Wireless power transfer (WPT) technology, also known as contactless power transfer (CPT) technology, refers to electrical energy transformed by an emitting device into other forms of energy, such as electric field, magnetic field, microwave, laser, waves, etc., in the space of the non-contact transmission distance, and then through the receiving device again transferring the other forms of energy into electricity, realizing the wireless transmission of power, and implementing the complete electrical isolation between the power and electric equipment.

Broadly speaking, any system not directly connected to the supply and receiving electric power by contact transmission belongs to the category of wireless power transmission (WPT) technology. According to the implementation of the power transmission mechanism, wireless transmission technology in general can be divided into magnetic field, electric field, microwave, laser, and ultrasonic technology [28].

• Magnetic field technology. Based on the principle of electromagnetic induction coupling, it is a wireless power transmission technology that realizes the wireless transmission of electric energy through the non-contact mode between power acquisition equipment and power supply by integrating modern power electronic energy conversion technology, magnetic field coupling technology and modern control theory [29]. High-frequency alternating current is passed through the transmitting coil, the transmitting coil generates a high-frequency magnetic field, the receiving coil close to the magnetic field induces the electromotive force, and the non-contact transmission of electric energy is realized by inductive coupling. Power transmission from milliwatts to several hundred kilowatts can be realized through resonant capacitor compensation, and the transmission efficiency can reach more than 90%. Today, WPT has been commercially used to charge electric vehicles [30], electronic products [31] and biomedical systems [32,33,34]. The advantages of wireless charging are safety, convenience and reliability, and the charging process is fully automated.
• Electric field technology. Capacitive coupling to tube lighting by Tesla in 1891 was the first public testing of WPT to power a load [35,36]. Capacitive wireless power transfer (CWPT) has been well studied [37] since then. The biggest advantage over the magnetic field technology is that the medium between the metal plates can also be metal. Therefore, it has certain advantages in some cases where electricity is transmitted through metal. The electric field is used for energy transmission, and the transmission distance is short. The transmission medium between plates and a small change of transmission distance will greatly affect the stability of power transmission [38]. Compared with magnetic field technology, the plate voltage is higher, and there is a strong electric field around the plate. Its environmental safety is also a problem to be solved.
• Microwave technology. Wireless power transfer using microwaves has been investigated since the 1950s [39]. Microwave technology [40,41] can transmit hundreds of kilowatts of power within the scale distance of hundreds or even thousands of kilometers. The key problems limiting its application at present are low transmission efficiency and only linear transmission with no other obstacles within the linear range. In recent years, people are increasingly interested in RF energy collection technology, and microwave wireless energy transmission [42] is being actively studied. However, due to the low efficiency of long-distance energy transmission, the feasibility of microwave wireless energy transmission in applications such as AUV charging is still under discussion.
• Ultrasonic technology. This technology has been recently proposed in [43,44,45,46]. Ultrasonic technology uses efficient electro-acoustic energy conversion, transducer and circuit matching, acoustic matching, and acoustic energy convergence to carry out long-distance wireless power transmission [47]. Due to the high frequency and short wavelength of ultrasonic wave, the transmission direction is good. This method does not produce electromagnetic interference, and also is not affected by electromagnetic interference, but the transmission power is small, and needs a certain transmission medium such as air, water.
• Laser technology. Optical power transfer based on laser sources was first introduced for the application of solar power satellites [48]. The laser technology [49] consists of a laser transmitter and a laser-electric energy conversion component. This method has good direction and high energy density. However, the technology is still not mature enough because it requires high precision and low efficiency.

It can be seen that the AUV under seawater can be charged wirelessly with the charging platform at close range, so compared with other wireless power transmission modes, the magnetic field coupling mode has inherent advantages in underwater wireless power transmission. By analyzing the system loss model under seawater conditions, selecting the appropriate topological structure, optimizing the working frequency and selecting a reasonable control mode, the wireless charging demand of AUV under seawater can be met and the ideal use effect can be achieved.

Magnetic field coupled radio energy transmission technology has become a hot topic in recent years. In the field coupled radio energy transmission, there are two main technologies: induction and magnetic coupled resonance. Both of them use the principle of electromagnetic induction and are two different manifestations of the same technology. Inductive wireless power transfer (IWPT) is based on the transformer principle, and its transmission distance is small, only millimeters. Magnetically coupled resonant wireless power transfer (MCR-WPT) systems can achieve high power and efficient transmission of electric energy at both short and medium distances, and are especially suitable for electrical energy supply of underwater vehicles and other electromechanical equipment in marine environments. The basic structure and working principle of underwater MCR-WPT technology are introduced as an example.

The typical topology of an MCR-WPT system is shown in Figure 1. It is mainly composed of a power frequency alternating current, rectifier filtering, high-frequency inverter, primary side compensation, electrical energy transmission coil, electric energy receiving coil, secondary side compensation, rectifier filter, buffer control circuit, and load. After the electrical energy of the submarine base station undergoes high-frequency inversion, the output is sent to the transmitting coil, and under the action of magnetic coupling resonance, the receiving coil and the transmission coil produce coupling resonance, and the received electrical energy can be used for battery charging and other electrical energy replenishment requirements after rectification filtering, so as to realize the contactless transmission of electrical energy from the submarine base station to the underwater vehicle.

A typical MCR-WPT system is a two-coil structure consisting of a power transmission coil and a power receiving coil. Krus et al. [50] proposed a four-coil structure consisting of two resonant coils, one power supply excitation coil connected to the power supply, and one load coil connected to the load. This structure can perform power supply matching and load matching, and realize the isolation of power supply and power transmission coil, and the isolation of load and receiving coil. In [51], a three-coil structure is used in an underwater MCR-WPT system: a resonant relay coil is added to the energy transmission and receiving coils. The transmission distance is effectively improved by adding an auxiliary coil to the coil gap, which is more demanding on the installation and use of the structure, but provides a new idea to solve this problem.

In an MCR-WPT system, the power supply is used to send electrical energy when the frequency of the coil is the resonant frequency of the system; resonance will occur on one side of the sending coil, which will generate a large current in the coil and establish a strong magnetic field. Due to resonance, the electric field energy stored in the capacitor on one side of the sending coil is constantly exchanged with the magnetic field energy in the inductor coil. On one side of the receiving coil, the alternating magnetic field induces current in the receiving coil because the magnetic fields of the receiving and sending induction coils are coupled with each other. When one side of the receiving coil also resonates, the magnetic field energy of the induction coil and the electric field energy in the capacitor constantly exchange energy, thus realizing the wireless transmission of electric energy from the sending end to the receiving end and then to the load. The power transmission principle of the system in the four-coil structure and the three-coil structure is similar.

Since the permeability of seawater and the permeability of vacuum are extremely close, it can be considered that the coupling ability of the transmission coil of the coupled system in the two environments of air and seawater is consistent, but at the same time it should be noted that the conductivity of the air is extremely small, and it can be approximated that the air is not conductive, so there is no eddy current loss problem. When seawater is used as the transmission medium, seawater has good conductivity and large conductivity, and a high-frequency alternating magnetic field generates a vortex electric field in seawater, which in turn produces vortex current and eddy current loss, and part of the energy is absorbed by seawater. Therefore, in order to accurately describe the electromagnetic induction system under the seawater medium, ref. [51] adds an additional equivalent resistance and equivalent capacitance to the air-to-air model as shown in Figure 2, which reflects that the actual seawater medium changes the impedance value of the coupler coil at the air gap and produces additional active power loss. However, at the same time, it is also seen that the seawater medium only changes the numerical size of the parameters in the system, and does not change the transmission mechanism of the original system.