Research On Key Technologies Of NPC/H Five-Level Converter Finite Control Set Model Predictive Control

NPC/H five level converter combines the characteristics of the three level NPC topology and the cascade H bridge topology,and has broad application prospects in the medium and high voltage fields.Finite-Control-Set model predictive control(FCS-MPC)has advantage of visual modeling intuitive,easily understood and good at dealing with multiple constraints and multi-objective optimization,meanwhile,without current PID controller and PWM modulation links,helping to further enhance the dynamic and static control performance of variable-frequency speed control system.Thus it has a broad application prospect in high power multilevel converters.This paper focuses on excessive predictive calculation,difficult setting of weighting factor,low switching frequency control and other issues in the face of the NPC/H five level converter finite-control-set model predictive control(NPC/H-FCS-MPC),and the main work and achievements are listed as follows.1.Research on low switching frequency control strategy of NPC/H-FCS-MPC.On the basis of detailed analysis of the 5L-NPC/H topology,the design of FCS-MPC controller is carried out with traditional weighting factor method from the perspective of reducing the computation complexity and simplifying the design of optimal performance function.This paper proposes a simplified control strategy based on logic switch vector optimization method,and the performance of the FCS-MPC system is analyzed in the whole modulation range.On the basis of this,a novel FCS-MPC low switching frequency control strategy is proposed,which is based on the theory of finite set model predictive control.This strategy does not increase the burden of system operation,and has no relevant weight factor setting problem.It can be extended to other finite set model predictive control systems.Simulation and experiment show that the strategy in the entire modulation range can effectively reduce the switching frequency;the switching frequency can be controlled within 125 Hz to 400 Hz,and always running around 200Hz;When the current modulation system is about 0.81,the highest decline is up to 48%;And the control performance is basically not affected with only a slight increase in the harmonic distortion rate.2.Research on a novel finite control set mode predictive control strategy based on hierarchical optimization.For the first time,this paper proposes the design method of NPC/H-FCS-MPC system based on hierarchical optimization method combined with satisfactory optimization,and conducts detailed analysis on the design methods and characteristics of different coordinate systems.A new finite control set model predictive voltage control strategy based on hierarchical optimization is proposed,the weighting factor is avoided through the optimization of the control objectives of the hierarchical optimization design;by changing the multiple current prediction of the voltage vector into a single voltage prediction of reference voltage,the forecast operation is reduced to a minimum;and the minimum common mode voltage output control is realized.Simulation and experimental results show that the proposed method has good control performance,and the output common mode voltage is maintained at 1/6 times of the DC side voltage.On the basis of this,a detailed analysis and research was made on the parameter stability domain of the finite set model predictive control system and its development method,and an incremental predictive voltage control strategy for finite control set model is proposed.Simulation analysis and experimental results show that the stability domain of inductance parameters remained unchanged,meanwhile the stability domain of resistance parameters improved significantly.3.Research on Variable-frequency Speed-regulation Control System based on NPC/H-FCS-MPVC.In view of the deficiency of the traditional mixed flux observer,an improved hybrid flux observer is proposed.The improved hybrid flux observer combines the advantages of the current model and voltage model,and uses the transition function which contains the phase compensation to make the flux switching path approach the ideal path.It not only ensures the accuracy of flux observation in the whole speed regulation,but also decreases the sensitivity of the flux observer to the parameters of the motor.And then the characteristic of flux observer is analyzed in detail by using frequency response function.In order to ensure that the speed regulating system of NPC/H five level inverter is not damaged due to the damage of individual switch devices,a double arm fault-tolerant operation control strategy based on a finite control set model predictive voltage control was proposed,and the feasibility of fault tolerant operation and realization method were studied in detail.Simulation and experimental results show that the dynamic performance of the speed regulating system under the double arm fault-tolerant operation mode is basically equivalent to that when the three phase of inverter is in normal operation,which can meet the half-speed emergency operation of speed control system in full load.The algorithm of this fault-tolerant control strategy is simple and easy to be implemented.The research results of this paper lay a theoretical foundation for improving the dynamic and static performance of the NPC/H five level converter frequency conversion speed regulation system,and this will help to promote the application of finite control set model predictive control technology in multilevel converters.