**1. Introduction**

The promotion of electric vehicles [1] (EVs) is key to the realization of sustainable transportation. Initially, the main power transfer technologies involved battery exchange and wired charging. While battery exchange is operationally convenient, battery storage safety is still a major challenge. Wired charging, although avoiding storage safety issues, also brings some inconveniences to the utilization of EVs, such as aging wires, the risk of electric shock, and poor contact [2,3]. For this purpose, new technologies to improve comfort and safety are being investigated, of which wireless power transfer (WPT) technology is certainly one of the most appealing [4–7].

However, one of the crucial challenges of WPT systems when applied to EVs relates to the electromagnetic field (EMF) safety issues that can be caused by human exposure to severe EMFs [8–10]. During the transmission of power from the transmitting coil to the receiving coil, a portion of the EMF is radiated around the WPT system, called EMF leakage. The generation of EMF leakage not only has an impact on the devices and parts around the WPT system. It also jeopardizes the health and safety of human beings by generating current and heat inside the human body, which can cause irritation to muscles, nerves, tissues, and organs [11–13]. Therefore, the electromagnetic safety issue become an essential and critical point in the design process of WPT systems, which must ensure that the EMF leakage levels comply with the International Commission on Non-Ionizing Radiation Protection (ICNIRP) standards and guidelines [14].

Many studies have been conducted in the past few years on electromagnetic shielding technology to reduce EMF leakage from WPT systems [15–18]. Currently, there are three main shielding measures to reduce EMF leakage: passive shielding, resonant reactive current loop, and active shielding [19,20]. Features of these shielding technologies are shown in Appendix B. The present study focuses on passive shielding technology [21,22]. This suppresses electromagnetic radiation by using metallic materials to generate an EMF in the opposite direction to the one generated by the coupling coil in the form of eddy currents [23].

**Citation:** Li, Y.; Zhang, S.; Cheng, Z. Double-Coil Dynamic Shielding Technology for Wireless Power Transmission in Electric Vehicles. *Energies* **2021**, *14*, 5271. https:// doi.org/10.3390/en14175271

Academic Editor: Adel El-Shahat

Received: 26 July 2021 Accepted: 18 August 2021 Published: 25 August 2021

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Despite its simplicity of operation and ease of implementation, this technology has obvious drawbacks. The use of metallic materials leads to an increase in the weight of the system, reducing the coupling coefficient and also increasing the losses [24]. Therefore, an improved passive shielding technology, resonant reactive current loop, has been proposed [25]. Its operation is based on the principle that, by placing a closed-loop coil with matching capacitance around the transmitting and receiving coils, respectively, a canceling EMF opposite to the incident field is generated to significantly reduce the EMF leakage of the WPT system. Resonant reactive current loop overcomes the shortcomings of traditional passive shielding and achieves good shielding with a small additional volume and less impact on the power transmission efficiency.

Although the method is simple in structure, the shielding coil achieves only 53% shielding effectiveness against EMF leakage, owing to the fact that the power of the shielding coil is derived from the induced EMF, yielding a limited cancellation of EMF [26]. For cases with higher shielding requirements, active shielding technology [27,28] has obvious advantages over the two technologies mentioned above. The principle of active shielding technology is to eliminate EMF leakage by generating a canceling EMF with a vector direction opposite to the incident EMF. In this technology, the active shielding coil arranged at the periphery of the transmitting coil is provided with an independent power supply, and satisfactory shielding effectiveness is achieved by adjusting the power supply.

In previous studies, a single active shielding coil around the transmitting coil was commonly taken into consideration. Therefore, the radius of the active coil should be greater than the radius of the transmitting coil [29]. With regard to the current strength and phase magnitude, the current strength should be selected based on the EMF calculation of the WPT system, while the phase should be opposite to the phase of the transmitting coil current. Therefore, the active coil generates a canceling EMF in the opposite direction of the EMF of the transmitting coil, which leads to a weakening of the EMF leakage [30]. However, the total EMF received by the receiving coil is the sum of the EMFs of the active shielding coil and transmitting coil. The algebraic sum of the two EMFs in opposite directions is significantly lower than the EMF when only the transmitting coil is present. Therefore, the addition of the active shielding coil negatively affects the transmission performance of the WPT system.

To minimize the degradation in transmission performance from the active shielding coil, multiple active coils can be considered. In this case, the geometry of each active coil, its placement, and parameters such as current strength and phase need to be discussed in more depth.

In addition, in previous WPT systems for EVs, the shielding function is usually limited to static shielding, i.e., discussing the shielding effectiveness at a fixed transmission distance [31,32]. However, for different EVs, the distance between the chassis of vehicles and the ground transmitting coil is not consistent. As a result, the power transmission distance during the charging process is inconsistent, resulting in varying EMF leakage levels.

In order to improve the wide applicability of shielding technology, this paper proposes an improved active shielding technology—the double-coil dynamic shielding scheme—for different transmission distances of EVs. Firstly, the active shielding structure with double shielding coils is discussed. Secondly, a modeling analysis of the improved, dynamically shielding WPT system is performed. Finally, simulations and experiments are performed on the proposed shielding structure to verify the effectiveness of the double-coil dynamic shielding scheme.
