Hybrid HVDC Transmission with LCC and Multi-Terminal Full-Bridge Modular Multi-level Converter

摘要

This paper investigates the control and operation of an HVDC hybrid topology utilizing a line commutated converter (LCC) and the more recently introduced modular multi-level converter (MMC) technology. The hybrid LCC-MMC system studied in this paper uses the LCC as the rectifier to perform AC to DC conversion, while a multi-terminal connection of full bridge MMCs (FB-MMC) are used as inverters to perform DC to AC conversion. HVDC systems conventionally utilize the same converter technology at both the rectifier and inverter station. Hybrid LCC-MMC systems, which use different converter technologies at each station, is a new concept which has yet to be physically implemented. The hybrid LCC-MMC system leverages the features of both LCC and MMC technologies. The LCC is more economical and has a higher power capacity, while the MMC has a smaller space requirement, provides control of reactive power, and is able to connect to weak AC systems. With the future prospect of DC grids overlaying existing AC grids, the hybrid LCC-MMC topology represents a potential candidate to integrate remote energy sources to the grid. In this paper, a control strategy is proposed for the hybrid LCC-MMC which performs the following: 1.) power flow control by implementing ignition angle control at the rectifier and current control at the inverter; 2.) FB-MMC converter control which includes capacitor voltage control of the MMC sub-modules, DC current control and AC current control; and 3.) DC link fault ride through capability by defining characteristic curves that limit and regulate the DC currents at each station during a fault. The proposed hybrid LCC-MMC utilizing FB-MMCs provides accurate power flow control and power sharing amongst converter stations, while offering the ability to ride through DC-pole-to-ground faults on the HVDC line as verified by PSCAD/EMTDC simulations.