Research On Control Strategy Of Modular Multilevel Converter Based HVDC

In recent years,flexible DC transmission technology based on Modular Multilevel Converter(MMC)has attracted wide attention from national and international scholars in related fields due to its advantages such as high waveform quality,low loss and well expansibility.In the field of High Votage Direct Current(HVDC),MMC has gradually replaced the traditional Voltage Sourced Converter(VSC)which only has two or three levels.However,as an emerging technology,the control strategy research of MMCHVDC is still in the development stage.At present,there are some problems of MMCHVDC in the aspects of its steady-state control strategy,transient control strategy,system control strategy and AC/DC fault interaction influence caused by fault ridethrough control strategy.This paper focuses on the control strategy of MMC-HVDC and the impact of fault ride-through control strategy on AC side protection.This paper first studies the station control strategy of MMC-HVDC: 1)Based on the typical double closed-loop steadystate control strategy of MMC,the sliding mode control(SMC)is introduced into the design of MMC controller,which improves the response speed and robustness of MMC.In order to solve the chattering problem of conventional sliding mode control,an improved sliding mode control strategy is proposed.Finally,some simulation examples are carried out to verify the effectiveness of the proposed method.2)The MMC transient control based on the traditional instantaneous power theory can not simultaneously suppress the double frequency fluctuation of active and reactive power under unbalanced grid voltage.Thus,a new MMC transient control strategy which combines new instantaneous power theory and improved sliding mode control is proposed.Finally,the effectiveness of the new control strategy is verified by many simulation cases based on PSCAD/EMTDC.Secondly,the system control strategy of MMC-HVDC has been studied in this paper.In order to optimize the control performance of the traditional DC voltage droop control,two improved droop control strategies are proposed based on station headroom and system loss which also combine with improved sliding mode control.To combine the advantages of the two strategies,a comprehensive droop control strategy based on the actual operating state is proposed,which can simultaneously consider station headroom and system loss.Finally,the simulation cases based on PSCAD/EMTDC are carried out to verify the feasibility of the proposed three methods above.Finally,this paper combines the traditional sequential network theory and the fault ride-through control strategy of MMC station configuration in a MMC-HVDC project in China to analyze the fault characteristics of the AC side line of the project.Then,according to the AC fault characteristics,the influence of the access of the MMC station on the directional pilot protection based on sequence component,current differential protection and distance protection configurated on the AC line is analyzed.Based on the simulation model of the project built in the PSCAD/EMTDC,the correctness of the AC fault characteristics and the applicability of various traditional AC protections are verified.