Research On Control Strategy Of MMC-HVDC Transmission System

Modular multilevel converter high voltage direct current(MMC-HVDC)transmission system has high degree of modularity,good quality of output waveforms and can separate controlling active power and reactive power.Because of it can fit in high voltage and high power requirements.MMC-HVDC transmission system has a best application prospects.And there are plenty of practical engineering putting into operation.This article takes MMC-HVDC transmission system as research object,in view of problem of circulating current and sub-module voltage unbalancing under normal operation and asymmetric arm-bridge operation.MMC-HVDC transmission system model predict control strategy has been researched,which combines proportional-integral controller and model predict control strategy,according to ac current tracking objective function,sub-module capacitor voltage balancing objective function and corresponding weight coefficient builds up overall objective function,switching state has been adjusted by circulating current suppression compensative voltage level.This strategy can keep sub-module capacitor voltage balancing and suppressing circulating current.When MMC-HVDC transmission system under arm-bridge unbalanced impedance operation,arm-bridge current is been analyzed.This article points out that ac side current cannot been separated equally in unbalancing arm-bridge,which causes circulating current not only containing second frequency also fundamental-frequency,and leading to fluctuation of dc side current.In the light of this problem,model predict control strategy for asymmetrical arm-bridge impedance has been presented.This method combines sorting prediction time sub-module capacitor voltage,and best switching state has been chosen according to overall objective function and circulating current suppression objective function,which can accomplish suppressing unbalancing circulating current and minimize dc side current fluctuation.Under condition of MMC-HVDC transmission system existing redundant sub-module,modular multilevel converter redundant fault tolerant mathematical model has been built.A redundant fault tolerant model predict control strategy has been proposed,which set sub-module capacitor voltage balancing objective function in accordance with the number of normal sub-module.This strategy can adjust find best switching state,and can tracking ac current,sub-module capacitor voltage balancing and suppressing circulating current of fundamental-frequency and second frequency.MMC-HVDC transmission system simulation model based on MATLAB/SIMULINK has been built,simulation analysis have been carried out under normal operation condition,asymmetric arm-bridge impedance operation condition,and asymmetric arm-bridge number of sub-module operation condition separately.And simulation results verify the feasibility and effectiveness of proposed control strategies.