Non-interruptible energy transfer algorithm applied to multi-terminal VSC-HVDC with modular multilevel converter

This paper presents a design method to provide failure-tolerant operation of converter in the multi-terminal voltage-source-converter high-voltage direct current system (VSC-HVDC). It might be connected to an offshore wind farm with modular multilevel converter (MMC) topology. To fully implement the multi-terminal VSC-HVDC, three controllers, which are the system operator controller, supervisory controller, and MMC controller, are designed. For the sustainable energy transfer in case that the DC bus voltage control of a converter is failed, the communication-based power balancing algorithm is then proposed. That is, the voltage droop control and the constant flux control in offshore wind farm converter are used for a back-up operation to regulate a constant DC bus voltage. In particular, the wind power reduction via the constant flux control is comprehensively analyzed. To evaluate the performances of proposed algorithm, several case studies are carried out by time-domain simulation based on the power systems computer aided design/electromagnetic transients including DC (PSCAD /EMTDC^®) software. The results show that the offshore wind farm consistently supplies the active power to the multi-terminal VSC-HVDC even when the different terminal in a severe fault condition is to be blocked according to its current regulation.