Research On High-Efficiency Modulation And Control Technology Of Single-Phase T-Type Three-Level Inverter

In the 21st,the global consumption of fossil energy continues to increase,especially coal and oil,resulting in worsening energy depletion and environmental pollution.As the largest energy consumer and producer,China is highly concerned about the air pollution,and has taken the lead in putting forward the goal of carbon peak and carbon neutrality.During the two sessions,Premier Li Keqiang once again stressed the importance of the two carbon targets,urging the energy sector to speed up its transformation and establish a new energy structure featuring clean,electrified,digital and standardized energy.As the main force of energy production,new requirements is set to the power industry due to the two carbon targets,which greatly promoted the development of the new energy industry.Distributed photovoltaic power generation system is one of the most promising new energy generation methods.With the implementation of preferential policies such as photovoltaic poverty alleviation and national subsidies,the cost of distributed photovoltaic power generation has been declining and the capacity has been growing,and the distributed photovoltaic power generation industry has entered the fast lane of development.As the interface of energy conversion in distributed photovoltaic power generation system,the performance of single-phase inverter directly affects the efficient,safe and stable operation of the system.Traditional single-phase two-level inverter is difficult to meet the needs of efficient operation of distributed photovoltaic power generation system due to its poor output waveform quality and large filter volume.Single-phase three-level inverters have many advantages such as low switching stress and low waveform distortion,etc.,which have attracted wide attention and become an international research hotspot.Scholars at home and abroad have done a lot of work on single-phase three-level inverters.However,there are still some key problems to be solved.First of all,the single-phase three-level inverter is faced with the problem of loss reduction and efficiency improvement.The switching loss increases with the increasing switching frequency,which seriously affects the operating efficiency of the system,and the inherent midpoint potential balance problem of three-level inverter increases the difficulty of modulation to improve the efficiency.Secondly,in order to solve the problem of light load and low efficiency of distributed photovoltaic power generation system,relevant scholars proposed a solution of parallel operation of single-phase inverter.However,the circulation introduced into the parallel operation of the inverter will not only cause the current waveform distortion,but also increase the additional line loss,and even lead to the system overcurrent,which seriously threatens the safe and stable operation of the inverter.These problems seriously restrict the efficient operation and large-scale application of single-phase three-level inverters in distributed photovoltaic power generation field.In order to solve the above problems,on the basis of summarizing the relevant work of scholars at home and abroad,this paper takes single-phase T-type three-level inverter as the research object and aims to achieve efficient operation.The research focuses on loss reduction and efficiency increase of single operation and circulation modeling and suppression of parallel operation.The main work is as follows:1.Aiming at the problem of loss reduction and low efficiency,the modulation method to reduce switch loss is analyzed emphatically.Firstly,this paper introduces the carrier modulation method based on the bias component injection,and analyses the injection under the condition of different bias component,single phase T three level inverter switch and switch loss,based on this,puts forward the improvement of carrier modulation method based on maximum offset component injection,to make the switch frequency of traditional carrier modulation method is reduced by half,then reduce the switching loss.At the same time,the proposed modulation method can realize the balance control of the midpoint potential.2.In view of the parallel operation of single-phase T-inverters,the port degradation characteristics of the single-phase T-inverters are firstly analyzed,and based on this,an improved parallel equivalent circuit of the single-phase T-inverters is proposed.Then,based on Kirhofer’s voltage and current law,a mathematical model of the single-phase T-type three-level inverter parallel system is proposed,and the excitation sources of the circulation under the condition of symmetric and asymmetric filter inductance parameters are analyzed emphatic.It is found that under the condition of symmetrical filter inductance parameters.the circulation in the parallel system contains only high frequency components,while under the condition of asymmetric filter inductance parameters,the circulation in the parallel system contains not only high frequency components,but also low frequency components.Among them,the low frequency component of circulation is the main cause of extra line loss,which needs to be suppressed.In order to solve the circulation suppression problem of single-phase T-type three-level inverter running in parallel under the condition of asymmetric filter inductance parameters,a control method to realize the circulation suppression and midpoint potential balance at the same time is proposed.Then,based on the mathematical model of the circulation,a beat-free circulation controller is designed.The simulation results show that the circulation model is correct and the circulation suppression method is effective.3.An experimental platform of single-phase T-type three-level inverter based on dSPACE is built,and the proposed algorithm and model are verified by using the experimental platform.The experimental results prove the effectiveness of the proposed algorithm and the correctness of the proposed model.