A Review on Interoperability of Wireless Charging Systems for Electric Vehicles

: Wireless charging technology has become an important development trend in future electric vehicle (EV) power supply technology due to its safety, ﬂexibility, and convenience. With its indus-trialized development, interoperability has become an essential technical link. The interoperability of wireless charging systems refers to the ability of output performance to meet speciﬁed indicators when different transmitter and receiver devices are matched. This study reviews the research status of the interoperability of EV wireless charging technology. First, the deﬁnition and composition of the interoperability of wireless charging systems are brieﬂy given. Then, the article provides a review of standards about interoperability so far. The interoperability of various magnetic couplers and compensation topologies is also analyzed and summarized. After that, the paper reviews the existing interoperability evaluation methods. Finally, this work highlights the existing problems and challenges in current research of interoperability. We hope that this work will contribute to the further development of EV wireless charging technology.


Introduction
The destruction of the natural environment and damage to human health caused by greenhouse gases, toxic smoke, and dust due to the use of fossil energy is increasingly significant.There is an urgent need to replace traditional fuel vehicles with electric vehicles charged using renewable energy sources [1][2][3].However, the speed of development of the charging infrastructure is unable to keep pace with the increasing demand for EVs.Wireless power transmission (WPT) has therefore gradually become an important development trend due to its greater flexibility, convenience, safety, and intelligence compared with traditional contact charging [4][5][6][7][8][9].In low-power fields, wireless charging technology is already widely used in mobile phones [10,11], wearable equipment [12,13], implantable medical devices [14,15], and smart home products [16,17].However, wireless charging systems can also play an irreplaceable role in high-power scenarios such as electric vehicles [18][19][20][21][22][23], unmanned aerial vehicles (UAV) [24][25][26][27], unmanned underwater vehicles (UUV) [28][29][30][31], electric ships [32], and aerospace devices [33].Especially in the field of EVs, wireless charging technology will be an important part of intelligent transportation due to its unmanned and automatic charging [34][35][36].
Wireless charging has a more complex system structure than contact charging due to its need for more electric energy converters.A wireless charging system usually includes a power factor correction (PFC), inverter, primary topology, transmitting coil, receiving coil, secondary topology, rectifier, and load [37,38], as shown in Figure 1.In providing these components, different suppliers may adopt different technical routes [39,40].Even with the same technological path, fabrication tolerances and component parameters may still lead to different charging characteristics.Therefore, the charging capabilities between transmitters and receivers of different suppliers has becomes a critical research issue [41,42] for several reasons.First, in order to regulate charging equipment, relevant wireless charging standards have been proposed, which specify charging indicators and introduce the concept of interoperability.The interoperability of wireless charging systems refers to the ability of output performance to meet specified indicators when different transmitter and receiver devices are matched.Failure to meet the specified indicators means that the corresponding transmitter and receiver cannot interoperate.Poor interoperability will lead to low charging efficiency during product matching, resulting in a significant waste of power or even failure to charge.Especially in public places, the lack of interoperability will make many vehicles unable to use the charging device, resulting in severe hindrances in developing the EV wireless charging industry.Therefore, interoperability between products has become a critical factor for the industry [43], which needs to be ensured through excellent standards.

Secondary coil
Second, the interoperability problems center on the differences between the technical routes of the primary and secondary sides.Therefore, analyzing the interoperability between different technical routes and providing improvement schemes is necessary.Significant differences in technical routes are embodied in communication, magnetic couplers, and compensation networks, as shown in Figure 2. Communication interoperability is determined by communication mode and protocol [44][45][46].Relevant research and technical solutions have been approaching maturity.Therefore, the interoperability of different magnetic couplers and compensation networks is the focus of this paper.Third, the interoperability evaluation method is also essential in interoperability research.The interoperability evaluation method tests and judges the interoperability between two devices.It will help further improve the tested equipment.An excellent interoperability evaluation method can better promote the interconnection between different products.

Interoperability of magnetic couplers
Considering the above three aspects, this paper reviews the research status of the interoperability of EV wireless charging technology in terms of standards, interoperability analysis, and interoperability evaluation.The rest of the sections are organized as follows.A review of standards regarding interoperability is presented in Section 2. Section 3 analyzes and summarizes the interoperability of various magnetic couplers and compensation topologies.A review of the existing interoperability evaluation methods is summarized in Section 4. Section 5 highlights the prospects and challenges for wireless charging system interoperability.

Process of Interoperability Standardization
Standards are an important way to ensure interoperability.When formulating standard schemes and requirements, interoperability must be considered.It is necessary to ensure that the recommended schemes are interoperable with mainstream schemes and to restrict some products from entering the industry to improve the overall level.
The process of interoperability standardization is shown in Figure 3.In 2013, the International Organization for Standardization (ISO), one of the standardization organizations, first addressed the communication problems of EV wireless chargers in ISO 15118 [47].Then, the Society of Automotive Engineers (SAE) started to set standards for EV wireless charging.In 2016, the first version of recommended standards for EV wireless charging technology by SAE was published, numbered SAE J2954.Thus far, SAE J2954 has been revised four times, the most recent in 2020.SAE J2954 provides industry specifications for wireless charging of light-duty and plug-in electric vehicles, including minimum performance, safety, electromagnetic fields, electromagnetic compatibility, interoperability, and testing.It divides the system power level into WPT1~WPT3 (Maximum input 3.7, 7.7 and 11.1 kVA) and the transmission distance into Z1~Z3 (VA Coil Ground Clearance Range 100-150, 140-210 and 170-250 mm) [48].SAE J2954 specifically stipulates the interoperability of EV wireless charging in Chapter 8 for the first time.Meanwhile, the interoperability requirements of different power and Z classes are set, as demonstrated in Tables 1 and 2.
--  Another major organization setting standards for wireless charging is the International Electro-technical Commission (IEC).In 2016, IEC 61980 was issued and it consists of three parts: general requirements in 61980-1, special requirements for system activities and communication in 61980-2, and special requirements for the off-board side in 61980-3 [49].IEC 61980-3 also specifies interoperability requirements for off-board power transmission in Chapter 8, which is similar to SAE J2954.Requirements for on-board wireless charging equipment are specified by ISO 19363 issued in 2017 [50].This specifies the safety and interoperability requirements of electric vehicle wireless charging.
In 2020, China began issuing national standards on EV wireless charging systems, numbered GB/T 38775.GB/T 38775.6 and GB/T 38775.7 specify ground and on-board equipment interoperability requirements [51,52].Thus far, Parts I-VII has been released, including general requirements, communication protocols, electromagnetic compatibility, interoperability, etc.China's national standards to date are generally consistent with international standards on general requirements.However, in terms of the recommended technical solutions, the Chinese national standard differs from the international standard in the structure and parameters of the coil and topology.For example, China's national standard recommends a topology of double-sided LCC while the international standard SAE J2954 recommends a tunable matching network, which will be described in Section 3.
In 2022, the China Power Supply Society issued a standard for high-power wireless charging (≥WPT4) drafted by Harbin Institute of Technology, numbered T/CPSS 1001-2022 [53].It specifies communication requirements, safety requirements, auxiliary function requirements, etc.

Interoperability of Different Coils
Magnetic couplers play the most critical role in wireless chargers.The magnetic coupler is mainly composed of a coil, magnetic core, and shielding plate, as shown in Figure 4.The coil structure is a key factor in determining interoperability.There are three basic coil types: circular, rectangular, and Double D (DD).Their interoperability is of great significance.Coil interoperability is usually based on two aspects: the magnetic flux distribution on the physical grade and the coupling coefficient or mutual inductance on the numerical grade.For the interoperability of circular and rectangular coils, the research of [54] shows that when circular and rectangular coils with the same outer diameter are used as the transmitting coil, the mutual inductance M varies with the offset in an equal trend.As the transmitter, the rectangular coil can provide a higher coupling coefficient and transmission efficiency, proving the excellent interoperability between the circular and rectangular coils.Other authors [55] studied the coupling characteristics of square and circular coils.The mutual-inductance characteristics of the square transmitting coil and circular receiving coil (interoperability of square and circular) at various vertical distances are described by using simulations.The results show that the square-circular coupler provides better mutual inductance and efficiency than the circular-circular coupler.

Primary litz wire
Researchers at the University of Michigan, USA, researched the interoperability of circular and DD coupling coils.They pointed out that the coupling zero point appeared in the y direction (the lateral direction of vehicles' driving direction), and the interoperability increased rapidly with the increase of the y-direction offset [56].It was also found that there was a significant difference in the coupling coefficient between the circular and the DD type when they are used as the transmitter and receiver.The coupling coefficient between the DD transmitter and circular receiver decreased by about 50% when the maximum offset was in the y direction.In contrast, that between the circular transmitter and DD receiver decreased by less than 25% at the same offset, indicating better interoperability of the latter combination.
The interoperability of rectangular and DD coils is affected by the relative position of coils, as shown in Figure 5.The interoperability test conducted by the University of Stuttgart for the rectangular and DD (specified in SAE J2954) shows a better performance of the rectangular coil as a transmitter than the DD type.The decline rate of the coupling coefficient decreases by more than 50% with misalignment [57].With the aid of finite element analysis, the magnetic field emission during the operation of rectangular and DD coils was studied by Oak Ridge National Laboratory [58].The results suggested that the DD transmitter needed magnetic shielding, while the rectangular transmitter needed aluminum shielding.However, the previous research was based on finite element simulation or prototype testing.It is difficult to summarize the general interoperability rules under arbitrary parameters.To deal with this problem, Harbin Institute of Technology proposed a method based on fitting magnetic flux distribution, which converts the coupling coefficient into a normalized function related to size.Then the changed rules of interoperability under different parameters can be obtained [60].
Some special coil structures are proposed to improve the basic coil shortage, which usually has an excellent performance in interoperability with the traditional coil.The magnetic coupling of DD and circular coil combinations without an offset is very low, resulting in poor interoperation performance.Hence, Auckland University proposed a DD-Quadrate pad (DDQP) [61].They analyzed the system output when the DDQP was used as the receiver to interoperate with the circular and DD.Due to the addition of orthogonal coils in DDQP, the coupling zero point in the y direction moved to 70% of the coil length, significantly improving the interoperable range.The situation of the DDQP as the transmitter was studied in [62].The charging range was more than five times larger than that of the basic coil.However, the two orthogonal coils in the DDQP require two additional sets of independent rectification and filtering circuits, which increases the complexity of the receiver.
Furthermore, the University Malaya connected the DD and circular coils in series to form an enhanced DD type [63].Aligarh Muslim University proposed a "4D" coil with a four-coil structure [64].Both eliminated coupling zeros, and the interoperability range was further improved.However, the above structure has dramatically changed the basic coil, and adding a Q coil will bring more power losses.Therefore, compared with the DDQP, the bipolar pad (BPP), which reduces the wire consumption by about 25%, has become a frequently used scheme to improve the interoperability of couplers.Southwest Jiaotong University has studied the interoperability characteristics of the BPP as a transmitter and receiver [65].As the receiving end, the BPP has the same coupling zero point and charging range as the DDQP.As the transmitter, the BPP can control the coil current to adapt to the magnetic flux demand of different receiving coils, so it has the premise of interoperability with different receiving coils.Moreover, [66] proposed a threecoil structure named the Tripolar Pad (TPP).Furthermore, [67] studied the interoperability of the TPP with the circular pad (CP) and the BPP.The results show that the coupling between the TPP-CP and TPP-BPP couplers increases by driving each independent coil with currents at different amplitudes and phases.The structure was further improved in [68].The effective charging of vehicles with varying chassis heights on a charging pile can be achieved.Furthermore, [69,70] proposed a three-phase coil structure and verified its magnetic interoperability with the SAE J2954 WPT3-recommended scheme.The interoperability characteristics of basic and special coil structures are shown in Table 3.The interoperable charging range of different coils is related to coil size, position, transmitter type, and other factors.It is difficult to ensure interoperability only through designing parameters.Although most special coils have expanded the charging range and improved interoperability, they have added more complex structures and lack control strategies to ensure stable power and efficiency.Therefore, there is still a significant gap in research on improving the interoperability of different coils.

Interoperability of Different Topologies
The compensation network plays the role of impedance matching and reactive power compensation.Even if the coils have good interoperability, an unreasonable compensation network will still lead to an impedance mismatch.As a result, the load cannot fully absorb the energy, leading to a decrease in active power received by the load [71].Therefore, the compensation network must be considered when assessing interoperability.
To date, the compensation networks widely used in EV wireless charging systems mainly include series (S), parallel (P), and LCC.The research on interoperability between them is relatively complete.Scholars from the Ontario University of Technology in Canada compared the combination of basic compensation topologies (S and P) in terms of power, efficiency, stability, etc. [72].In the four varieties under voltage source inputs, if S is used at the transmitter, the output power of the two topologies is the same under different loads.In contrast, if P is used at the transmitter, the output power of both topologies at the receiver will decrease significantly.Moreover, only the S-S topology has frequency stability (the load change does not affect the resonant frequency), and the other three combinations will deviate from the resonant state due to the load change.Therefore, S and P do not have good interoperability.
The double-sided LCC (DSLCC) compensation topology proposed by the University of Michigan provides a new solution for improving topology interoperability [73], as shown in Figure 6.Researchers from the Hefei University of Technology compared and analyzed the topology characteristics of S-S and DSLCC.The results show that DSLCC topology is superior to S-S topology in coping with parameter drift, device stress, efficiency stability, etc. [74].The team further studied the interoperability between LCC and S, P [75].The LCC-S, LCC-P, and S-LCC combinations could achieve the same power output under equal coil parameters.P-LCC could also output the same power under specific coupling coefficients, as shown in Table 4.The Chinese Academy of Sciences research also proves from reflected impedance that when the equivalent load of S compensation is close to the optimal load of LCC compensation, the two functions in the system can be considered equivalent [76].In conclusion, DSLCC is easier to interoperate with other topologies than S and P. It has been included in China's national standards.Notably, SAE J2954 recommended a tunable matching network (TMN), as shown in in Figure 7.In this topology, variable inductors and capacitors are used to achieve power regulation and resonance maintenance [48].TMN topology is more flexible than LCC topology.When it interoperates with LCC, TMN as the receiving side shows better interoperability.

Interoperability Evaluation Method 4.1. Evaluation Based on Power Efficiency
When conducting an interoperability test, the test equipment must be at the specified charging position to transmit the rated power with the rated efficiency.In SAE J2954, efficiency is defined as the energy ratio from the grid to the battery, which cannot be less than 85% at the rated working point and 80% at the maximum offset point.Therefore, the evaluation method based on power and efficiency is the most common and intuitive to determine interoperability, which has been widely used in industries.Figure 8 shows the interoperability test of Qualcomm's different products [77].The researchers conducted tests on circular, solenoid, and DD coils.They used the multi-dimensional movable bench shown in Figure 8a to simulate different charging positions.An 85 kHz fixed frequency inverter was used to supply power to the transmission pad and the electronic load was used to simulate the battery on the secondary side.Whether the interoperability requirements are met can then be judged by the efficiency test results of different products, as shown in Figure 8b.The test results are consistent with the theoretical analysis and simulation results.Figure 9 shows the interoperability testing of products by SEW, IPT Technology, and other German enterprises [78].Different enterprises' vehicle assembly (VA) and ground assembly (GA) products were cross-tested, as shown in Figure 9a.The test involved two GA products, one reference device and four VA products, and was completed at a working frequency of 140 kHz.The four VA products were first tested for interoperability with the reference device, and then tested with two GA products.Vehicle A and Vehicle B were equipped with DD coils, while Vehicle C and Vehicle D were equipped with smaller solenoid coils.The efficiency test results shown in Figure 9b intuitively show differences in product interoperability.There is no obvious difference in the efficiency test results of the four vehicles, and they all meet the requirements specified in the standard.Furthermore, Witricity and Qualcomm cooperated with institutions such as the Korea Advanced Institute of Science and Technology (KAIST) to supplement the power factor, electromagnetic exposure level, and human safety tests based on this method, enriching the connotation of interoperability testing [79,80].However, the port's external energy and internal impedance characteristics are very complex in EV wireless charging systems.The power-efficiency evaluation method can only characterize whether the external characteristics meet the requirements but cannot reflect the impact of the internal parameters, nor can it provide a theoretical basis for product optimization design.

Evaluation Based on Coil Parameters
Considering that the power-efficiency method requires the tested system to work at full power output, Momentum Dynamics Corp., USA, explored the evaluation method based on the coil's parameters.The coupling coefficient k and quality factor Q of the magnetic couplers were used to evaluate the behavior of EV wireless charging systems [81].After simplifying the magnetic coupler with the loose coupling transformer model, power and efficiency can be expressed by k and Q, as shown in Figure 10.The evaluation results can be obtained only through weak-current measurement.Compared with the power-efficiency method, the k-Q method does not require full power output and reduces requirements for auxiliary equipment.However, some defects of this method cause it to be rarely used:  I.The k-Q method can only obtain the highest efficiency rather than real-time efficiency, which cannot reflect the impact of other parameters (compensation topology, equivalent load, etc.) on efficiency.
II.The measurement of k and Q is not straightforward.Under the effect of the magnetic core, Q is affected by the excitation current, resulting in a significant error under weak current measurement [82].Therefore, it is not easy to comprehensively evaluate interoperability through k and Q.

Evaluation Based on Port Impedance
Considering the complex testing work and the failure to reflect the effect of magnetic parameters on interoperability, the Karlsruhe Institute of Technology (KIT) and Mercedes Benz first proposed an evaluation method based on port impedance mapping [83].According to impedance mapping theory, the whole circuit on the secondary side is equivalent to an impedance connected in series on the transmitting coil, called the reflected impedance.This can be expressed as: where ̟ is the working angle frequency, M is the mutual inductance of the magnetic couplers, L VA and r VA are self-inductance and internal resistance of the secondary coil, and Z e is the equivalent load considering the compensation network and power converter in the secondary side.On this foundation, they defined VA port impedance Z VA and GA port impedance Z GA as shown in (2) and Figure 11a: where L GA and r GA are self-inductance and internal resistance of the primary coil.Z GA and Z VA contain enough factors used to evaluate the transmission power of magnetic couplers.Therefore, they can be used to evaluate the interoperability of wireless charging systems.The evaluation method based on port impedance can then be established by specifying the standard value regions, as shown in Figure 11b.Figure 12 shows the behavior of port impedance in SAE J2954.The port impedance of the tested product is located in the corresponding region, indicating that the rated power can be output during interoperability.Since port impedance includes self-inductance, mutual inductance, load, and other parameters, the location of impedance value in the complex plane can guide the design of parameters to improve interoperability.However, cross-testing is unavoidable, and the workload of impedance testing is still huge.On this basis, BMW and the Technical University of Munich (TUM) first introduced the concept of "benchmark equipment" [84], which was included in the recommended test scheme by SAE J2954.This method takes the benchmark equipment as the rule and converts the cross-test of VA and GA equipment in the traditional method into the test between VA (GA) and benchmark GA (VA) equipment.On the one hand, this dramatically reduces the test workload caused by too many products under test.On the other hand, the introduced benchmark equipment is conducive to standardization and specification.Further, TUM extended the impedance method to obtain a method for designing systems [85].Operational boundaries were mapped to each interface to establish the interoperable impedance region, helping component design and evaluation.Harbin Institute of Technology defined the relevant characteristic factors based on the original port impedance mapping and divided the impedance standard value region (shown in Figure 13), reducing the misjudgment rate of the original method at the impedance boundary [86].Tsinghua University and Tiangong University also proposed corresponding improvement measures for the port impedance evaluation method [87][88][89][90], effectively expanding the application scope of the port impedance evaluation method, as shown in Figures 14 and 15    However, the impedance angle is difficult to measure accurately at high frequencies.To solve this problem, [91] proposed an impedance measurement method based on power decomposition, as shown in Figure 16.The input power was decomposed to calculate the phase difference, significantly improving the measurement accuracy.Moreover, Ref. [92] also proposed a novel method to measure high-power VA impedance.By selecting several different test ports and parameters to reconstruct the port impedance expression, a measurement method with lower uncertainty was realized.

H1
H1 H2  Recently, Ref. [93] used the port impedance method to evaluate the interoperability of a 50 kW magnetic coupler.They further considered the influence of electromagnetic compatibility and misalignment.Over time, the port impedance method has gradually become the mainstream interoperability evaluation method.

Existing Problems and Expectations
After the review of current research, existing research on interoperability could be further promoted in the following aspects: I. The core criterion of the port impedance interoperability evaluation is whether the measured impedance value is within the reference region.Therefore, the boundary of the impedance reference region is of vital importance.However, the calculated impedance region boundary function is a little different from the actual equipment, due to the vehicle and ground environment, installation height, fluctuation of electrical parameters, and so on.However, the complex influence of the above parameters on the reference region makes it difficult to express by mathematical formulas or obtain by exhaustive methods, resulting in lower accuracy of the evaluation results.Therefore, quantifying the influence of environmental parameters on the boundary and further determining the tolerance region of impedance is necessary.
II. Interoperability between devices of different power frequencies needs to be considered.The study by [94] uses finite element analysis and network circuit analysis to compare two different coil systems (WPT1 and WPT2) regarding interoperability aspects.It is shown that proper tuning of the primary side helps to increase interoperability.However, the operating frequencies of the two systems studied are identical.Due to the management requirements of the radio frequency band, some high-power systems may be required to work in different frequency bands from low-power systems.The International Telecommunication Union (ITU) has proposed two high-power candidate frequency bands (22 kHz (19-25 kHz) and 60 kHz (55-57 kHz and 63-65 kHz)) and a medium-power candidate frequency band (80 kHz (79-90 kHz)) for EV wireless charging devices.In addition, the current regulations of China's State Radio Office also require that wireless charging equipment with rated power over 22 kW should work at 20 kHz while other equipment works at 85 kHz.However, the impedance between devices with different frequencies is difficult to match.It will lead to much reactive power in the charging process, making it difficult to meet the interoperability requirements.Therefore, the interoperability of wireless charging systems with different frequencies needs further research.
III. Batteries of electric vehicles are the largest, lowest-cost, and safest energy storage systems [95,96].With the energy storage demand, the vehicle-to-grid (V2G) charging function of the charging system has become increasingly important [97][98][99][100][101][102].On this foundation, vehicle-to-grid interoperability needs further analysis.In addition, the integration between the conductive charger and the wireless charger also needs to be considered.This could help to improve the compatibility of electric vehicle chargers and reduce the system costs.For example, [103] proposed a novel DC-DC topology, introducing the receiving coil to the on-board conductive charger, as shown in Figure 17.It provides a feasible solution for integration between the conductive and wireless chargers.Meanwhile, this is also an exploration of interoperability between plug-in chargers and wireless charging devices.However, there are still few studies on the integration of plug-in electric vehicle chargers and wireless chargers.The existing research only verifies the feasibility of the technical scheme, but is not mature enough to support industrial application.The integration method of plug-in EV chargers and wireless chargers can be further explored.

Conclusions
Electric vehicle wireless charging is an automatic charging technology and has be-come an important development trend.Research on the interoperability of wireless charging is the key to realizing the interconnection of various technical routes.This paper presents the current research on the interoperability of EV wireless charging technology in standards, interoperability analysis, and interoperability evaluation.
I. A review of standards for interoperability is presented.The current interoperability standards are generally complete, but the interoperability requirements for higher power levels still need to be defined.
II.The interoperability of various magnetic couplers and compensation topologies is analyzed and summarized.The interoperability analysis of compensation topologies has been sufficiently in-depth.However, there is still a significant gap in research on improving the interoperability of different coils and V2G interoperation.
III.A review of the existing interoperability evaluation methods is summarized and the port impedance method has gradually become mainstream.However, the method to determine the tolerance area is still unclear.Further research is needed to determine the appropriate tolerance regions to improve accuracy.
We believe that with the efforts of researchers, the research on interoperability will be further improved to promote the industrial development of EV wireless charging technology.
Author Contributions: Summary of existing problems, K.S.; writing-original draft preparation, Y.L.; review of evaluation based on coil parameters and port impedance, X.Z.; writing-review and editing, J.J.; review of topology interoperability analysis, C.S.; review of coil interoperability analysis, G.Y.; review of interoperability evaluation based on power efficiency, F.Y.; review of relevant standards, H.L. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement:
No new data were created or analyzed in this study.Data sharing is not applicable to this article.

Figure 2 .
Figure 2. Three kinds of interoperability in wireless charging systems.

Figure 8 .
Figure 8. Interoperability testing by Qualcomm based on the power-efficiency method (a) Multidimensional movable bench; (b) System efficiency test results.

Figure 9 .
Figure 9. Interoperability test and results of products by German companies (a) Cross tests between VA and GA; (b) Efficiency test results.

Figure 10 .
Figure 10.Relationship between k-Q and transmission efficiency.

Figure 11 .
Figure 11.Interoperability evaluation method based on port impedance (a) Schematic of port impedance; (b) Z GA standard mapping region.

Figure 12 .
Figure 12.The behavior of the port impedance. .

Figure 13 .
Figure 13.The reference impedance regions based on characteristic factors proposed by Harbin Institute of Technology (a) Region of Z GA ; (b) Region of Z VA .

Figure 14 .
Figure 14.Reference region of inverter port impedance proposed by Tsinghua University.

Figure 15 .
Figure 15.Improved impedance region proposed by the Tiangong University.

Figure 16 .
Figure 16.Power decomposition algorithm for measuring port impedance.

Figure 17 .
Figure 17.The integration between the conductive charger and the wireless charger.

Funding:
This research was funded by [National Natural Science Foundation of China] grant number [52277006, 51977043], [Southern Power Grid Corporation Wireless Power Transmission Joint Laboratory funded by the Opening Foundation Projects], [State Key Laboratory of Smart Grid Protection and Operation Control], Public Open Project of Automobile Standardization of China [CATARC-Z-2022-01330].

Table 1 .
Interoperability requirements of various power classes.

Table 2 .
Interoperability requirements of various Z classes.

Table 3 .
Interoperability of several types of coil structures.

Table 4 .
Interoperability of several types of coil structures.