Sometimes life seems to be a series of "gotchas," and there doesn’t seem to be an exception for owners of electric vehicles. If EVs attain the kind of wide-spread popularity their promoters hope for, buyers of Chevy Volts, Nissan Leafs, and similar vehicles could find unexpected bills for pole-mounted distribution transformers showing up in their mailboxes.
The reason is explained by Prof. Saifur Rahman of Virginia Tech’s electrical and computer engineering department. The distribution transformer on the pole outside your house typically handles about three to five homes and has a relatively small (25 kVA) capacity. That figure can be less than the maximum theoretical load (usually about 6 kVA) from each of those houses if all their major electrical loads were switched on simultaneously. But the scheme works because it’s unlikely that every house on the block will run the clothes dryer, A/C, water heater, and other major appliances at the same time.
Add EVs into the mix and the whole scenario changes. It is not unlikely that five EV owners could arrive home from work at roughly the same time and plug their cars in for a recharge. “Smaller EV chargers as for Volts or Leafs can pull up to 40A. They are like adding another load from an air conditioner, water heater, or dryer,” says Rahman. “Today, when you buy an EV, the car dealer will inform your power company, and the power company will do a survey before giving the OK to install a charger.”
In a case where multiple EV owners live on the same block, the extra load will force the utility to install a bigger transformer. The question is, of course, who pays for it. In many cases, the bill goes to the guy whose EV charger caused the potential overload.
“I know a Tesla owner in California who had to pay $10,000 to have his transformer upgraded,” says Rahman. “That’s a bigger EV, and its charger draws 100 A. But he’d just shelled out over $100,000 on his Tesla, so spending another $10,000 was not a big deal for him.”
Of course, less well-heeled EV owners might not be quite as sanguine about such an unexpected bill. And they might be even less cheerful if they have solar panels installed on their houses with the idea of using them to charge their EVs in an emergency. Major storms in the northeast last year that knocked out power for several days to wide-spread areas show that lengthy periods with no juice are a real possibility.
“A big worry is how to charge the EV so you can get out if you lose power for a couple of days. But in this country, your solar panels are not allowed to produce ac current if the grid is down,” explains Rahman. “That rule is for safety. You don’t want someone working on the power lines to be electrocuted from your solar cells.”
Fortunately, all these problems are solvable. Rahman has a National Science Foundation grant to, among other things, come up with a system able to automatically sense and make decisions about residential loads so multiple EVs don’t blow out transformers. And solar cells could safely power local loads in emergencies with the right kind of regulations. “These are issues of rules, not of technology,” he says. EE&T