With the developing trend towards large capacity

high voltage and long distance for flexible DC technology

challenge and higher requirements are put forward for the existing flexible DC real-time simulation technology. In order to meet the control and protection testing requirement for the large capacity

high voltage and long distance flexible DC system

a new simulation method is proposed. Firstly

the equivalent modeling method of modular multilevel converter(MMC)suitable for variable sub-module topology and module level fault is developed. Then the modeling algorithm is realized based on the field programmable gate array(FPGA). The FPGA carries on communication with Thevenin equivalent circuit in real-time digital simulator(RTDS)through GTFPGA. Thus

a flexible and user defined flexible DC model is built in RTDS. The model developed using FPGA technology provides an effective way to research control and protection characteristics for the large capacity

high voltage and long distance flexible DC system. And it provides an effective real-time simulation test system and important simulation technology support. This work is supported by China Southern Power Grid Company Limited(No. CSGTRC-K142031).