Enhanced independent pole control of hybrid MMC-HVDC system

Xiang, Wang and Ling, Weixing and Xu, Lie and Wen, Jingyu (2018) Enhanced independent pole control of hybrid MMC-HVDC system. IEEE Transactions on Power Delivery, 33 (2). pp. 861-872. ISSN 0885-8977

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    Abstract

    This paper presents an enhanced independent pole control scheme for hybrid modular multilevel converter (MMC) based on full bridge sub-module (FBSM) and half bridge sub-module (HBSM). A detailed analysis of power distribution between upper and lower arms under asymmetrical DC pole voltages is presented. It is found that the fundamental AC currents in the upper and lower arms are asymmetrical. To enable operation under asymmetrical DC pole voltages, an enhanced independent pole control scheme is proposed. The controller is composed of two DC control loops, two AC control loops and circulating current suppression control based on current injection. Six modulation indices are presented to independently control the upper and lower arms. With this controller, the DC voltage operating region is significantly extended. To ride through pole to ground DC fault without bringing DC bias at the neutral point of interface transformer, a pole to ground DC fault ride through strategy is proposed. Feasibility and effectiveness of the proposed control scheme are verified by simulation results using PSCAD/EMTDC.