Model Predictive Control Method For High Power Three-level PWM Rectify System

PWM rectifier is an important interface for the high power equipment to connect the grid.The core control objective of PWM rectifier is balancing the AC and DC power as a bridge between the high power equipment and the grid.However,when power is transferred from AC to DC,some energy is consumed by PWM rectifier,that is normally called power loss.Meanwhile the switching loss is the major part of the power loss.Voltage oriented control(VOC)based on state-space averaging model,as a traditional PWM rectifier control method,cannot stabilize the PWM rectifier and ensure the quality of electric energy in a low switching frequency.Hence,based on the three-level PWM rectifier,we do some intensive research on the principle of rectifier model,predictive control theory,finite control set model predictive control(FCS-MPC)optimization method and some other related issues as follows:As the state-space averaging model cannot be used in lower switching frequency PWM rectifier.First,we analysis the discrete dynamic events of the PWM rectifier and the continuous dynamic events of the PWM rectifier.Second,we point out that the hybrid relationship between discrete dynamic events and continuous dynamic events.At last,we build the three-level PWM rectifier hybrid model based on finite state automaton(FSA).Meanwhile,the Lyapunov stability criterion is used for stability analysis of FSA hybrid model.Also,we point out that different control preference will lead to different control effect for PWM rectifier.The FCS-MPC method for three level PWM rectifier is proposed based on the analysis of hybrid model.The research is mainly focused on the prediction model design method and the cost function design method.First,the high precision discrete prediction model for three level PWM rectifier is built after analyzing the differential function Taylor series expansion principle.Second,the cost function for three level PWM rectifier's multi-objects is designed one by one,and the weight coefficient of cost function is analyzed.In addition,when FSC-MPC algorithm is employed in digital signal processor system,there exist control delay problem.To solve this problem,we propose a FCS-MPC delay compensation method by the second predictions.At last,the performance test results of three-level PWM rectifier FCS-MPC system is given,including: steady-state test,dynamic-state test,switching frequency statistics and a comparison with the VOC method,etc.We propose a novel FCS-MPC cost function design method based on satisfactory optimization to solve the local optimum problem.The novel cost function can make sure the global optimization of three level PWM rectifier by the layer-by-layer optimization.In addition,in order to solve the conservative problems of the conventional FCS-MPC single-step prediction,we propose a multi steps FCS-MPC method based on trend extrapolation which can effectively improve the prediction horizon.However,multi-steps prediction will increase the computational burden.Hence we build a switching sequences search tree and a branch-and-bound method for multi-steps satisfactory optimization FCS-MPC.The computational burden of multisteps satisfactory optimization FCS-MPC can be decreased effectively.When PWM rectifier is controlled with FCS-MPC method,the steady-state switch sequence is random which will cause the low frequency harmonic problem in grid side.In order to solve this problem,we propose a selective harmonic elimination MPC(SHEMPC).First,the SHE-PWM off-line solution is adopted as FCS-MPC search starting point.Second,a new cost function is designed to amendment SHE-PWM pulse time.Third,the output of SHE-MPC switching sequence is accomplished by rolling optimization.In addition,the traditional virtual impedance method based on LCL filter is no longer useful.So,we propose a virtual power injection method for FCS-MPC and SHE-MPC.The equivalence between virtual power injection method and capacitor branch injecting series resistance is also verified.Meanwhile,the experiment results show that the proposed method is appropriate for FCS-MPC and SHE-MPC.Finally,2MW back-to-back double three-level PWM rectifier platform is designed,and the proposed methods is verified by experiments.