Research On Three-level Rectifier Based On Modulated Model Predictive Control

With the development of power electronic technology,a large number of power electronic devices have arisen both in industrial field and people’s daily life.As a converter with many advantages,such as sinusoidal network side current,unit power factor and bidirectional energy flow,PWM rectifiers have been widely used in recent years.In this paper,the control strategy of a neutral point clamped(NPC)three-level rectifier is studied.First of all,the topological structure and working principle of a NPC three-level rectifier are introduced in detail,and the mathematical model of the rectifier is established both in three-phase static coordinate system and two-phase static coordinate system.The basic space voltage vectors of a NPC three-level rectifier are analyzed,they are divided into four categories: large vector,medium vector,small vector and zero vector,the effect on the neutral point potential of different kinds of space vectors are discussed.Secondly,this paper analyses the basic principle of model predictive control(MPC).Based on the principle,a traditional MPC strategy applied to a NPC three-level rectifier is introduced,the predictive model is established,and the value function is designed.For the problem that the control accuracy of traditional MPC is not high enough,an improved MPC strategy is introduced,which effectively improves the control accuracy.The strategy selects the most appropriate switch state to apply to the rectifier by finding the optimal control trajectory in two control period.Thirdly,to solve the problem of the unfixed switching frequency of rectifier under traditional single-vector MPC,and to improve the control accuracy,a modulated model predictive control strategy is proposed in this paper,which combines the concept of synthesizing vector with MPC strategy.In each control period,a group of vectors is selected by rolling optimization,it contains four space voltage vectors,which get synthesized according to a certain duty cycle,and the corresponding switch states are applied to the rectifier accoding to the seven-segment type switch sequence.The proposed strategy fixes the switching frequency and effectively improves the control accuracy,but it also increases the computing burden of the processor.In order to reduce the amount of calculation,this paper introduces a simplified method based on the original modulated model predictive control strategy,which uses deadbeat prediction to calculate the reference voltage vector and judge its sector,effectively reduces the selection range and improves the operation efficiency.Finally,in order to verify the correctness of the modulated model predictive control strategy proposed in this paper,the hardware and software of the experiment platform of a NPC three-level rectifier are designed.Experiments are carried out in the steady and dynamic environment,respectively,proving the correctness of the control strategy proposed in this paper.