Research On Cascade Multilevel Rectifier

In recent years,cascade multilevel rectifier,as a typical representative of multi-level structure,has been widely used in power electronic transformers,reactive power compensation,multilevel inverters and other high-voltage and high-power occasions because of its simple control,high reliability,modular design and other advantages.For the control objective of cascade multilevel rectifier,the control of cascade multilevel rectifier can be divided into two parts: output voltage control and voltage balance control.In this paper,the control strategy and the improvement of dynamic characteristics of cascaded multi-level converter are studied.Firstly,the topology structure of cascaded multilevel rectifier is introduced.The working mode of single-phase PWM rectifier under different PWM modulation modes is introduced in detail.The phase-shifted carrier technology of cascaded multilevel rectifier is introduced.Finally,the given Mathematical model of cascaded multilevel rectifiers.Secondly,the overall control strategy of cascaded multilevel rectifier is introduced.The double closed loop control strategy of voltage outer loop and current inner loop is introduced in detail.The design formula of voltage outer loop and the parameter selection scheme of current inner loop are given.Aiming at the problem that the low bandwidth of traditional voltage outer loop controller leads to poor dynamic characteristics of the system,an improved voltage outer loop controller and a ripple voltage feedforward control strategy are proposed.The rationality and effectiveness of the proposed scheme are verified by simulation and experiment.Finally,the simulation model and experimental platform of cascaded multi-level converter are built,and the simulation and experimental waveforms under load balancing and load unbalance are given.Finally,the voltage balance control strategy of cascaded multi-level converter is introduced,the proportional pulse compensation voltage balance strategy is introduced in detail,and the design formula of voltage balance controller is given.The simulation and experiment prove that the proportional pulse compensation method can achieve the output voltage balance of cascaded multi-level converter under unbalanced load of each unit.Compensation voltage balancing strategy,when the load of a unit is switched,there will be a large voltage difference between the output voltage of each unit and a long balancing adjustment time,and the dynamic characteristics are poor.Then a voltage balancing strategy based on load current feedforward is proposed,and the correctness and effectiveness of the proposed scheme are verified by simulation and experiment.