Control of series-connected diode rectifier and MMC for three-wire-bipolar-structure based HVDC

With the increased penetration of renewable sources and the increased load demand, how to upgrade the power transfer capacity draws lots of attention. Converting the existing AC transmission lines to DC is one of the most attractive solutions. This paper proposes a three-wire-bipolar-structure based high-voltage-direct-current (TWBS-HVDC) for wind power transmission, where the rectifier station consists of 12-pulse diode rectifier and a modular multilevel converter MMC (DRMMC) which are connected in series at the DC-side. Such a system has the ability to increase the power transmission capability and more importantly, reduce the investment during the transmission solution conversion. A control strategy is also proposed, especially for the DR-MMC, to enable the reliable operation of the TWBS-HVDC for wind power transmission. With the control strategy, the wind-farm-side AC voltage and frequency are well manipulated and the DC voltage sharing between the DR and MMC is well control. In addition, the mode switch from HVDC to HVAC is also investigated. Simulation results in PSCAD/EMTDC verify the feasibility and effectiveness of the proposed hybrid DR-MMC based TWBSHVDC system.