A Novel Five-level Inverter For 1500V String Photovoltaic Grid-connected System

With the rapid development of human society industrialization,energy shortage and environmental pollution have become a social problem that needs to be solved urgently.It is estimated that the three major fossil energy reserves in the world are available for human use only in a hundred years,so many countries in the world regard renewable energy as an important research direction in energy field.This paper focuses on the field of photovoltaic power generation system.A novel five-level grid-connected inverter is proposed.The five-level inverter topology,the corresponding modulation strategy and the method of balancing the floating capacitor voltage are researched.Firstly,this paper introduces the development status of the photovoltaic industry,makes a classification of the solar power generation system.Then it focuses on an important part of the photovoltaic system-multi-level grid-connected inverter.This paper summarizes the traditional multilevel inverter topology,including three-level inverter topology,five-level inverter topology and high-level inverter topology.At the same time,various modulation strategies for multi-level inverter topology are also summarized,including space vector modulation strategy,carrier modulation strategy,and specific harmonic elimination modulation strategy.Based on these research,a new five-level inverter topology oriented to 1500 V string photovoltaic grid-connected system is proposed.The current can flow through fewer components when the output voltage is in its high voltage-level,so the conduction loss can be fundamentally reduced.Next,an improved modulation strategy suitable for this topology is proposed.By combining the carrier-stack modulation strategy and the state machine,the switching loss of the inverter is minimized.What’s more,the real-time balance of the floating capacitor voltage is maintained by the floating capacitor voltage balance strategy.After that,the loss mathematical model of the five-level inverter is established.Through specific scenario,the loss distribution is calculated and compared with the traditional five-level inverter.In addition,this paper also proposes a method for pre-charging the capacitor voltage in the inverter,which can ensure that the voltage stress of each component does not exceed the voltage stress under normal operating conditions when the inverter is started.Finally,the feasibility of the proposed five-level topology,modulation strategy and capacitor voltage pre-charging method are verified by simulation model and experimental prototype,and the simulation and experimental results are analyzed.