Study On The Three-phase Three Leg Four Level Inverter And SVPWM Algorithm

Multilevel inverter is widely applied in the field of high voltage for its low output voltage distortion and good waveform quality,low switching loss and low voltage stress.But some problems exist,such as complex structure,difficult to modulate,difficult matching of the same device,high switching frequency,and existing active circulating current among cascaded units.Therefore,a three-phase three-leg four-level inverter and its modulation algorithm are researched,and effectiveness of the inverter is proved by MATLAB/simulink simulation.The topology studied in this paper is based on the three-phase three-leg four-level inverter.Only one power supply is added to the DC side,when the ratio of the two power supply is set into 2:1,the output voltage is synthesized into four levels.Compared with the conventional four level inverter,the number of power switches is decreased and no need to clamp with diodes or capacitors in the topology studied in this paper,makes the structure simpler and the cost is greatly reduced,at the same time,the output waveform distortion rate is lower than that of the three level inverter.Using the structure of three level,the basic modulation vector number of the four level inverter is equal to the three-level,but the number of vector length is more when it is less than the traditional modulation of the four level.No redundant vector except zero in the vector modulation diagram for the characteristics of the four level inverter topology,the selection of the switching state becomes simple,but the simultaneous action of several bridge arms will be caused while switching the states;when the asymmetry of the region division appeared for the absence of the vector,the computation of algorithm will be increased.Therefore,a variety of SVPWM modulation methods are discussed in this paper,and an optimal modulation mode which makes the structure with the lowest switching frequency and the best quality of the output waveform is found.The feasibility of the topology in three-phase three-leg four-level inverter is verified.All kinds of space vector modulation algorithm are analyzed,verified and compared with the three level NPC inverter in the MATLAB / simulink and RTLAB semi physical verification platform.The results show that the four level topology is feasible and the eight-zone modulation strategy has better effect,lower harmonic content.