Start/Stop Control Strategies Of Modular Multilevel Converter Based HVDC System

Trans Bay Cable Project, the first modular multilevel converter based HVDCtransmission system set up by Germany’s Siemens, was officially put into operation in2010.With countries being also planning to build MMC-HVDC projects, the study of MMC-HVDChas important practical value. In order to reduce the undesired impacts and damages on theMMC-HVDC system and the connected AC system during the start-up procedures,appropriate start-up control strategies should be dedicatedly designed. Only takingsteady-state control strategies without other auxiliary measures will have a seriousover-voltage and over-current phenomenon, even system oscillations, affect the normaloperation of the AC system, and endanger the safety of the machines. Therefore, the researchof this process is important.In order to ensure the capacitor voltage of sub-modules in series stability, and avoidMMC maintenance downtime because of less quantum modules in failure, three possibleredundancy protection schemes have been proposed. Finally, the best redundancy protectionprogram is selected through simulation analysis by Matlab/Simulink.In view of the different forms of the connected system on both ends of the converterstation, this paper puts forward two kinds of different start-up methods of converter station,which is defined as active mode and passive mode. For active start-up mode, this paperproposes a scheme for reducing the impact on the AC and DC system while the other oneMMC is being unlocked. For passive start-up mode, this paper presents a direct voltagecontroller before the converter station connects to grid, and two auxiliary charging strategieswhich are to reduce the current surge because of the undercharge sub-modules when theinverter station is being unlocked. For the different forms connected to the inverter station inthe process of passive start-up, the inverter side controller after grid-connection must bechanged. Finally, the simulation results show the correctness and effectiveness of theproposed strategies.Finally, the whole outage process is divided into the energy feedback stage and resistorconsumption stage. Meanwhile, to maximize the energy feedback for the inverter stationwhen there is a passive network connected to the converter station, this paper presents a low-voltage unlocking strategy in the inverter side, which changes the number that should be putinto per phase. On the other hand, this paper uses certain auxiliary trigger strategies so that allsub-modules residual energy can be progressively lost in the form of resistance heating, andsimulation results verify its effectiveness.