Study On Three-level NPC Converter SVPWM And Its Point-potential Unbalance Control Strategy

The multi-level converter has the advantages of good pressure resistance,low output harmonic and equivalent switching frequency,which is more suitable for high voltage and high power than the current two-level structure.The research about the structure of the diode-clamped three-level converter is more common and its application is the most common.Therefore,this paper studies the spatial vector control method and its simplification about the structure,and explores the structure' inherent problem of midpoint potential imbalance of to study control strategies to against it.First of all,this paper summarizes the background of the origin of multi-level structure and sums up the practical significance of studying multi-level structure,and expounds its development history and research status as well.Analyze the advantages and disadvantages of the three basic structures of multi-level structure by taking the three-level structure as an example,and take three-level structure as the research object.And then analyzes the basic working principle of the main circuit structure of the selected object,leads the space vector and the three-level space vector control method.This paper analyzes the principle and control process of the three-level space vector algorithm,and constructs a new spatial vector control method based g-h coordinates instead of the original method based ?-? coordinates,which greatly simplifies the calculation.The two methods are simulated in MATLAB to verify the conclusion.Three-level structure is difficult to avoid the midpoint of the problem which will produce all kinds of hazards if it is not controlled.Based on the space vector,this paper summarizes the different effects of different vectors on the midpoint potential in the three-level vector,which is the fundamental of the control strategy.After summarizing the research status of the midpoint control strategy and analyzing the advantages and disadvantages of various methods,the midpoint charge control algorithm and the virtual vector algorithm are taken as examples to analyze the advantages and disadvantages,which are simulated to verify the conclusion.In order to simplify the control and ensure sufficient effect,this paper proposes a new hybrid modulation algorithm.The new algorithm consists of sector selection control algorithm based on svpwm with simplified three-level and virtual vector algorithm with variable virtual vector.In the new algorithm,the sector selection control algorithm will be putted to use in the low-key system,and virtual vector algorithm will be used in the high-profile system.Thus control could be simplified,while ensuring adequate control results.Finally,the simulation system is established in MATLAB,and the correctness and feasibility of the algorithm are verified.