The Research And Design Of Three-phase Four-leg Inverter Based On Three-dimensional Space Vector Control

When the three-phase load was symmetrical,generally, thetopological structure was three-phase three-leg, the three-phase outputvoltage was also symmetrical. However, with the development ofeconomy, the appearrance of a large number of asymmetric andnon-linear loads, which required such an inverter with relatively higherload capacity, but the traditional three-phase three-leg inverter withasymmetrical and non-linear loads was difficult to output the symmetricalthree-phase voltage. In order to solve the problem brought by three-phaseinverter with asymmetrical loads, such a new topological structure ofthree-phase four-leg inverter was proposed. The topological structure canreduce systemic mass and volume, and it can output the symmetricalthree-phase voltage under the situation of three-phase asymmetrical load.First, the causes of the three-phase three-leg inverter under thesituation of asymmetry of the three-phase output was analysed, and in theview of this situation, several kinds of solutions were proposed to solvethe asymmetry of the three-phase outputunder the unbalanced loadconditions, with the analysis of the advantages and disadvantages ofseveral kinds of solutions, it reached an conclusion of the superiority ofthe three-phase four-leg inverter on topological structure. Then it made anintroduction of several kinds of controlling methods of three-phasefour-leg inverter, such as PWM control, hysteresis current control, spacevoltage vector control. This paper focused on the space voltage vectorcontrol of the three-phase four-leg inverter, and solved the problem ofasymmetry of three-phase output voltage under the situation of theunbalanced load, the solution can reduce the harmonic component, andimprove the use ratio of DC voltage, easier to achieve the digitizaion.It made an detailed introduction of principles and methods of twokinds of three-dimensional space vector modulation at the coordinatesystem of αβγ and abc, it reached an result that three-dimensional spacevector modulation (3D-SVPWM) in the abc static coordinate system hasan advantage of simple calculation, easier to understand and analyze, and easier to realization of an digital signal processor and so on. And then, itestablished an mathematical model of the three-phase four-leg inverter,and decoupled the system, and the symmetrical component method wasused, it decomposed the three-phase output into positive, negative andzero sequence component, and they were respectively to control in thecoodinate system of positive sequence d-q, negative sequence d-q andpositive sequence d-q, and the systemic control block diagram of athree-phase four-leg inverter was established.An systemic simulation model was created in the simulationsoftware of MATLAB/Simulink, it conducted systemic open-loopcontroland closed-loop control simulation in all varieties of three-phaseload conditions, and the result of simulation and theory reached the same.An experimental platform of the three-phase four-leg inverter was built,using chip TMS320F28335of Texas Instruments as the core,programming,the systemic experiments of open-loop control andclosed-loop control was conducted. It can be seen from the result ofsimulation and experimental waveform that three-phase four-leg inverterin the case of asymmetry of three-phase load can keep three-phase outputmaintain symmetry, accelerate the system respond, lower the THD ofthree-phase output voltage. which indicated that the system is relativelysuperior.