Control of Hybrid HVDC Transmission System with LCC and FB-MMC

Nowadays, a Line Commutated Converter (LCC)-HVDC is a mature technology with decades industry experience. Even though it's with several advantages such as higher reliability, lower cost, and higher efficiency, it presents some drawbacks such as large converter station size and lack of black starting capability. So the LCC-HVDC is not suitable for some applications in which black starting capability is necessary or high power density is highly emphasized. A LCC-Voltage Source Converter(VSC) hybrid HVDC transmission system combines the advantages of both technologies. In this paper, a comprehensive control strategy of a LCC and Full Bridge Modular Multi-level Converter(FB-MMC) hybrid HVDC transmission system, including converter control schemes, power flow control strategy, black starting strategy, and DC short circuit fault ride through strategy is proposed. Also, DC short circuit fault characteristics depending on AC grid types of the FB-MMC are analyzed and appropriate ride through strategies for each case are established. Validity of the proposed control strategies are verified by computer simulation and tested at reduced scale prototype set-up.