Research On Several Problems Of Three-Level Grid-Connected Converter Control

With the development of power electronic technology,the three-phase controllable voltage source grid-connected converter is widely used in new energy grid-connected power generation,power quality control,HVDC and other fields due to its unique advantages such as adjustable power factor,bidirectional energy flow,fast voltage regulation on the DC side,and not large support of DC capacitance.The performance of the grid-connected system depends on the relevant technology of the grid-connected converter.Especially under unbalanced grid voltage or the grid voltage is affected by the outside world,the grid-connected system can not control the quality of the output current well and there will be low-frequency harmonic components in the grid current,which indirectly aggravates the distortion of grid voltage.In serious cases,the equipment which connected to the grid will be damaged.Therefore,it is very important to study the grid-connected converter in case of unbalanced grid voltage.Firstly,the topology of NPC three-level grid-connected converter is analyzed and the mathematical model of three-phase three-wire system under three-phase stationary coordinate system,two-phase stationary coordinate system and two-phase rotating coordinate system is established.And a phase-locked method of converter based on positive sequence fundamental voltage in two-phase stationary coordinate system is analyzed.Secondly,two kinds of converter modulation strategies under unbalanced DC voltage are introduced,and a neutral point potential balance algorithm of three-phase three-wire converter is introduced.The neutral point potential self-balance mechanism of three-phase three-wire converter and the output influence of the dead zone based on three-level converter are analyzed.Then,a discrete-time control method of grid-connected converter based on model predictive deadbeat control(DBC)is analyzed in two-phase stationary coordinate system.Based on the "two-step prediction" of a given current,a current-weighted correction method is used to solve the control delay compensation problem,reduce the error caused by sampling and improve the stability of the system.Finally,starting from the instantaneous power model of unbalanced grid voltage,and deduces the current setting under the constraints of different control targets in two-phase stationary coordinate system and two-phase rotating coordinate system,respectively,based on the positive and negative sequence components of grid voltage.Value,and the two kinds of coordinate system under the control system were analyzed,the final selection of two-phase stationary coordinate system model experiments.The current inner loop uses improved deadbeat control.When the grid voltage drops,there is no need to separate positive and negative sequence of the grid-connected current,which reduces the calculation and overhead of the program.Experiments are carried out on the experimental platform to verify the feasibility of proposed control methods.