Design of rotatory transformer for wireless power transfer in the X-rotor wind turbine

Operation and Maintenance (O&M) of offshore wind turbines is, by far, the largest cost element of the levelized cost of energy (LCOE) of an offshore wind farm project [1]. The quest for reducing the LCOE has led to the recent deployment of the EU-funded X-rotor project [2]. The X-rotor is a novel hybrid wind turbine that retains some of the advantages of Vertical-Axis Wind Turbines [3]. A key feature of the X-rotor is the use of secondary rotors, mounted in a X-shaped vertical axis structure.

One requirement in the X-rotor operation is the capacity to transfer the power generated by secondary rotors from a rotating structure to the turbine tower. Given the well-documented impact of slip-rings in offshore wind turbine O&M [4], the necessity of MW-level wireless power transfer (WPT) arises to retain the advantages of the X-rotor system.

This research focuses in the magnetic design and finite element simulation/verification of a MW-level, medium-frequency Rotary Transformer (RT) interfaced by a 3-phase (3-hp) dual-active-bridge (DAB) converter for WPT. The research presents the design of a rotary transformer and the selection of the parameters such as operating frequencies, current density, windings area, air-gap length, and others for the proper functioning of the system. The efficiency is quantified using finite element simulation and numerical calculations. Furthermore, its thermal performance at rated power levels is analysed and quantified using finite element simulation. Furthermore a 3 phase dual active bridge power electronic topology for the rotary transformer is used to manipulate the wireless power flow at the levels of efficiency required by the XROTOR project. The analysis of the topology includes electrical operation and energy conversion losses.

Results indicate that the combined efficiency of the rotary transformer with its associated power electronic converter turned to be 96.53% for a 1MW system.