A New Magnetic-Linked Converter for Grid Integration of Offshore Wind Turbines Through MVDC Transmission

A medium-voltage ac network is commonly used to interconnect wind turbines to the offshore substation, which can create significant power losses, as the distance between wind turbine and offshore substation can vary from 5 to 9 km. The copper-conductor-based high-voltage dc (HVDC) or high-voltage ac (HVAC) transmission lines are generally used to transmit the electrical power from the offshore substations to the onshore grids. With the recent advancement of the high-temperature superconducting (HTS) cables, it is now possible to replace the traditional HVAC/HVDC transmission lines with a medium-voltage dc (MVDC) cables. A new MVDC converter is proposed in this paper to convert the generator low ac output voltage to MVDC directly. The proposed new technology not only eliminates the requirement of low-frequency heavy and large step-up transformers and line filters, but can also eliminate the voltage conversion stages to allow the entire power transmission networks to use MVDC HTS cables. A compact, lightweight, and efficient high-frequency common magnetic link is used in the proposed converter. The magnetic link can inherently solve the isolation problem. In this paper, a scale-down prototype converter is developed in the laboratory to validate the feasibility of the proposed MVDC converter.