A Modular Multiple DC Transformer Based DC Transmission System for PMSG Based Offshore Wind Farm Integration

In this paper, a modular multiple DC (MMDC) transformer based DC transmission system, which is used for permanent magnetic synchronous generator (PMSG) based offshore wind farm grid connection, is proposed. The MMDC transformer is composed of input-parallel output-series (IPOS) DC/DC submodule groups, each of which consists of several input-series output-series (ISOS) LLC resonant converters (i.e., submodules). In the offshore wind farm, the electrical power generated by PMSG is converted to DC power by diode rectifier. Compared with the existing offshore wind farm grid-connection schemes, the proposed system has advantages of fewer conversion stages, higher efficiency and lower cost. The mathematical relationship of the system is studied, and then one kind of control strategy is proposed. Furthermore, a detailed DC transmission system simulation model containing a 160 submodules (LLC resonant converters) based MMDC transformer is built in PSCAD/EMTDC. Finally, a down-scaled experimental prototype of the DC system, which includes 9 submodules, is presented. Both simulation and experimental results validate the feasibility of the proposed DC system and effectiveness of the control strategy.