Research Of Improved Model Predictive Control Strategy For MMC-HVDC

Modular Multi-level Converter(MMC)has the advantages of low switching loss,good power quality,and high integration.Therefore,it exhibits extremely superior engineering application prospects and is widely used in the field of high-voltage and large-capacity power transmission.At present,the modular multilevel converter-high voltage direct current transmission(MMC-HVDC)has become a transmission and distribution solution with broad application prospects,and is widely used in offshore wind power grid connection,island power supply,New energy grid connection,passive network power supply and grid interconnection and other fields.The control strategy plays an important role in ensuring the normal operation of MMC-HVDC.Classical double closed-loop vector control has the characteristics of high control accuracy and fast response speed,but it needs to design the controllers and adjust the parameter.Model predictive control has the advantages of intuitive modeling and no need of controllers,but the solving process of predictive instruction is complex.In this paper,based on the idea of functional equivalence,the double closed-loop vector control is equivalently divided into two predictive links by synthesizing the advantages of the two control strategies in two-phase stationary coordinates and the improved model predictive control is formed.Two different control command acquisition methods are studied to avoid the problems of large computational load and complex adjustment of weight factor in traditional predictive control.At the same time,by discretizing the MMC mathematical model of the modular multilevel converter technology,the MMC valve side voltage cont rol command output prediction model is established to achieve the equivalent function of the inner loop controller,and the inner loop control function can be concentrated into one controller.Power tracking,negative sequence current suppression and power fluctuation suppression.Considering the superiority of model predictive control in MMC applications and the ability of multi-objective constraints,weight coefficients are introduced,and a multi-objective model predictive control strategy based on a two-phase static coordinate system is proposed as the MMC-HCDC system in the event of an asymmetric failure The control strategy,which can flexibly adjust the control goals by changing the weight coefficients of different control goals,and achieve different control effects to meet the needs of various working conditions.Finally,a 21-level MMC-HVDC system was built based on PSCAD/EMTDC,and the two control strategies were simulated and analyzed.The simulation results verified the feasibility and effectiveness of the control strategy.