Challenges of Charging Plug-in Electric Hybrid Vehicles

Wed, 02/02/2011 - 9:41am
Clayton Pillion, Microchip Technology Inc.
Clayton PillionMost people arrive home and instinctively plug a cellular phone in for a quick battery charge, knowing that it charges immediately and will finish promptly. We take the charging process for granted. As engineers, we know this charging process requires a simple battery-charging circuit and applied power. Unfortunately, the same can’t be said about electric vehicles, as their power requirements present challenges to consumers and utilities.

Power requirements for charging an electric vehicle can reach 6.6 kW (typical), as consumers opt for the quickest charging mode — AC Level 2 — in order to complete a charge cycle in three hours or less. The AC Level 2 charging mode specifies a nominal 220 V and up to 80 A supply, resulting in a 3.3 kW or 6.6 kW load in three hours. From the utility’s perspective, this is a substantial challenge, since a typical west-coast home consumes approximately 2-3 kW over a 24-hour period. As shown in Figure 1, for each vehicle that is charged nightly at the Level 2 rates, the utility sees an additional load equivalent to one to three houses.

Figure 1. AC Level 2 charging is equivalent to one to three houses during peak loading. (Microchip)

From a macro level of the grid, adding 1-3 houses isn’t a problem and is, in fact, analogous to constructing new houses or a strip mall. But electric vehicles will be added to existing neighborhoods where the “last mile” infrastructure, such as transformers and power lines, are already sized and installed based upon housing sizes. An example is the “typical” North American transformer that supplies six to eight houses on the same residential block. Adding a Level 2 charger would generally be fine, although it will cut into the planned overhead and safety margin. Should a second resident add a Level 2 charger, the transformer would now be faced with not only a 30 to 40 percent load increase, but the additional 6.6 kW to 13.2 kW being condensed into a three-hour span. Clearly, this would be a major concern for utilities, since neighborhood-based equipment could fail, causing scattered outages with associated safety risks for residents.

Overnight charging is often discussed as a reasonable option but, unfortunately, it is not a perfect solution. Sure, there may be more power available, since night-time electrical loads are considerably lower, and lower temperatures allow equipment to deliver more power; but this doesn’t guarantee sufficient power for one charger, let alone multiple chargers. An additional fact to consider is that many utilities rely on lighter night loads to allow the transformer to cool down, otherwise preventing thermal runaway.

Will utilities be forced to research neighborhood infrastructure for each Level 2 charger installation? If left untouched, the impact may be costly. Utilities may be faced with unplanned upgrades to scores of equipment that are purchased with expectations of decades of useful life, not to mention the labor cost to remove and install the equipment.

One of the leading solutions for addressing this problem is to coordinate the charging events. Each charger would communicate with a higher-level system that has awareness of the power-distribution infrastructure and real-time loading. For example, imagine that two neighbors on the same transformer arrive home within 30 minutes of each other and start charging their vehicles. A centralized coordinator can examine the current loading on the transformer (and upstream equipment), and determine the start time and charging levels possible. Should transformer loading change during the charge cycle, the coordinator would send updates to modify charging levels, or even initiate a demand-response/load-shedding event, causing the charger to delay.

Industry standards are being developed to ensure that chargers and coordinators can communicate on a common network. The key communication paths in this network are between the vehicle and charger, and between the charger and the utility. For the purpose of this article, assume that the AC Level 2 chargers are external to the vehicle.

Communication between the vehicle and the chargers is ideally suited for Power-Line Carrier (PLC) technology, because a fixed, hardwired connection exists as defined by SAE J1772. PLC systems operate by superimposing a high-frequency modulated carrier signal on the DC or low-frequency AC power line. The carrier signal is then de-coupled and demodulated at the receiving end, to recover the information. Jean-Pierre Fournier, president of Ariane Controls, said, “PLC technology is challenged by varying impedance, considerable noise and high signal attenuation on traditional power lines. However, the charger-to-vehicle connection has impedances and noise sources that are well understood and fixed, making for a highly robust PLC connection.”

Two proposals for SAE standards for the physical-layer communications are being discussed. One proposal is for the J2931/2 standard, which defines in-band signaling over the control pilot. The other is the J2931/3 standard, which defines power-line communications for plug-in electric vehicles using the power wires. The messaging protocol is being defined in the SAE J2847 standard. As illustrated in Figure 2, the goal of these standards is to ensure that chargers from various manufacturers can communicate with various utilities. 

Figure 2. Communications network between charger and utility.

The communication between the charger and the utility can be accomplished using many established and robust technologies with various benefits and cost structures. A charger connects to the home-area network and uses the existing Internet connection to communicate with the utility’s servers. The Wi-Fi 802.11 protocol is the most common solution. Another option is to connect using a proprietary wireless network, with hardware that bridges the proprietary wireless connection to a hard-wired Ethernet connection on the Internet gateway/router. Connecting directly to a smart electrical meter is another option for utilities that have deployed meters with wireless technology, such as those based upon the ZigBee protocol. Lastly, a charger could incorporate long-range wireless technology, such as cellular or WiMax, to send and receive messages with the utility. The communications capabilities and associated user interface are key areas for charger manufacturers to add unique value (see Figure 2).

Utilities, automakers, and charger manufacturers are working together to enable fast charging of plug-in electric vehicles. This is nothing new. In prior decades, the industry has faced similar challenges when households began adding new products such as a televisions, centralized HVACs, or computers. As the standards are drafted and ratified, and products move into mainstream status, consumers will take for granted a simple and smooth electric-vehicle charging effort much like they do with today’s cellular phones.

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