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Abstract The IPT system has been studied for underwater applications such as autonomous underwater vehicles (AUVs) and subsea sensors. However, it rarely comparatively shows the performance of the IPT system in air, freshwater, and seawater. Based on the fore‐mentioned research background, this paper presents a survey of the properties of the IPT system in different mediums. Here, a 100 W power‐level experimental IPT prototype is built and tested. The resonant frequency is set at 300 kHz with a gap range from 10 to 190 mm. The comparison is focused on the efficiency, mutual inductance, coupling coefficient, coil resistance, and quality factor of the IPT system. The IPT system is placed in air, freshwater, and seawater with the same settings. What's more, the magnetic fields of coupling coils in air, freshwater, and seawater are presented in this paper. This paper could be acted as a reference to optimize the IPT system and facilitate future IPT research for underwater applications by analysing the performance of the IPT system in different mediums. The 3D Ansys Maxwell simulation of the IPT system is also given here to study the magnetic fields.

To achieve simple and reliable conversion from constant current (CC) charging mode to constant voltage (CV) charging mode in wireless power transfer systems, this paper proposes a hybrid topology equipped with a reconfigurable rectifier. An AC switch is adapted to simultaneously realize the change from half-bridge rectification to full-bridge rectification and achieve the shift from a topology with capacitors in the series compensated primary and secondary sides to a topology with capacitors in the series compensated primary side and the inductor-capacitor-inductor (LCL) compensated secondary side. Notably, communication between the transmitter and receiver sides is not necessary in the proposed method. Furthermore, the zero phase angle characteristic is successfully maintained in both charging modes. The experimental results obtained using the 48 V/0.7 A experimental prototype, which was built to validate the relevant theoretical analysis, show that output current/voltage can be achieved in CC/CV charging mode independent of load resistance. Notably, the maximum power transfer efficiency of the system during the charging process reached 95.33%.

Underwater wireless power transfer (UWPT) systems are appropriate for battery charging of compact, submerged devices without a complicated and expensive sealing structure or human contact because the power source and load are not physically connected. For the shore-based power supply situation, the underwater power conversion and junction technology should be required to drop down shore-based voltage to the target voltage for the underwater energy supply of the UWPT system. This paper proposes a lightweight, high efficiency and power density underwater power conversion connector system for the UWPT system, in which the LLC resonant converter is constructed with SiC transistors. The full load range zero-voltage switching (ZVS) and load adaptive characteristics have been achieved. The optimized RC level shift driver is adopted to highly reduce the switching loss of SiC transistors. Shore-based voltage of 1000 V was converted to the target voltage of 375 V for the UWPT system. The highest measured efficiency is over 98% at a load power level of 1500 W underwater conditions.

In order to solve the problem of power transmission efficiency reduction resulting from misalignment in the Wireless Power Transfer (WPT) system for Autonomous Underwater Vehicle (AUVs), a novel coupling structure with strong tolerance to misalignment is proposed. A solenoid coil is selected as the transmitting coil, and the receiving coil is composed of dual combined planar coils. The WPT system can still maintain stable output under uncertain axial misalignments for AUVs. The magnetic field distribution of the proposed magnetic coupling structure is analyzed theoretically, and the distance between the coils in the dual combined planar receiving coil is optimized. The theoretical analysis shows that the proposed solenoid-dual combined planar coils coupling structure can effectively maintain a stable mutual inductance between the transmitting coil and receiving coil under different axial misalignments compared with solenoid-unipolar planar coil coupling structure. An S-S resonant compensated WPT experimental prototype is built to verify the output characteristics of the proposed magnetic coupling structure. Compared to the magnetic coupler with the unipolar planar coil, it is validated by experiment that the proposed magnetic coupler substantially enhances the stability of power transmission efficiency and output power when axial misalignment occurs. The power transmission efficiency decreases by 6.74% when axial misalignment increases from 0 to 40 mm in saltwater. Meanwhile, the variation of output power is less than 4.15%.

In this paper, a wireless power transfer (WPT) system with a compact planar magnetic coupler for an autonomous underwater vehicle (AUV) is proposed. A passive induction (PI) coil is integrated into the circular transmitter (Tx) coil to build a uniform magnetic field (UMF), which can guarantee the stable output of the WPT system under uncertain radial and axial misalignments for AUV. Based on normalized magnetic induction intensity distribution analysis, a UMF constructing method with a PI coil is given, aiming to eliminate the fluctuation of magnetic field intensity, and the PI coil design principles and flow chart are obtained. The theoretical analysis shows the proposed integrated coil can effectively enhance the radial misalignment tolerance compared with a conventional circular spiral coil. The zero-phase angle (ZPA) input condition can be achieved by adjusting the series capacitor connected with the Tx coil in S-S compensation topology. Experimental results show that the proposed magnetic coupler containing an integrated coil significantly improves the stability of output power and power transfer efficiency within the possible radial and axial misalignments compared with a conventional coupler. It was demonstrated that the output power changes less than 5.5% and the power transfer efficiency maintains at approximately 84.5% in arbitrary radial positions within the possible working region with an axial transfer distance of 50 mm in saltwater.

In the marine environment, the inductive power transfer (IPT) technology offers many benefits such as increased flexibility and complete rejection of the winding problem. However, for underwater vehicles, the impact of seawater flow vibration will affect the stability of the power transmission. In this paper, a symmetrical double coil structure named 1×1×1 system is proposed to reduce the eddy current loss (ECL) and enhance the power transmission stability of the system under the impact of seawater flow vibration. Firstly, based on the discretization of the electric fields excited by each turn of two transmitting coils, the novel calculation method proposed is that the synthetic electric field at each discrete point is the sum of the electric field vector. Then the synthetic electric field and induced voltage of the 1×1×1 system are simulated by the finite element simulation software COMSOL to verify the calculated results. Finally, experiments are conducted to illustrate that the 1×1×1 system has good ability to resist Z-axial interference and reduce the impact of the seawater flow vibration.

Inductive power transfer (IPT) realizes no physical connection and make it possible to be a substitute for conductive charging in autonomous underwater vehicle (AUV) power feeding applications. However, in marine environment, the seawater stream vibration could bring disturbance and change the primary and secondary parameters, which reduces the stability of the power transmission system. This paper proposes a concentric circular ring structure applied in AUV’s IPT system for resisting the disturbance of ocean current. The study focuses on its misalignment tolerance. It is found that the 3rd coupler (19:24 turns) has better tolerance on axial misalignment among the three couplers with different secondary winding turns. It improves the coupling coefficient reduction from 34.7% to 23.6% when the axial misalignment varies from 0 to 50 mm and the radial displacement almost has no influence on coupling coefficient if radial misalignment is much smaller than the diameter of coils. Experiment results show that the system efficiency drops from 93.2% to 82.4%, when the axial misalignment changes from 0 to 50 mm.

Underwater wireless power transfer (WPT) has gained a lot of popularity in recent years. In the seawater environment, the electrical conductivity is non-negligible compared with that in the air. Therefore, the eddy current loss is generated by the high-frequency alternating currents in the coils. The eddy current loss of the aligned coils has been analyzed while the eddy current loss of the misaligned coils has not been explored yet, which is crucial because misaligned coils are more common in practical applications. In this paper, an analytical model for the eddy current loss of a coreless WPT system in the seawater with misalignments is established with Maxwell’s equations. The theoretical expressions of the electric field intensity and the eddy current loss are derived. Then the eddy current loss is analyzed under different misalignments and frequencies. It is found that the efficiency in the seawater remains stable at a frequency range of 215.5 kHz to 248.4 kHz. Moreover, the efficiency is relatively unchanged under a small lateral misalignment and decreases sharply when the lateral misalignment keeps growing. An underwater WPT prototype is built and the experiments verify the theoretical analysis.