Start-Up and Shut-Down Control Strategies for MMC-Based Multi-Terminal HVDC Systems

Abstract

This paper presents start-up and a shut-down control strategies for modular multilevel converter (MMC) based multiterminal high-voltage direct-current (HVDC) systems (MMC-MTDC). An AC system charging method with voltage slop control and a DC line charging method with sequentially unblock control are firstly discussed. Based on the above two charging methods and considering the strength of the connected AC systems, a start-up control scheme suitable for MMC-MTDC systems is described. The scheduled shut-down control strategy is divided into three stages: in energy feedback stage, four methods including increasing modulation index, adjusting transformer tap, introducing third harmonic voltage injection and inserting redundant submodules (SMs) are presented; and the second stage is energy dissipation stage, in which the SM capacitor voltage is discharged to its lower limit; The SM capacitors are fully discharged by SM internal resistor in the third stage. Finally, a simulation model of five-terminal MMC-HVDC is realized in PSCAD/EMTDC. The results show that the system can be start-up smoothly as well as be shut-down rapidly and reliably, which verifies the feasibility of the proposed control strategies.