Study On SVPWM Method Based On 60° Coordinate System For Three-Level Inverter

This research is follow-up study of the fund project "Harmonic Suppression Technology of Power Grid"(20101402120004),which inherits the research methods and achievements in three-level inverter of research group.A SVPWM control strategy of three-level inverter is designed in this paper,which is transformed from rectangular coordinate system into 60°coordinate system.This paper discusses the control method of SVPWM and the solution of neutral point voltage unbalance.The reasonability of the control strategy is verified by MATLAB simulation and experiment,and this strategy has the advantages of stable output voltage,good sinusoidal voltage,fast dynamic response and low total harmonic distortion.It has great significance in solving the problems of three-level inverter which have complex algorithm,large amount of calculation and so on.Now the specific research content are as follows:The defects of the traditional SVPWM algorithm are analyzed,such as the complexity of regional judgment,the large amount calculation of acting time and the redundancy of time series,so this paper proposes the SVPWM control algorithm in 60° coordinate system.After the transformation of 90°coordinate system into 60° coordinate system,considering reference voltage vectors are synthesized by the basic voltage vectors,this paper adopts per-unit transformation and presents a simple calculation method of reference value in order to simplify the mathematical calculation process.After determining the location of reference voltage vector,the rotating module control strategy is proposed and the rotation module equation is established which is based on large sectors with 60°division principle.It means that the reference voltage vectors in other large sectors can be rotated to first large sector so that the judgment of small sectors and the calculation of the basic vector time can be simplified greatly,and the system operation speed is improved.On the basis of the 60° coordinate system,this paper analyzes the reason of the midpoint current flow leading to the midpoint imbalance.The virtual vector method is established to solve this problem.Five small sectors are used to replace six small sectors,and the action sequence and action time are analyzed and calculated.The mathematical model of VSVPWM is completed.On the MATLAB simulation platform,the simulation model is built based on the mathematical model.The simulation results verify the effectiveness of SVPWM and VSVPWM algorithm in 60°coordinate system.Compared 60° SVPWM with traditional SVPWM control method,SVPWM control algorithm with VSVPWM neutral point potential control method,results show that the proposed control strategy can effectively improve the system dynamic response ability,solve the problem of neutral point potential unbalance.Based on inheriting the experimental circuit,in order to achieve the purpose and effect,the hardware circuit is improved,the program of this control strategy is written with C language on the CCS3.3 platform and the relevant debugging is carried out.Finally,the whole system debugging is carried out on the experimental platform.The experimental results are consistent with the theoretical analysis and the simulation results,which proves the rationality of the hardware design and software compilation.The validity of optimal control strategy is proved by experimental verification of SVPWM and VSVPWM neutral point potential balance strategy in 60° coordinate system.