Determining Time-of-Use Schedules for Electric Vehicle Loads: A Practical Perspective

Analyses have shown that electric vehicle (EV) loads may considerably affect the secondary service voltage quality. One of the ways to mitigate voltage drop concerns is to use a time-of-use (TOU) pricing scheme. A TOU pricing scheme utilizes the off-peak generation for EV charging, thus deferring any immediate grid upgrade and improving the grid sustainability. This paper evaluates various aspects of EV charging under a TOU schedule, with off-peak rates starting at hours ranging from 8 P.M. to 3 A.M. The study is conducted using an actual residential distribution circuit. A best practical time to begin the off-peak rates is determined so that the effects of EV charging on the secondary service voltages are minimized while ensuring that EVs are fully charged by 7 A.M., thus maximizing both grid and customer benefits. The analysis suggests that the best time to begin off-peak rates is between 11 P.M. and 12 A.M. Furthermore, the analysis also suggests that setting up TOU off-peak rates at the latter half of the peak load demand, for example, at 8 P.M., is detrimental to the distribution circuit voltage quality. The result indicates that the existing utility TOU scheme may exacerbate voltage drop problems due to EV load charging.