An investigation into the impact of electric vehicle load on the electric utility distribution system

Abstract

The electric utility's interest in electric vehicles lies in the anticipated/expected benefits beyond the simple increase in energy sales. It is expected that the EV load will be contained within system off-peak hours without affecting the peak demand, thus increasing the sale of low cost electricity. However, the impact of electric vehicle charging on the energy and power demand is determined not only by the number of EV's in use and their usage pattern, but also by the number of EV's being charged at an instant and the charging profile of the battery module. The daily energy consumption by an EV will be limited by the range/cycle and the charging time, while its impact on the system demand will depend on the hour and pattern of charging. The case studies in this paper have revealed several important issues regarding the impacts EV load may have on utility distribution systems. Firstly, it is not adequate to have only sufficient generation capacity during off-peak hours to assure a system's ability to abs'orb EV loads without adverse effects. The constraints at the distribution level must be studied properly. Secondly, a sizable EV load can introduce a new peak in the early off-peak period. It may have scheduling implications, and completely throw any load management programs off balance. Thirdly, at the present state of EV technology, including those of battery modules and chargers, a typical distribution system may not be able to supply EV loads beyond 20% penetration level. This constraint is created by the long (up to 12 hours) charging cycle of batteries. INTRODUCTIONIncreasing environmental concerns, the consequent regulatory requirements, and the fjeo-politics of oil have made the fuel independent electric vehicle (EV), with low and controlled emissions, increasingly more attractive as practical and ecoqomical alternative to the conventional gasoline car. The present state of EV's and the current developmental activities in this field are summarized by Moore in a recent paper [I]. The key issue of adequate battery system for EV's to compete with the range and performance of conventional cars remains to be fully addressed. While the recent cooperation among automakers (e.g., the formation of US Advanced Battery Consortium) is expected to produce advanced EV batteries, the support of electric utilities will be essential in developing appropriate infrastructure standards and technology to make EV's attractive to the consumer.It is expected that the EV load will be contained within system off-peak hours without affecting the peak demand. From the electric utility operation aspect, this potential to fill the valley in the load curve will result in more electricity sales for the same system capacity. This implies more effective utilization of all equipment in the system, thus potentially reducing the per unit cost of electricity. However, such system wide expectations may not be easy to achieve. The authors believe that it would be necessary to study the impact of EV's on the system operation at sub-station lev...

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