The electrification . is an ongoing technical paradigm shift in the auto industry. In this change, wireless power transmission (WPT) for electrical vehicle (EV) charging is a new enabling feature to wirelessly transfer power. However, its potential impact on other systems has not been investigated in depth. As aggravating circumstance, the frequencies for WPT-EV are also used by radio communication systems or services. Since WPT-EV is a new technology, there is a lot of work going on in the world in evolving the technology itself, but also in investigating interference in other systems and setting emission limits. There is, for example, a working group within the ITU-R that is examining the possible impact of WPT-EV on the radio communication services operating in the same or adjacent frequencies. There are several proposed frequency bands for WPT-EV with different characteristics.
The automotive industry is undergoing substantial changes in drive train technology . In 2017, for the first time, more than 1000,000 new electric vehicles (EVs) were registered. More than 60% of these EVs were registered in China. Different studies forecast that in ten years, 20% of all newly sold cars will be EVs and in twenty years, around 25% [1]. In this development, new types of charging is proposed, and one of the new forms of charging is wireless power transmission (WPT).
Technologies to transmit electric power wirelessly have been developed since the 19th century, beginning from induction technology. Since the Massachusetts Institute of Technology innovation on non-beam wireless power technology in 2006, technologies of WPT under development vary widely; e.g. transmission via radio-frequency beam, magnetic field induction, resonant transmission, etc. WPT applications are expanding to mobile and portable devices, home appliances and office equipment, and electric vehicles. Nowadays, resonant WPT technologies are coming out to the consumer market. The automotive industry looks at WPT for EV applications in the upcoming future.
In the forecast [2], it is expected that the wireless charging for the electric vehicle market will grow at a compound annual growth rate (CAGR) of 46.8% from USD 16 million in 2020 to USD 234 million by 2027. In particular, the European region is expected to hold the largest market share, much due to availability of well-developed infrastructure enables the incorporation of wireless charging infrastructure in this region. The forecast also states that the implementation of stringent emission norms, increasing focus on R&D activities, and rapid technological changes will drive the wireless charging for electric vehicle market. Stationary charging is projected to be the largest contributor in the wireless charging for electric vehicle market, by charging type, during the same time period.
A substantial growth of the future use of WPT is also presented in [3], shown in Table 1. The growth of WPT the coming years between 2021 to 2025 will be 15 % and will increase even more over the years 2026 to 2030.
Since the charging powers can range from a few kW up to the order of 200 kW, the risks for radio interference must be carefully investigated not to harm wireless services. In order to examine this possible impact of WPT-EV applications on existent radio communication services operating in the same or adjacent frequencies, WRC-15 agreed that ITU-R should study this impact via its Resolution 958 (WRC- 15) as one of the urgent studies required in preparation for the 2019 World Radiocommunication Conference (WRC-19).
Studies of coexistence between WPT and radio services have started within the International Telecommunication Union (ITU).
There are a number of standards regulating different aspects of wireless charging. A recent standard is SS-EN ISO 19363:2021, regulating different aspects of safety and performance of WPT-EV systems. For emissions, other standards often refer to CISPR 11 that includes limits on radiated magnetic and electric fields. In CISPR 11 equipment is classified in two groups, where group 2 includes equipment that generate RF energy intentionally. For frequencies below 150 kHz limits are not specified yet. For WPT- EV systems, the operating frequency is below 150 kHz and specifying limits on the radiated EM-field is important to ensure coexistence with other systems.
Brief description of the technique
WPT uses the basic principle from transformer coupling via a magnetic core. A transformer consists of a primary and a secondary side. Each side uses a coil that is wired around a magnetic core. The transformer is electrically fed with a high-frequency power supply at the primary side and the output is used for charging the rechargeable energy storage system (RESS) at the secondary side. In WPT for EV, the transformer is broken up into two pieces, separated by an air gap, see Figure 1 and 2. The primary side is typically located at the ground and the secondary side is located in the vehicle. The vehicle is positioned over the primary side so that the two sides are electromagnetically connected. Special circuitry is used to match the two sides so that resonance is achieved during the charging process.
With this solution, an efficiency factor in the order of 80 – 95% is possible to achieve [3], depending on frequency band used. Thus, about 5-20% of the power fed to the primary side will be lost as e.g. heat and radiated power. It is therefore of high importance that the resonance is achieved accurately during charging, not to increase the unintentional radiated electromagnetic interference. The resonance peak is typically narrow with respect to frequency variation. Other physical variations can also affect the electromagnetic properties of the coupling. It is therefore of high importance that this resonance can be monitored and adjusted if needed. As a comparison, the efficiency factor of a transformer coupling without this air gap could be in the order of 99%. Thus, breaking up the circuit and introducing the air gap leads to large losses. These losses can be reduced, but not fully eliminated. This fact also means that WPT is not an optimal technology with respect to power efficiency in general, compared to if e.g. a physical wired connection is used. The minimum power transfer efficiency must be greater than 85 % to fulfill ISO 19363:2020, for center alignment of the system.
Technical parameters and frequency bands
Technical parameters and frequency bands
In Table 2, proposed power levels and frequency bands are shown for different kinds of applications [3]. The values are not decided for these, and other proposals exist. It is mainly in the 79-90 kHz band where the development is ongoing and where there are available products in the market.
Radiated emission levels from WPT
Emission limits regulate the maximum values of an electromagnetic field at a certain distance (or at several distances) at different frequencies. For WPT systems there are several standards that may apply. A survey of three different standards and regulations of WPT for EVs charging can be found in [4], comparing IEC 61980–1, SAE J2954 RP and ISO/PAS 19363:2017 and concluding that the regulations have many common points, but the limits on EMC differ. IEC (International Electrotechnical Commission) prepare and publicise of international standards for all electrical, electronic and related technologies. SAE International, previously known as the Society of Automotive Engineers, is a U.S.-based association that develop standards. ISO (International Organization for Standardization) is composed of representatives from various national standards organizations that sets international standards.
Of those three regulations, only SAE J2954 RP is currently having a limit for frequencies below 150 kHz stating that in the operating frequency range 79–90 kHz the field limit peaks up to 82.8 dBμA/m.
Besides the specific standards for wireless power transfer systems, there are standards for short-range devices (SRD) including inductive loop systems. There is also an ongoing work within CISPR with a standard for different classes of WPT systems for vehicles.
Within Europe, there are two ETSI harmonized European standards that treat WPT-systems:
- one for wireless power transmission systems (ETSI EN 303 417) and
- one for short-range devices including induc-tive loop systems (ETSI EN 300 330).
Both standards treat general systems for wireless power transfer systems used also for other purposes than vehicle charging. Hence, the limits are probably going to be revised to include high-power WPT systems. Limits for spurious emissions from WPT-systems are specified in the two ETSI harmonized European standards described earlier [2,3] from 9 kHz and above. However, it is possible that these limits will be adjusted for WPT-EV applications and permit higher values.
Within CISPR, there is an ongoing work to develop radiated emission limits for WPT-EV. The work with the emission limits is currently not finalized. There are proposed limits that are used for coexistence analysis within the ITU. In CISPR, the limits are expressed for Class A and for Class B systems depending on intended use and power. For Class B systems is the limit for medium power WPT-systems 72 dBμA/m at 20 kHz and 67.8 dBμA/m at 85 kHz measured at 10 m distance. For Class A systems, the limits for high power WPT-systems are 107 dBμA/m at 20 kHz and 102.8 dBμA/m at 85 kHz.
Impact of WPT interference
To exemplify how the interference level might increase due to the radiated electromagnetic (EM) fields from WPT, the received interference level from the WPT source is here compared to the interference level caused by atmospheric noise. The reason for the comparison with atmospheric noise is that the atmospheric noise constitutes a substantial part of the noise always present at the receiver at lower frequencies and therefore creates a lowest level of what a radio system may receive. By analysing at what distances a WPT station may be located and still exceed the always present atmospheric noise can hence be used as a measure of how devastating the WPT might interfere. Consequently, the longer distance from a receiving antenna the WPT need to be at, the worse interference it is.
The analysis is performed at two frequencies: 20 kHz, which is the frequency band proposed for heavy duty vehicles, and 85 kHz, which is proposed mainly for passenger vehicles. At these frequencies, it is assumed that the WPT installation exactly fulfils either the CISPR 11 standard for Class A or Class B equipment.
The emission standards have a limit on the magnetic field strength at 10 metres distance from the WPT system. For a frequency of 20 kHz, the highest limit is 107 dBμA/m for Class A equipment and 72 dBμA/m for Class B equipment, according to the proposed CISPR 11 standard.
Analysis of the propagation of the EM field radiated from WPT is somewhat complicated by the fact that its power is dominated at lower frequencies and hence the analysis may need to be done in the near field region of the WPT source. The interference level is converted to other distances by assuming that the attenuation of the electrical field in the near field is assumed to be 40 dB/dec. In the far field region, the attenuation is assumed to be 20 dB/dec up to distances longer than about 100 km, where the attenuation is larger.
The atmospheric noise is described in an ITU recommendation on radio noise as a noise factor in terms of minimum and maximum values exceeded with a probability of 99.5% and 0.5% [5]. In this analysis, the emissions from WPT-systems are compared to those two levels of the atmospheric noise. From the noise factor, the electric field strength in a receiving antenna is calculated for the measurement bandwidth in the standards (200 Hz for frequencies from 9 kHz to 150 kHz).
Figure 3 shows the electric field strength as a function of distance from the WPT for the frequency 20 kHz and for the maximum limits from the proposed CISPR 11 standard. Furthermore, the atmospheric noise levels are shown as the minimum value (red line, the level that is exceed with a probability of 99.5%) and maximum value (black line, the level that is exceed with a probability of 0.5%).
The figure shows that an operating WPT station emitting 72 dBuA/m (class A) needs to be located at a distance larger than 27 m from a potential receiving antenna to not exceed the 0.05%-level of the atmospheric noise. To exceed the 99.5%-level of the atmospheric noise, which is lower than the 0.05%-level, the distance needs to be over 250 m. If the operating WPT station is a class B equipment and emit 107 dBuA/m, the distances need to increase to still be in level of the atmospheric noise. It is clear that the WPT system may impact radio systems at quite long distances. Furthermore, the figure also shows the change of attenuation model for the near- and far field at a distance of approximatively 2 km.
Figure 4 shows the corresponding results for 85 kHz. The emission limits are lower at 85 kHz than for 20 kHz, but at the same time the atmospheric noise (0.5 % and 99.5 %) is also lower at higher frequencies. In addition, the near field limit is also shorter for higher frequencies. The interference from an operating WPT station exceeds the 0.05- and the 0.95% levels of the atmospheric noise for distances up to about 80 m and 15 km for Class B and 600 m and 300 km for Class A, respectively.
Conclusions
Wireless power transmission (WPT) for electrical vehicle (EV) charging is a new and enabling feature for the development of the electrification of the automotive market. For this purpose, new standards are under development to include WPT-EV systems, although some are not yet finalized. The fact that WPT transfer high power, there is also a risk that the electromagnetic emission from WPT stations in operation may interference with radio communication services, especially at lower frequencies. From the analysis and results shown here, it is clear that a WPT station that emits in line with the CISPR 11 requirements may increase the interference levels at a receiving antenna at quite large distances.
Sara Linder, Karina Fors and Kia Wiklundh Swedish Defence Research Agency (FOI)