A Novel Strategy on Smooth Connection of an Offline MMC Station Into MTDC Systems

Themodular multilevel converter-based multiterminal HVDC (MMC-MTDC) technology has been applied to the Nanao project, which was commissioned successfully in China, and has shown many advantages. However, how to connect an offline MMC station into an operating MTDC system smoothly without expensive dc circuit breakers presents a huge challenge. Making full use of the structural characteristics of MMC, the proposed strategy implements the smooth connection by sequential operations of the dc disconnectors (DSs) at the positive and negative polarities, without the need for changing system parameters or hardware. The charging circuits of MMC are studied fully after every operation of DSs, and a four-order state equation is built to explore the potential maximum surge current after closing the DSs. Case study and simulation are performed based on the PSCAD/EMTDC model of the Nanao three-terminal MMC-HVDC system to validate the effectiveness of the proposed strategy. Besides easy operation and high reliability, the strategy neither influences the operating system nor increases any cost.