Practical and Implementation Issues of Wireless Power Charging of EV’s
By: John M. Miller, JNJ Miller plc
Edited By: Xiaodong Shi
The field of wireless charging of plug-in and battery electric vehicles (PEV) has grown exponentially in recent years to the point that today several companies offer commercial wireless chargers as aftermarket products for integration into light duty passenger vehicles . Wireless power transfer (WPT) can be viewed as a revolutionary step in PEV charging because it fits the paradigm of vehicle to infrastructure interconnection and communication – wirelessly. Benefits of WPT are well known as a convenient, flexible, safe, and potentially automated means of passenger vehicle charging that has the potential to completely eclipse today’s conductive charging. There are no cables to trip over, no heavy plugs and cabling to wrestle with during inclement weather, and no concerns about inadvertent disconnection or theft.
Wireless transmission dates to Heinrich R. Hertz (1857 – 1894) a student of Herman L.F. von Helmholtz (1821 – 1894) and developer of the Hertz antenna receiver. Most notably, it was Nikola Tesla (1856 – 1943) who promoted wireless transmission of power using electrostatic induction from a high tension induction coil. It was in 1891 that Tesla demonstrated his wireless power transmission apparatus to the American Institute of Electrical Engineers, predecessor to the IEEE, at Columbia College. More famous however is his Wardenclyffe tower, a facility he financed through venture capital during the years 1898 to 1901, first as a trans-Atlantic wireless telegraph and then for wireless power transmission until funding was cut off. Most recently, WPT concepts gained widespread interest when André Kurs, Marim Soljačić and others at MIT (Science 2007) demonstrated transmission of 60 watts across 2.5 meters to illuminate a light bulb. Other pioneers such as Professors Grant Covic and John Boys of Auckland University in New Zealand developed inductive power transfer (IPT) for automobile and bus charging. IPT was scaled to 10’s of kW power levels and applied to rail, shuttle, SUV, and passenger vehicle charging in-motion by researchers at the Korean Advanced Institute for Science and Technology (KAIST) over the past several years . As a result of growing interest in WPT and the emergence of aftermarket products, the Society of Automotive Engineers (SAE) in 2010 formed the SAE J2954 Wireless Charging Task Force to address the need for comprehensive standardization of WPT.
WPT FUNCTIONALITY – BASIC
The most basic WPT charging system, such as some 3.3 kW chargers offered as aftermarket options today, consist of four functional blocks: i) grid-connection and input rectifier, ii) High frequency (HF) power converter, iii) WPT coupler, iv) vehicle side rectifier (plus filter, cabling, and disconnect). Not shown in this figure is a common tendency today to route the rectified WPT voltage shown in figure 1 into the vehicles on board charger (OBC) and thereby utilize the existing battery charging algorithm. WPT is also future compatible because it reflects more the architecture of DC fast charging than of cable connected OBC. Therefore, the WPT architecture shown in figure 1 may have its power scaled to > 22 kW and be compatible with fast charging if the vehicle is so equipped.
|Figure 1 : WPT essentials: the most basic implementation|
To be compatible with conductive charging (i.e., SAE J1772) the overall power flow path should be greater than 90% efficient. This means that utility power from a 220VAC line at 3.7 kW (17.7 Arms, 0.95 PF) and 90% efficiency will result in 3.3 kW delivered to the rechargeable battery. Other on-board systems must also operate such as DC-DC converter, battery management unit, and the radios so some provision must be made in power transfer to accommodate these loads and what influence that choice will have on the efficiency target.
The private and secure communications channel, perhaps dedicated short range communications (DSRC), brings a host of challenges, not the least of which is keeping the identity of the vehicle under charge secure along with the credit card information of its owner.
Figure 2 illustrates a WPT stationary charging installation consisting of base station charging electronics, a floor mounted (or embedded) primary pad, and a vehicle equipped with secondary (or capture) pad, a rechargeable battery, and wireless communications. Prof. Joachim Taiber of Clemson University International Center for Automotive Research (CU-ICAR)  summarizes the communications challenges of WPT charging by noting that vehicle electrification has been followed by the connected vehicle and that we are moving toward an era of automated vehicles.
|Figure 2: Stationary WPT charging|
WPT functionality is similar to the OBC found in PEV’s with the notable exception that the HF transformer used for electrical isolation is expanded into the WPT coupler. Since the couplers gap is large (100 to 250 mm) the compact and tightly magnetically coupled HF transformer of the OBC now must scale to a sizeable pad (300 to 700 mm diameter). Details on WPT coupler, system design, and power regulation can be found in [4-6]. Figure 3 summarizes this as a HF coupler matched on both input and output with tuning components that enhance magnetic field coupling for large gaps. The remaining functions shown are common in all OBC’s, including grid-side power factor corrector (PFC), the HF power inverter, and the load side rectifier, filter, and sensing electronics.
|Figure 3: WPT charger as evolution of on-board-charger (OBC)|
Just as in the OBC case, a WPT charger must satisfy grid power quality requirements for power factor (PF > 95%), and total harmonic distortion THD < 5%. These two specifications mean that WPT input current is within 18 degrees of the line voltage and very near sinusoidal.
PARKING AND ALIGNMENT
With WPT charging a PEV must park in reasonably good alignment with the primary charging pad for most effective and efficient power transfer. C.P. Yang  reports on two studies completed at the University of Warwick, U.K., showing that 1 in 10 people park within 25% of acceptable WPT alignment in a parking space regardless of pulling-in or backing-in. In a related study these same researchers found that 3 in 20 people were successful in aligning the front of the vehicle with the charging pad than if center mounted.
|Figure 1: WPT charger as evolution of on-board-charger (OBC)|
Figure 1 shows the concept of parking and alignment aide for WPT charging. When a PEV equipped with WPT charging coil approaches a primary pad of the charging station it may be possible to use a primary pad located homing beacon for the vehicle align with. In addition, some manner of human-machine-interface (HMI) display in the passenger cabin would provide the driver with approach heading and corrections until the vehicle is in near perfect alignment with the primary pad. For more information on alignment the interested reader is referred to the SAE J2954 standards committee . It is important for WPT operating at kW power levels to have good alignment, typically within +/- 100mm of exact alignment in order to minimize leakage fields. Leakage fields consist of HF magnetic fields that fringe beyond the footprint of the charging coils and may extend outside the perimeter of the vehicle under charge where people may be standing. HF magnetic fields will induce currents into conductive objects that in turn contribute to heating. Furthermore, electric fields will be established by induction that may interfere with on-board electronics such as keyless entry, tire pressure monitoring system, and other electronics. This concern also applies to vehicles parked adjacent to the one undergoing WPT charging.
Manufacturers of WPT systems today recognize these issues and have designed the charging pads with sufficient magnetic flux guides and shielding so that leakage fields are well below international limits . The authors  have documented field measurements on a WPT equipped battery electric transit bus that is in revenue service in Chattanooga, TN.
The need for standards in the case of WPT is essential, and in no area more important than interoperability, and of course safety. But it is interoperability that will enhance safety and facilitate lowest cost to not only vehicle manufacturer but consumer. There are five major interoperability criteria:
• Secondary coil mounting location on the vehicle
• Center frequency selection
• Alignment and obstacle detection
• Private and secure communications
• Leakage field mitigation
|Figure 2: WPT secondary coil mounting options|
Figure 2 shows three candidate locations for secondary coil mounting on a PEV: central mounted which provides the highest fore-aft transition zone for leakage fields to attenuate before emanating from beneath the vehicle. Front mounting is preferred by some, and as the University of Warwick study shows is easiest for drivers to align. However, front mounting places the secondary pad within the crush zone where even a minor fender bender may damage it, and the same for rear mounted. On the plus side, front mounting moves the HF magnetic field zone further from cabin occupants as this may become more of an issue as WPT evolves to quasi-stationary and vehicles charge while at stop lights, by special charging lanes on highways such as lay-bys and other provisions where occupants are in the vehicle. The main issue with rear (and to some extent central) mounted secondary coil is that PEV battery packs are typically chassis mounted beneath the trunk space, down the central alley beneath the passenger compartment, and rarely beneath the front. This is because PEV’s are dominantly front wheel drive and the traction motor and its transaxle are mounted there.
It is within the purview of the SAE to issue guidelines, recommended practice, and standards on WPT center frequency selection, alignment types, obstacle detection requirements, and internationally harmonized leakage field limits. For light duty passenger vehicles WPT charging at power levels < 22 kW are to operate at 85 kHz with incremental frequency variation to accommodate gap and load changes.
Very likely the most demanding requirement will be on the communications channel for WPT. This wireless channel must convey back to the WPT base station all the messaging necessary to maintain regulated power flow and the ability to deal with contingencies such as rapid termination in case of a fault in the power flow. Other contingencies may arise from utility rate structure, time of use, and load shedding. At present and with the cooperation of the U.S. Department of Transportation the 5.8 GHz band is being looked at for DSRC in WPT systems. This activity is part of the connected vehicle trend and likely to see very rapid implementation progress over the next few years. This is important because WPT guidelines will roll-out in the 2014 to 2016 time frame.
The following two subsections present insight into WPT charger technology.
WPT FULL FUNCTIONALITY
A full function WPT system is defined as one consisting of a five stage cascade of power processing blocks as shown in figure 3. This system meets all requirements of grid-side power quality of PF and total THD by utilization of a controlled rectifier stage that maintains high power factor and does double duty as an adjustable voltage source to the HF bridge. The HF stage generates the 85 kHz current to excite the coupler. Primary side regulation is shown and consists of the commands for voltage and frequency adjustment communication via the radio. A HF isolation transformer mounts within the WPT base station to provide electrical isolation of the HF cable and primary pad from the utility line. Depending on installation and regulatory requirements it may be necessary to have this isolation. In fact, having it makes the WPT functionally on par with OBC designs that utilize isolation.
The WPT coupler itself consists of the primary pad and secondary pad mounted on the PEV. It is beyond the scope of this article to distinguish between voltage source and current source WPT. Suffice to say that most stationary charging systems today are of the voltage source type in which the HF inverter uses voltage control to manage power flow through the coupler. A current source design by comparison is typical of transferring power to moving vehicles whether they are material handling in a warehouse or of the online electric vehicle (OLEV) shuttle type. Current source operation provides regulated HF current to a primary loop that one or multiple pick-ups may obtain charging from. The coupler of course requires tuning components in order to maintain resonance at the specified center frequency. It must be pointed out here that some practical concerns arise in the choice of coupler design and tuning capacitors. For the series-parallel (S-P) tuned WPT it is advisable to mount the primary tuning capacitor within the primary pad enclosure so that all high voltages resulting from resonant operation are contained. In this way the HF cable feeding the primary pad may operate at only 200 Vac and 40 Arms. To understand this consider two cases, both have a primary pad (coil) of inductance 120 H and are excited with 40 Arms of current, but A) at f=20 kHz and B) f=85 kHz. For these two cases the series tuning capacitor values and corresponding reactances are: A) C= 0.53 F, XC= 15.1, and B) C=29.2 nF, XC= 64. Therefore the component voltage stress with an applied 40 Arms current in case will be: A) Upk = 852 V, B) Upk = 3.63 kV. This is dramatic and the reason why higher frequency WPT requires multiple tuning capacitors placed in series.
|Figure 3: Full function WPT|
The remainder of the functionality in figure 3 is the same as the basic version. The practical and implementation issues are the following.
• A five stage cascade means that each block in figure 6 must be on the order of 97% efficient to meet an overall 85% target
• Loss minimization entails use of appropriate bandwidth Litz cable in all the HF stages, and most importantly in the cables and primary pad
• Ferrite materials used for inductors in the PFC stage, as core in the HF transformer, and as flux guides in the coupler must be low loss and low cost
• For voltage source operation the PFC output DC voltage must be restricted to a low value until the secondary is present, otherwise excessive currents will flow. For current source operation a similar, but less sensitive condition applies to voltage control
• A DC contactor is recommended in the WPT secondary so that high voltage (HV) battery potential is not left on the secondary pad during non-charging operation of the vehicle. PEV HV batteries are nominally 370 Vdc when under charge.
This article has described the present status of wireless power transfer charging of electric vehicles and some of the practical and implementation issues. It has been pointed out that private and secure communications is as important as the power handling components so that all aspects of vehicle battery charging parallel that of conductive charging. Furthermore, a description of the WPT functional cascade was provided to highlight the implementation requirements so that utility power quality and electrical safety are maintained. An example was presented on primary side tuning component stress to put into perspective what the implications are when scaling to higher power levels. Lastly, the need for high efficiency was shown to be dependent on material selection and architecture.
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