Series-connected-based Offshore Wind Farms With Full-bridge Modular Multilevel Converter As Grid-and Generator-side Converters

Series-connected-based offshore wind farms can  eliminate bulky central offshore converters, transformers, and platforms. In a series-connected-based offshore wind farm, the DC voltage of the converters should be changed within a large range to control the DC link voltage (grid side) and maximize wind power acquisition (generator side). In this study, the full bridge-  ased modular multilevel converter (FB-MMC) is proposed as grid- and generator-side converters. The DC voltage of the FB-MMC can vary in a large range because the full-bridge submodule (FBSM) is used. The control method of the FB-MMC in a series-connected-based offshore wind farm is different from those of traditional ones. The DC current control and capacitor voltage balance control are proposed in this study, with which the FB-MMC can operate normally in  the series connected offshore wind farm. Moreover, the capacitor ripple voltage in the generator-side FB-MMC is studied. The result shows that the capacitor in the FBSM can be designe  in rated power and frequency, and it will be feasible when the power and frequency decrease. Simulation  results verify the feasibility of the proposed configuration and the corresponding control algorithm.