When it comes to the impact of plug-in electric vehicles on the grid, most of the attention has focused on the potential problems these cars might cause in the suburbs. Getting much less attention are the potential problems that might arise when people take their electric vehicles to work in urban areas.

Unlike the suburban drivers who charge their cars at night when power demands are at their lowest, daytime charging will occur during the time of peak power demands.

Studies by the Electric Power Research Institute, the National Renewable Energy Laboratory, and others find that the load from electric vehicles in urban areas could be substantial. A 20 percent adoption rate of electric vehicles (with half using 120 volt, 20 amp Level 1 chargers and the other half using 240 volt, 40 amp Level 2 chargers) could increase the load by 150 megawatts or more in small cities like Atlanta, Boston, and Baltimore. And larger cities like New York and Los Angeles would need to provide an additional gigawatt of power just for the electric vehicles.

To accommodate these additional loads during the day, utilities have limited options. Adding such significant capacity is challenging. Building new urban substations is an extremely expensive proposition. Adding capacity to existing substations is often not possible since most typically use every available inch of space and they are land-locked, preventing physical expansion.

So what else can be done? One idea proposed at last year’s IEEE Conference on Innovative Technologies for an Efficient and Reliable Electricity Supply is to take advantage of unused step-down transformer capacity, which is already installed but sitting in reserve in case of problems.

The proposed solution would be to tie urban substations together using superconducting cables to make more efficient use of the installed capacity that sits in reserve in case of transformer downtime. By tying substations together, each substation is no longer required to contain its own spare capacity but may share it among substations.

This approach might help utilities meet the daytime power load increases that would come if many drivers use electric vehicles and expect to charge them at work in cities during the day.