Design and Loss Analysis of Loosely Coupled Transformer for an Underwater High-Power Inductive Power Transfer System

To prevent magnetic flux leakage to the maximal extent, increase the coupling coefficient of high-power inductive power transfer (IPT) system, and reduce the electromagnetic radiation in water medium, a novel underwater loosely coupled transformer (LCT) based on the semiclosed magnetic core structure is proposed in this paper. Power loss of the devised LCT is investigated comparatively in three media including air, freshwater, and seawater. It is shown that winding loss and core loss in the three media are basically identical. However, additional eddy current (AEC) loss in various water media is different for the sake of the difference in conductivity. In particular, AEC loss is extremely low in freshwater and relatively significant in seawater. Furthermore, when the operating frequency exceeds a certain value, AEC loss will be greater than core loss and winding loss, and become the major factor in restricting transmission efficiency of LCT in seawater. Finally, theoretical analysis and simulation are demonstrated by the underwater experiment on an IPT prototype system, which can transfer 10 kW at 91% maximal transmission efficiency and over a 25 mm air gap.