Multi-degree-of-freedom control framework for HVDC transmission converters

This paper systematically investigates the multi-degree-of-freedom (Multi-DoF) control framework of high voltage direct current (HVDC) converters based on their topological characteristics and control requirements. First, this study reveals that the control degrees of freedom (CDoF) of a converter are determined by its topology. Specifically, a line-commutated converter (LCC) possesses only one CDoF, a voltage source converter (VSC) has two, and a modular multilevel converter (MMC) can theoretically achieve up to eight. However, only six CDoF are practically necessary for MMC. Despite this, existing MMC control strategies utilize only four CDoF, limiting control flexibility. To overcome this issue, this paper proposes a six-degree-of-freedom (6 DoF) control framework for MMC, which enables asymmetric and precise regulation of DC voltage and SM capacitor voltage compared to the existing MMC control. Based on this framework, corresponding control strategies are designed for MMCs in HVDC systems with different control objectives. Finally, a two-terminal MMC-HVDC system is built on the PSCAD/EMTDC to verify the effectiveness of the proposed 6 DoF control.