Control And Stability Analysis Of Three-phase Four-wire NPC-type LCL-filtered Grid-connected Inverter

With the rapid development of economy,the social demand for energy keeps increasing,and the energy crisis is becoming increasingly serious.At the same time,the public constantly strengthen their awareness of environmental protection and distributed generation which is represented by renewable energy is continually spreading in the power system.All these factors effectively help alleviate the energy shortage and environmental pollution problems.The grid-connected inverter,as the interface connecting the grid,can not only be used to realize power transmission,but also can be used in power quality management occasions.To meet the application demand of high power level,it is developing towards the direction of being multi-level,large capacity and multi-module.Therefore,it is of great practical significance to study the topology of grid-connected inverter and its current control technology.Firstly,this paper analyzes the research status of grid-connected inverter and its control technology.Taking the three-phase four-wire NPC-type LCL-filtered grid-connected inverter as the research topological object,this paper establishes its main circuit mathematical model in abc frame,αβ frame and dq frame respectively.Based on the circuit mathematical model in abc frame,this paper makes a comparation between the two grid-connected current control modes of LCL-filtered grid-connected inverter and puts forward the design methods of their control parameters respectively.In consideration of safety issues in practice,this paper chooses the ICC method.Aiming at the amplitude and phase deviation of ICC mode,an improved ICC control strategy based on command current compensation is proposed in this paper.That is,the command current is compensated,and the control structure is optimized by using the relationship between the system state variables,so that the number of sensors is reduced to 2groups,so as to save the system cost.Futher the traditional discrete differential operator is improved to compensate the calculation delay to further improve the stability of the control system.Then,in order to enhance the functions of the three-phase four-wire NPC-type LCL-filtered grid-connected inverter,and for realizing the function integration of power transmission and power quality management,this paper detailedly analyzed the working principle of the grid-connected inverter when performing various load current compensation.At the same time,in order to suppress the influence of the background harmonic,the second-order complex-vector feedforward scheme of the filter capacitor voltage is proposed.That is,the unit active power vector of capacitor voltage is calculated with the general vector principle,and its fundamental component is obtained by the SOCVF-function,and then the corresponding command current is obtained with the instantaneous power theory.In addition,the DC voltage control method is carried out to ensure the performance of the power quality management.In the last part,this paper verifies the feasibility of the proposed strategies by simulation,and builds an experimental prototype of the three-phase four-wire NPC-type LCL-filtered grid-connected inverter.Meanwhile the algorithm of the control strategies above is realized with the TMS320F28377 DSP.Experimental results validated the effectiveness of the proposed control strategies.