Research On DC Side Voltage Balance Control Of Three-Phase Hybrid Cascade Multilevel Rectifier

With its advantages of low harmonic content and low switching stress,multilevel converters have important applications in the fields of new energy power generation and train traction systems.Compared with clamped converters,cascaded multilevel converters are widely used in high-power applications.In order to solve the neutral point problem of the traditional three-phase cascaded multilevel converter,this article studies a three-phase hybrid cascaded multilevel converter topology.This structure effectively reduces the volume of the converter and is easy to realize the modular operation of the system.The topology and working principle of three-phase hybrid cascade rectifier are firstly introduced in detail,and the switch mathematical model is given.Then an open-loop simulation is carried out to verification working principle by using carrier phase shift modulation strategy.In addition,the equalization control of DC side voltage of three-phase hybrid cascade multilevel rectifier is studied,and the reason of DC side voltage imbalance is analyzed.In view of the problem that when the number of cascade modules increases,the traditional centralized control scheme will increase the interconnection lines between the controller and the main circuit,lengthen the lines,and reduce the system stability margin,a quasi-distributed voltage equalization control scheme is proposed in this article.With the addition of the voltage equalization control module on the basis of the traditional double closed loop,the main controller only needs to send out the phase information of the power grid.The unfinished function of the control system is implemented in the sub-processors of each module,which can improve the modularization level of the control system and reduces the number of connections between modules,which is conducive to the development of the system in the direction of high power.In view of the imbalance problem between the three phases of the system,a new control stragey combine the zero-sequence voltage injection method and the quasi-distributed voltage equalization control scheme is developed.Three-layer control structure is adopted to realize the phase-to-phase and intra-phase balance control of the rectifier.The feasibility of the control scheme is verified by simulation.On the basis of theoretical analysis,an experimental platform of three-phase hybrid cascaded multilevel rectifier is built and verified experimentally under quasi-distributed voltage equalization control scheme and three-layer control scheme based on dc components injection method.the results reveal that the control scheme can achieve DC-side voltage equalization and three-phase current balance,which proves the correctness of the theoretical analysis.