Researchers probe grid capacity limits caused by EV charging load

In a new report, researchers at the U.S. Department of Energy’s Pacific Northwest National Laboratory evaluated the capacity of the power grid in the western U.S. over the next decade to handle growing fleets of EVs of all sizes, including trucks. They found that through 2028, the overall power system--from generation through transmission--looks healthy up to 24 million EVs, about 9% of the current light-duty vehicle traffic in the U.S.

However, at about 30 million EVs, issues may arise at the local level. The PNNL researchers say that even one fast-charging EV can draw as much load as up to 50 homes. As a result, if, for example, every house in a cul-de-sac had an EV, one power transformer would not be able to handle multiple EVs charging at the same time.

For their study, "Electric Vehicles at Scale – Phase I Analysis:  High EV Adoption Impacts on the Western U.S. Power Grid", the authors used data about future grid capacity from the Western Electricity Coordinating Council. The analysis revealed the maximum EV load the grid could accommodate without building more power plants and transmission lines.

Current grid planning doesn’t adequately account for a mass influx of EVs. That omission exacerbates the already stressful situation that manifests itself in the much-discussed duck curve.

As explained in the report, the duck curve is a 24-hour profile of load on the power system, and usually occurs in areas with a lot of photovoltaic rooftop installations. The curve is based on moderate load in the morning, low load during the day when solar units feed electricity into the grid and high load at night as people get home from work and the sun goes down.

When demand spikes, voltage falls. This swing is hard on system operations. And with more EVs plugging in to charge in the evening, the ramp-up becomes even steeper and drives up electricity costs.

Smart charging strategies that include avoiding charging during peak hours in the morning and early evening can smooth demand peaks and fill in the duck curve, according to the study. The approach could offer two upsides. First, it would take advantage of relatively clean solar power during the day. It also would reduce or eliminate the sharp evening ramps when solar power fades and other sources are dispatched to make up the difference.

The research showed that bottlenecks due to new EV charging appeared in parts of California, including Los Angeles, which plans to go all-electric with its city fleet by 2030. The greatest constraint came from the growth of fast-charging cars and commercial fleets of electric trucks. These vehicles can draw 400 amps through a circuit for as long as 45 minutes, instead of the 15 to 20 amps pulled over 6 to 8 hours by most EVs today.

Smaller cities with limited resources need help planning for their charging infrastructure and hosting capacity. In a follow-on study, researchers will take a closer look at ways to integrate EVs into local and regional power distribution systems across the nation.