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Electric Vehicles Present Challenges For Distribution Grids

Oct. 16, 2011 - Dr. Russell Lefevre - renewgridmag.com

Increased worldwide interest in electric-vehicle (EV) deployment has led many researchers to consider the effects wide-scale adoption will have on the electric grid. Most experts have concluded that there is enough generation and transmission capacity to accommodate the expected number of EVs, at least in the near term.

However, the distribution system could present a significant challenge, and the critical element is the transformer.

In the U.S., the average pole-mounted transformer serves seven to eight homes. Many who have examined the issues have concluded that even small numbers of EVs in a limited area could cause difficulties.

The effects will be very scenario-dependent, but many studies show potential problems. For example, Saifur Rahman, director of the Virginia Tech Advanced Research Institute, recently indicated that in the case of a 25 kVA transformer serving three houses - with each house quick-charging a plug-in hybrid vehicle at 6 p.m. - just flipping on a clothes dryer would severely overload the transformer.

Another concern - one originally highlighted a couple years ago at a California Energy Commission meeting - is that simply charging EVs at night, when power demand is significantly reduced, is not necessarily a solution.

The problem is that many utility distribution grids employ undersized transformers that are designed to cool overnight. If a high number of EVs charge at night, when transformers are supposed to be getting a break, the sustained excess current will eventually cook a transformer's copper windings, causing a short and blacking out the local loads it serves.

During the IEEE-USA Electric Vehicle Workshop held this spring in Austin, Texas, Karl Rabago, vice president of distributed energy services at Austin Energy, spoke about the anticipated influx of EVs and the need to prepare for the impact on utilities.

One projection indicates that Austin could have as many as 192,000 EVs on its roads in 10 years. To address the issues associated with this growth, the utility has initiated a program called Plug-In Partners, which is intended to support the installation of home charging stations within the city. The program provides a rebate of up to $1,500 toward the purchase and installation of a Level 2 (240 V) charging station.

For the utility, the most important condition of the agreement is customer participation in a charge-management pilot program. This program is specifically designed to enable Austin Energy to determine the actual effects EVs will have on the grid as more cars come online.

Also, Honda's Electric Vehicle Demonstration Program, launched late last year in Torrance, Calif., is designed to help utilities prepare for large numbers of EVs. The program includes research into customer behavior and usability, public charging infrastructure planning and sustainability initiatives. Stanford University and Google are also involved in the program.

Undoubtedly, the growing market for EVs will continue to spur innovation in areas such as battery technology and charging infrastructure, but there are still many challenges to overcome in order to achieve mass-market adoption.