Research On Optimization Control Method Of Modular Multilevel Converter

Modular multi-level converter(MMC)has many advantages such as high degree of modularity,free expansion of voltage and power levels,high power density,and flexible control structure.Since it is introducted in 2001,has received extensive attention,but it is still limited by its own topological characteristics in some special environments and applications.Based on those,the paper takes MMC as the examination object to research its system modeling,optimization control method,safe operation ability and related derivative problems.First,the related issues such as MMC topology,working mechanism,and model establishment are researched.The MMC main circuit topology and sub-module structure are discussed,its basic working principle is introduced.Based on this,the mathematical model of the continuous domain of MMC is established,and the correlations between its AC and DC sides and internal variables and its own characteristics are analysed.The influence of sub-module capacitance voltage fluctuation on the internal and output quality of the system are fully considered,and the submodule voltage fluctuation average value model(SCVF-AVM)is established.The value model(SCVF-AVM)analyzes the differences between the SCVF-AVM and the classic simplified model of the MMC are analyzed,its influence on the system’s characteristics of dynamic state and steady state is verified.Aiming at the problems that there are many internal and external control variables coupling with each other in MMC and the multi-objective cooperative control,so FPS-MPC method is proposed for MMC.The predictive operations are very complex of the classical model predictive control and collaborative optimization and other aspects of different control objectives are carried out.Firstly,the mathematical model of MMC discrete domain is established.The reasons why the operation has large amount,the switching frequency is not fixed and the sub-module control resources are not fully utilized when the traditional MPC method applied to MMC are analyzed.Different control objectives are prioritized,sub-module co-processor resources is made full use of,and the core processor’s computational load is reduced.In the optimization process of FPS-MPC,the restriction of safety constraints and the optimal expected voltage pick-up method are introduced.Under the premise of obtaining the same control performance,the number of elements in the search set and the number of predictions are greatly reduced.Aiming at the problem of low-priority control targets,a virtual sub-module mapping method is proposed to further reduce the frequency of sub-module switching and achieve global optimal control.Finally,the performance test results of the MMC converter FPS-MPC method control are given,including: steady-state performance,dynamic performance,and comparison analysis of security constraints constraint.Aiming at the serious pulsation of the capacitor voltage in the low-frequency state of the MMC drive system,an optimization control method of MMC frequency conversion is proposed.Firstly,the MMC energy model is established to analyze the reasons of the voltage fluctuation of the sub-module capacitor voltage in the low frequency state.The current method of low-frequency control of MMC is reviewed.For the problems of unbalanced energy of arms,large loss of traditional low-frequency control methods,and difficulty in designing circulating current controllers,a low-frequency control method based on bias voltage(IBCV2)is proposed which the multiplication components made full use of high-frequency common-mode voltage to reduce the amplitude of the high frequency circuit current.By using the FPS-MPC idea,an indirect control of the high and low frequency components of the internal circulation can be achieved by tracking the desired bridge arm bias voltage.To deal with different voltage fluctuations in sub-modules of different frequency bands,switch link of high and low frequency control mode is designed,and smooth dynamic process of MMC frequency conversion driving motor is achieved by real-time online correction of high and low frequency control components.At the same time,the effectiveness of this method for MMC variable frequency drive system is verified by the experimental results.Aiming at the problems that between the contradiction of the multiple control objectives and the unequal energy of the phase unit of the unbalanced state for MMC,the two parts of “external control” and “internal control” are focused on.In the "External Control" section,a flexible current predictive control method(FC-MPC)for unbalanced grids is proposed.The positive and negative sequence of active and reactive currents are described and normalized.At the same time,the tedious coordinate system change and parallel control architecture of the positive and negative sequence component are abandoned,the current on the MMC side directly is predicted and controled;according to the different control objectives proposed by the system,the adjustment factor is flexibly modified to achieve fast tracking and switching of different control targets.In the "Internal Control" section,an energy balanced predictive control method(EB-MPC)is proposed to analyze the causes of unequal energy in the phase cells,and the unbalanced part is compensated and controlled so that the mean value of the sub-module capacitance voltage achieve stable under unbalanced conditions.MMC is mostly used in harsh environments with high-voltage,high-powered,and high requirement for system security is raised.Therefore,a method of MMC sub-module faulty diagnosis based on Extend Kalman Filter technology is proposed.For traditional fault diagnosis methods,there are many objects need to be observed and the amount of computation is large,so each phase circulation is selected as the observation variable and an Extend Kalman fault state observation system is established to accurately monitor the system state and avoid interference of external noise and process noise and reduce the misdiagnosis phenomenon.Due to the traditional method that can only locate the open fault type of the submodule,a fault submodule positioning method based on port voltage discrimination(SPVD)is proposed.This method can quickly and accurately locate the location of submodules with different fault types for open and short circuits as well as more types of faults can be covered up and make detection and positioning more accurate by fault diagnosis system.Finally,the relevant experimental verifications of the research content of this paper are completed basing on the experimental platform of three-phase modular multilevel converter.