Research On Dynamic Circulating Current Suppression Strategy For Two T-type Three-level Grid-connected Inverters In Parallel

With the decrease of traditional fossil energy and the gradual improvement of energy structure,promoting new energy power generation is the only way to achieve sustainable development of energy.Photovoltaic grid-connected inverters,as the core device in the photovoltaic power generation system,have been developed rapidly.Among them,parallel connection of high-power inverters can achieve higher efficiency and more stable energy output while increasing capacity,which has become a key development direction.Many literatures at home and abroad have analyzed and solved the problem of steady-state circulating current suppression from the perspective of steady state,but most of them do not involve the problem of circulating current suppression in the dynamic operation of multiple inverters.However,in the practical application of the photovoltaic power generation,the impact of circulating current on equipment and power grid in the dynamic process of parallel operation of high-power inverters is a practical problem to be solved.Based on the application background of the photovoltaic power generation system,parallel operation of two T-type three-level inverters with the same capacity is taken as the research object,and the suppression strategy of dynamic circulating current is focused on.The specific research contents are as follows:Firstly,the working principle of a single T-type three-level inverter is described.Taking the a-phase bridge arm of the inverter as an example,the transient mathematical model of the equivalent circuit is established.A grid-connected current control algorithm of the T-type three-level inverter is studied,the principle of the control algorithm is explained,and the derivation process of the control algorithm is given.Then,the path of dynamic circulating current of two T-type three-level inverters in parallel operation is analyzed in detail,the mathematical model of dynamic circulating current of two inverters in parallel operation is established,the basic cause of dynamic circulating current of two inverters in parallel operation is analyzed,and the control strategy of dynamic circulating current of two inverters in parallel operation is studied.Furthermore,the influence of different filter inductance parameters of two inverters on the dynamic process of parallel operation of the inverter is discussed,an on-line identification method of inductance parameters based on model reference adaptive(MRAS)is studied.Based on the analysis of its principle,the reference model and adjustable model of the inductance parameter identification are selected according to the control block diagram and equivalent circuit model,and Popov hyperstability theory is used to design the adaptive law of online parameter identification based on MRAS,and the predicted inductance value is obtained.Through the combination of on-line identification method of inductance parameters and grid current control algorithm,the grid current tracking and circulating current suppression in the dynamic process are finally realized.Finally,simulation model of two T-type three-level inverters in parallel operation based on MATLAB/Simulink and the experiment platform with STM32F407ZG as the control core are built respectively to verify the online identification of inductance parameters of single inverter and the suppression strategy of dynamic circulating current of two inverters in parallel operation under various working conditions.The simulation and experimental results are both satisfactory,it is proved that the dynamic circulating current suppression strategy and the online identification method of inductance parameters for parallel operation of two inverters studied in this thesis are correct.