Self-Start Control With Grouping Sequentially Precharge for the C-MMC-Based HVDC System

The newly developed modular multilevel converter based on a clamp double submodule (CDSM), namely, C-MMC, has been identified as a good solution for high-voltage direct-current (HVDC) applications, due to its inherent unique dc fault blocking capability. This paper presents a self-start control strategy for the C-MMC-based HVDC link. The equivalent charging circuit for the uncontrolled precharging stage is derived and the behaviors of the CDSM are investigated. In addition, the selection of the current-limiting resistor is described. To fully charge the capacitors in submodules, a simple grouping sequentially controlled charge (GSCC) method is proposed. This method makes the capacitors be charged group by group so that each capacitor can share sufficient stored energy. In particular, detailed startup procedures are designed for the HVDC link under two conditions, that is, connecting to active grids and supplying the passive network. The feasibility of the GSCC scheme under cell failure conditions is discussed. The validity of the proposed strategies is verified by PSCAD/EMTDC simulations.