.On this basis,a novel seven switchs transformerless grid-connected inverter topology is proposed,this topology has superior performance of leakage current suppression.It is safe and reliable.At the same time,the double-frequency SPWM modulation strategy can be applied to it.One side,it can reduce the speed of the switch devices and the cost of equipments.It is helpful to the selection of devices.On the other hand,it can reduce the grid current ripple and the harmonic content fed into the grid.The improvement of the harmonic frequency of the output voltage is helpful to reduce the volume and the loss of the output filter.In any modes,the switchs are always staggered operation,which helps to balance the thermal stress and simplifies the difficulty of heat dissipation design.Thirdly,it has been found that the non ideal factor will lead to the instability of the common mode characteristics of the inverter,so the common mode leakage current can not be eliminated.In view of this phenomenon,a general theorem of diode clamping is proposed.The theorem is applied to all of the above topologies.The simulation results show this method can improve disturbance generated by high frequency common mode voltage under non rational factor effectively.At last,a general experimental prototype of the transformerless photovoltaic grid-connected inverter is designed.The common mode characteristics and efficiency of the four kinds of topologies are verified.