Research On The Leakage Current Of The Photovoltaic Cascaded H-Bridge Inverter

The transformerless cascaded H bridge inverter has many advantages such as high quality of output voltage,high efficiency,small bulk and weight and so on.Therefore,it has attracted a lot of attention from scholars and institutions.But without the isolation transformers,the leakage current has brought serious threat to the safety of human life and equipment.Therefore the cascaded H bridge inverter is studied in this paper and finding the solution to eliminate the leakage current is of great significance.In this paper,the background,the research significance and the research status of the leakage current are introduced firstly.Then the working principle,basic modulation and control strategy of cascaded H bridge inverter are introduced.In order to suppress the system leakage current,this paper studies from two aspects: modulation method and hardware schemes.This paper emphasizes the study of the modulation method,and establishes the equivalent common-mode circuit of the system,and analyzes the characteristics of leakage current in the symmetrical and asymmetrical filter inductance circuit respectively.Analysis shows that the leakage current can be eliminated through optimizing the modulation methods in the symmetrical circuit,but in the asymmetrical circuit the leakage current issue cannot be solved through modulation strategies.For this reason a new modulation strategy for the symmetrical circuit based on the multicarrier pulse width modulation is proposed which can eliminate the leakage current effectively.However,the MPDPWM will give rise to power unbalance of submodules.In order to ensure the stable operation of the system,a power balance strategy based on DC voltage error sorting is presented.With regard to the hardware methods,this paper conducts the research from the topology improvement and common mode filter.For the first approach,this paper introduces the improved H4 topologies and its working principle,and applies the H6 topology to the cascaded inverter,but it has little effect on the leakage current.For the second approach,this paper introduces the basic principle of common mode filter,a nd combines the cascaded H6 topology with the common mode inductor,which can suppress the leakage current effectively.Finally,the existing hardware solutions are summarized,and a novel common mode filter topology is disused.At last,the simulation model and experimental platform of the cascaded H bridge inverter are designed,and the parameters of the simulation and porotype are introduced in detail,and the designs of hardware and software are presented as well.The validity of the theoretical analysis and the effectiveness of the proposed modulation strategy and the power balance strategy are verified by experiments and simulations.