Modular Isolated LLC DC/DC Conversion System for Offshore Wind Farm Collection and Integration

Compared with conventional offshore wind farm with ac collection and high-voltage dc (HVDC) transmission system, the offshore wind farm with all dc system shows advantages with characteristics of higher power density, lower costs, and higher efficiency in many analyses. This letter proposed a novel all dc system for the offshore wind farm collection and integration. The presented dc system is composed of several medium-voltage (MV) dc collection systems and a dc transmission system. A novel modular isolated dc/dc converter (MIDC) topology is proposed for the MV dc collection system to raise the medium dc voltage to a higher level, while realizing the maximum power point tracking (MPPT) of the connected wind generators. Thorough comparisons of three dc/dc converter candidates are carried out, and the LLC resonant converter is found to be the optimal submodule (SM) of the MIDC. Furthermore, a two-layer control system for the MIDC is developed to obtain current and voltage sharing while achieving the MPPT of the wind generators in the meantime. A time-domain simulation model consisting of 160 SMs is developed using PSCAD, and a prototype with nine SMs is built for the experiments. The feasibility of the proposed pure dc system is verified by both the simulation and experimental results.