A multi-level coordinated DC overcurrent suppression scheme for symmetrical bipolar MMC-HVDC integrated offshore wind farms

Abstract

This paper proposes a multi-level coordinated control (MLCC) scheme for symmetrical bipolar modular multilevel converter based high voltage direct current (MMC-HVDC) integrated offshore wind farms. MLCC makes full use of the energy control capability of MMCs to effectively suppress DC overcurrent triggered by submarine cable breakage. This MLCC scheme consists of four control levels. The first level is for the offshore fault pole MMC. Once a DC submarine cable is broken, the fault pole MMC of the offshore converter station passively absorbs power from wind farms using the storage capacity of internal sub-module capacitors. This buys time for the rest of the control to start. The second level is for the onshore non-faulty pole MMC, which can actively raise the DC voltage to suppress DC overcurrent. The third level is for offshore non-fault pole MMC, which balances the power delivered to the DC system in real-time based on the power received from wind farms. The fourth level is for offshore wind farms, which reduce power to restore the non-fault pole submarine cable transmission power and current to the allowed state. The correctness and effectiveness of the proposed MLCC in different scenarios are verified using the PSCAD/EMTDC.