A Control Strategy For Three-Phase Four-Wire Three-Level VIENNA Rectifier Operating In Mixed Conduction Mode

Rectifier as the first appeared power electronic equipment has a wide application in both industrial and civil fields. Pulse-width modulation (PWM) rectifier, compared to the traditional non-controlled or phase-controlled rectifier, has the advantages of higher power factor, lower harmonic pollution and smaller size, is an important academic and industrial research area in power electronics field. Three-phase rectifier, often used in high power applications, its input current is greater impact on the grid. As the world's grids and electrical equipment have increasingly strict requirements for harmonic, the significance of study of three-phase PWM rectifier with high power factor, low harmonic, will become more and more important.Among the many three-phase rectifier topologies, three-phase four-wire three-level VIENNA topology has much concerned in industry, because of its unique structure and advantages of control and reliability. This study is right the use of this topology, combined with DSP digital control technology, to get a three-phase rectifier of high performance. In order to improve the rectifier's power density and reduce overall costs, this study also reduces the input inductor, so that the rectifier operates in mixed-conduction mode.As can be known from the mathematical models of the rectifier, respectively, in continuous and discontinuous conduction mode, the gains of duty cycle to inductor current are quite different, so that the traditional controller with only one set of parameters cannot satisfy different modes'system. Therefore, this research uses a feed-forward strategy to improve the current loop. In different operating modes, calculated ideal duty cycle will be added to the output of current loop controller. The burden of the current controller can be reduced, meanwhile, the system performance and stability in mixed-conduction mode can be improved. This research also uses an algorithm to compensate for the current sample in order to get the average current value in each switching cycle for different conduction modes.Based on the above method, this paper first shows the effectiveness of this control strategy by modeling analysis. And then the Simulink simulation results further illustrate and verify it. Finally, the experimental results get from a 5 Kw prototype platform demonstrate the practical effect of this control strategy. Both simulation and experimental results show that the new control loop can ensure a higher power factor and lower input current harmonics when rectifier operates in mixed-conduction mode.