## 1 Introduction  
In recent years, air pollution caused by traditional internal combustion engine (ICE) vehicles has been a major concern in many countries. The new energy automobile industry has been rapidly developing, and wireless power transmission for electric vehicles (EV) has also received much attention. Compared with traditional conductive charging, wireless power transmission can greatly improve the comfort and safety of the charging process. It can be highly integrated with smart electric vehicles and deliver a better experience for passengers. Generally, for EV charging using near field, wireless power transmission can be divided into inductive wireless charging, magnetic resonance coupling and permanent magnet coupling [1]. Among them, magnetic resonance wireless charging becomes the mainstream alternative due to the advantages of high transmission power, long transmission distance and high efficiency. The wireless charging system (WCS) transmits energy through electromagnetic field coupling. When transmission is engaged, the coupling coils generate a powerful magnetic or electric field, which may give rise to an amount of leaked electromagnetic field around the vehicle. The leaked field strength may not be neglected, and it has attracted the attention of some scholars. For magnet coupling charging, studies about magnetic fields mainly focus on two aspects. One is to study the magnetic field distribution of WCS working on normal conditions. When the coupling coils are aligned, characteristics of electromagnetic fields distribution around the coupling coils are studied [2; 3; 4]. Magnetic field under different charging modes like the constant current and constant voltage charging operations are compared [5]. Various types of coupling coils can also influence the magnetic field distribution of WCS [6] and optimizing the structure of coupling coils can suppress the leaked magnetic field to some extent [7]. In order to diminish the leaked magnetic field, some effective shielding methods have been proposed. The shielding methods can be divided into magnetic shielding [8], conductive shielding [9], active shielding and resonant shielding [10]. Some researchers have been optimizing the structure of compensation topology to improve the field, such as the combined topology of LCL and CL resonant networks [11] and multi-stage non-resonant topology [12]. The magnetic field generated by WCS may also have an impact on the human body. MRI (magnetic resonance imaging)-based whole-body voxel models [13] or other high precision homogeneous body models [14; 15] have been applied to these studies. On the other hand, in the charging process of an EV equipped with a WCS, the lateral offset of coils due to improper parking position and the deviation angle caused by the vibration is difficult to avoid, so considering the influence of offsets towards the magnetic field is meaningful. For the offset condition, some scholars have focused on improving the characteristics of WCS under different offsets [16], while others researched the electromagnetic field near the human body and compared it with the safety limits in the ICNIRP1998 (International Commission for Non-Ionizing Radiation Protection) and ICNIRP2010 guidelines [17]. In addition, some research has been carried out on the distribution of magnetic fields for various powers and offset distances. The electromagnetic field distribution of 18 kW rectangular coils under 75 mm longitudinal offset and 120 mm lateral offset is studied [18]. The magnetic field distribution of the human body model near the coupling coils under offset is also described. The variation of the magnetic field distribution of the 7.7 kW circular coils at 250 mm offset can induce an increase in the output voltage after offset [19]. The magnetic field safety region of the 22 kW circular coils with offsets of 100 mm and 200 mm is analyzed [20]. These studies analyze the characteristics of WCS by establishing 3D models.