Research On Parallel Control Strategy Of Three-Phase Four-Leg Inverter With Interconnecting Lines

With the development of new energy technologies,inverters have become more and more important in the field of new energy due to their high reliability in various power supply applications.The use of parallel technologies about inverters can effectively improve the redundancy,reliability and extensibility of the inverter system.On the other hand,the communication system based on CAN makes the application of parallel systems with interconnection lines more and more widely due to its characteristics of high reliability and real-time performance.However,the three-phase four-leg inverter has the advantages of high DC voltage utilization,adaptability to load unbalance,and flexibility of control strategies makes it more and more popular.Therefore,in this paper,the resesrch considers the three-phase fourleg inverter as the research object to start the study of multi-inverter parallel technology with the communication interconnection.First of all,this paper analyzes the topology and control method of inverter parallel system and contrasts the advantages and disadvantages between parallel systems with and without interconnecting lines.The results show that the parallel system with interconnecting lines has obvious advantages when the amount of inverter units are small and the distance between inverter units is very close.Secondly,this paper analyzes the model of a single three-phase four-leg inverter and compares the control methods based on the inverter mathematics in different coordinate systems.Then,a control technology of three-phase four-leg inverter based on third harmonic injection is designed.And the simulation results show that the control method is correct.Thirdly,this paper focuses on the double-parallel system of three-phase four-leg inverter.By analyzing the mathematical model of the inverter parallel system,this paper deduces the circulation current model of inverter parallel system and obtains the theoretical basis of the control strategy of power frequency circulation current,harmonic circulation current and DC circulation current according to the circulation model of the system.Furthermore,the influence on the output characteristics and the circulation current characteristics of the inverter parallel system when introducing the virtual impedance to parallel system is analyzed.And then,according to the result of theoretical analysis,a circulating current control strategy of threephase four-leg inverter parallel system is designed.Through the simulation analysis,the loop control strategy has a good control effect on AC and DC circulating current in parallel system.Finally,to solve a problem that the third harmonic injection method will introduce the interference in the circulation current of parallel system,an improved method for generating the third harmonic signal is proposed.And simulation shows the correctness of this improvement.Fourthly,with STM32F407 control chip as the core,a hardware and software platform is designed.The hardware experimental about the three-phase four-leg inverter system is carried out,and the experimental results show that the control strategy is correct.