The state-of-the-art wireless power transmission technology in electric vehicles

Since the 21st century, people have been paying attention to energy and environmental protection issues, and the world is accelerating the development and promotion of new energy vehicles. At present, the mainstream charging method of new energy vehicles is wired charging, but this charging method has certain safety hazards. So using wireless charging technology can effectively make up for this shortcoming. This paper starts with the development history of wireless charging technology, and expounds the birth and development of wireless charging technology. Then three common wireless charging technologies are introduced. This article will introduce the three wireless charging technologies from their principle, topology and advantages and disadvantages. Secondly, by comparing the three wireless charging technologies, it is concluded that IPT technology is the most mature and widely used technology at present, at the same time, CPT technology is also promising. Finally, the technical challenges and development prospects of wireless charging technology are discussed.


Introduction
As a result of the development of science and technology, the automobile has become the most widely used mode of transport in modern society, vastly improving human production and way of life, and playing a revolutionary role in economic and social development.Since the large-scale production of the automobile industry, crude oil, as the main fuel source for urban transportation, has been produced and widely used around the world, and its high energy density ensures the economy of transportation costs.However, its dominance has decreased dramatically over time due to sharply rising crude oil prices and growing awareness of the negative environmental impact of fossil fuel use [1].In order to solve these problems and promote the development of the transportation system in a more economical and greener direction, developing electric vehicles is gradually becoming an attractive and practical solution.
Electric vehicles are primarily composed of electrical drive system, power system, auxiliary system, control system, safety protection system and so on [2].When electric cars start driving, the power output from the battery is transmitted to a power converter, which in turn transfers it to a motor, whose torque drives the wheels forward or backward through mechanical transmission.Figure 1 shows the basic structure of pure electric vehicles.Currently, the method of charging electric vehicles is mainly wired charging, and electric power is transmitted by charging batteries [3].After several years of rapid development, wired charging technology has matured.Its transmission loss is mainly line loss, so the transmission efficiency is high.However, in actual use, there are many problems with wired charging.One of the main problems is that there are few public charging piles at this stage, it is not easy to find them, and it is not easy to occupy them after they are found, and the charging costs are relatively high.Another problem is that there are potential safety hazards.When using charging piles for charging, electricity leakage is prone to occur.This is a problem that cannot be ignored.Compared with wired charging, wireless charging technology has certain advantages in terms of safety, reliability, flexibility and adaptability, with better user experience.
The current WPT technologies are mainly composed of microwave power transfer (MPT), inductive power transfer (IPT), and capacitive power transfer (CPT) [4], as is shown in Figure 2. Different technologies have their own advantages and disadvantages, and they can be used in different ways.In this paper, the wireless charging technology of IPT, CPT and MPT will be introduced in detail, respectively from the principle, compensation network, advantages and disadvantages, etc.Finally, through the comparative analysis of different wireless charging technology application scenarios and advantages, the future direction of technology development is proposed.

Wireless charging technology
Wireless Power Transfer includes many technologies.As we will examine next, their applications are different.

Inductive wireless power transfer
The basis of inductive wireless charging technology is the principle of electromagnetic induction [1], which is the most common wireless charging technology.When the primary coil is connected to an AC power supply of a certain frequency, due to the principle of electromagnetic induction, the secondary coil will generate an induced voltage, thus transferring energy from the sending end to the receiving end, realizing the wireless transmission of electric energy.As shown in the general diagram of Figure 3. Since there is only inductance in the coupler, even a small amount of current requires a large amount of reactive power to achieve [5].The inverter has to provide all this extra power, which greatly limits the system's ability to transmit actual power.Therefore, compensating capacitors or tuned networks are used to provide reactive power.In order to improve transmission efficiency, it is necessary to take compensation measures for the transmitting and receiving circuits.Here, the resonant principle of LC circuit is used.The resonant topology of wireless charging system can be divided into four types, respectively series-series (SS), series-parallel (SP), parallel-series (PS) and parallel-parallel (PP) [3], as is shown in Figure 4. Four basic compensation topologies are examined.SS and SP are more economical when it comes to high power transmission.Wang and other people introduced the bifurcating phenomenon and studied four basic compensation topologies.They show that the primary capacitance CP depends on charge conditions in the PS and PP compensation topologies, but not in the SS and SP compensation topologies [2].The study of the compensation topology shows that the parallel resonance of the receiving end is suitable for charging the battery.The series resonance of the transmitter can reduce the requirement of the system on the power supply voltage rating.Consequently, SS and SP compensation topologies are better suited to varying load conditions because resonance is guaranteed.
However, this type of wireless charging technology has limitations: due to the limitation of the air gap between the two coils, the transmission distance is relatively short, and the farthest can only be achieved at the centimeter level [6], and the air gap also affects the power transmission efficiency.For the above reasons, this wireless charging method is mostly used in microelectronic products such as mobile phones, tablets, and electric toothbrushes.

Capacitive wireless power transfer
The capacitive WPT is a surface-based electrical field between the front and rear capacitors.The capacitor is located between the emission end and the reception end, and there are two closed plates parallel to each other.When the two plates are close together, an electrical field is generated and an induced current appears at the receiver end.Unlike resonance wireless chargers, capacitive WPT is a potential application choice because it enables efficient energy transfer, even with metal barriers [3].The general structure is shown in Figure 5.The capacitive WPT to charge an EV has many advantages: 1. Eddy current loss may be disregarded.First, in an IPT system, a high-frequency magnetic field creates eddy current losses in neighbouring metals, resulting in significant warming in low-and highpower applications [3].In particular, when a IPT system is used to charge the trolley, the power of the system may reach 10 kW, and the huge magnetic field will generate a lot of heat in nearby metal objects, which may cause a fire and damage life and property.However, in a CPT system, the electrical field is used for transmitting energy and eddy current losses are not an issue.As a result, when metallic materials are to be used, the CPT system can be applied.
2. Low cost and weight.In a CPT system, a capacitive coupling can be constructed with just a few metal plates.Sheet metal material and thickness have little effect on system performance.For example, aluminium can be used to set up a CPT system.2mm thick is enough to provide 2.4kW of power [7].As aluminum foil is much less expensive and lighter than copper Ritz wire, the cost and weight of the system can be significantly reduced.

Microwave wireless power transfer
Microwaves, also known as electromagnetic waves, use radio waves for power transmission, which is another relatively reliable wireless charging method [8].The most important part of this method is the antenna, which converts and transmits energy.After passing through the transmitting device, the electrical energy is converted into electromagnetic waves.After adjusting the angle, the receiving device receives radio waves and converts the electromagnetic waves into electrical energy.This general system is represented in Figure 7.The early ore radios were similar to this principle.They first converted electrical energy into electromagnetic waves, and then received electromagnetic waves and converted them into electrical energy.Although the transmission distance of this method is relatively long, the power and efficiency are relatively low, and the electromagnetic waves generated during the transmission process will have a certain impact on people.Therefore, this wireless charging method has not been widely used in the field of new energy vehicles.
MPT technology has the following limitations for charging electric vehicles: 1.The power conversion efficiency of MPT system is low, only 38% [6].
2. Microwave wireless charging technology has the characteristics of high frequency range and strict requirements for equipment [2].This does not apply to wireless charging of EV.

Comparison of wireless charging technologies
Above, we have elaborated three wireless power transfer technologies from the aspects of principle, topology network and advantages and disadvantages [9].The above three wireless charging technologies are compared, as shown in Table 1 It can be seen from Table 1 that in terms of efficiency and cost, IPT technology is the most studied technology today, and its advantages are indisputable.However, its short transmission distance has become a major factor limiting its development.For CPT technology, low cost and weight are its advantages.But it still has some disadvantages to solve, such as: low efficiency, short transmission distance and so on.MPT technology has the characteristics of high working frequency range and strict requirements on equipment.The more obvious defect is that the energy conversion efficiency of the system is very low.Therefore, although MPT technology has the characteristics of long transmission distance, it is not applicable to the occasion of short transmission such as EV [10].
In general, although the induction technology in the tram wireless charging is becoming mature, but it still needs to be studied and improved in many aspects.Some hybrid chargers have been proposed for greater efficiency.It is believed that in the future, wireless charging technology can be fully utilized in new energy vehicles and play a positive role in the development of society.

Conclusion
For electric vehicles, wireless charging technology is more convenient and safer than wired charging technology.By comparing three commonly used wireless charging technologies (IPT, CPT, MPT), we can see that IPT technology is the most mature technology at present.At the same time, there is room for improvement of CPT technology.Some hybrid technologies are proposed to better promote the development of wireless charging technology.The main problems focus on the cost, charging power and efficiency, transmission distance, these are the current research hotspot and focus.Developing a unified international standard for wireless charging and promoting the cooperation of international research teams can play a positive role in promoting the development of this technology.For the electric vehicle industry, wireless charging technology is of trans-era significance.It can not only save the cost of building charging piles and charging stations, but also bury charging power sources and transformers under the ground, making the charging of electric vehicles faster and safer.It is believed that in the future, with the progress of technology, wireless charging technology can be more widely used in all aspects.

Figure 1 .
Figure 1.The basic structure of pure electric vehicles.Currently, the method of charging electric vehicles is mainly wired charging, and electric power is transmitted by charging batteries[3].After several years of rapid development, wired charging technology has matured.Its transmission loss is mainly line loss, so the transmission efficiency is high.However, in actual use, there are many problems with wired charging.One of the main problems is that there are few public charging piles at this stage, it is not easy to find them, and it is not easy to occupy them after they are found, and the charging costs are relatively high.Another problem is that there are potential safety hazards.When using charging piles for charging, electricity leakage is prone to occur.This is a problem that cannot be ignored.Compared with wired charging, wireless charging technology has certain advantages in terms of safety, reliability, flexibility and adaptability, with better user experience.The current WPT technologies are mainly composed of microwave power transfer (MPT), inductive power transfer (IPT), and capacitive power transfer (CPT)[4], as is shown in Figure2.Different technologies have their own advantages and disadvantages, and they can be used in different ways.

Figure 2 .
Figure 2. Category of wireless power transfer system.

Figure 3 .
Figure 3. General structure of IPT technology.Since there is only inductance in the coupler, even a small amount of current requires a large amount of reactive power to achieve[5].The inverter has to provide all this extra power, which greatly limits the system's ability to transmit actual power.Therefore, compensating capacitors or tuned networks are used to provide reactive power.In order to improve transmission efficiency, it is necessary to take compensation measures for the transmitting and receiving circuits.Here, the resonant principle of LC circuit is used.The resonant topology of wireless charging system can be divided into four types, respectively series-series (SS), series-parallel (SP), parallel-series (PS) and parallel-parallel (PP)[3], as is shown in Figure4.

Figure 4 .
Figure 4.The four basic compensation topologies used in WPT.(a) Series-series.(b) Series-parallel.(c) Parallel-series.(d) Parallel-parallel.Four basic compensation topologies are examined.SS and SP are more economical when it comes to high power transmission.Wang and other people introduced the bifurcating phenomenon and studied

Figure 5 .
Figure 5.General system for capacitive wireless chargers.Figure6shows the schematic diagram of the most popular compensation topology for capacitive WPT.

Figure 6 .
Figure 6.Compensation topologies for capacitive WPT.(a) L Compensation.(b) LC Compensation.(c) LCLC Compensation.The capacitive WPT to charge an EV has many advantages: 1. Eddy current loss may be disregarded.First, in an IPT system, a high-frequency magnetic field creates eddy current losses in neighbouring metals, resulting in significant warming in low-and highpower applications[3].In particular, when a IPT system is used to charge the trolley, the power of the system may reach 10 kW, and the huge magnetic field will generate a lot of heat in nearby metal objects,

Figure 7 .
Figure 7. General system of MPT technology.

Table 1 .
. Comparison of three wireless charging technologies.