Offshore low frequency AC transmission with back-to-back modular multilevel converter (MMC)

This paper evaluates the use of a back-to-back modular multilevel converter (B2B-MMC) in offshore low frequency (LF) AC transmission. The scheme is compared to two alternative transmission technologies: DC and 50Hz AC. The MMC topology is further compared to a cycloconverter. A high level design of a ±120kV_dc, 250MW B2B-MMC system is described, with the total volume and weight of submodule capacitors estimated. The system performance is studied for both normal and fault conditions by computer simulation. Analysis shows that the dynamics of the controller that regulates the DC bus voltage may not be fast enough for the B2B-MMC, and feedforward control may be needed.