Southern Cal Edison finds that current electric vehicle driving patterns minimize impact on grid

I blogged about the Electric Power Research Institute (EPRI) report, Transportation Electrification: A Technology Overview, which provided projections of the commercial rollout of plug-in electric vehicles (PEVs), gives an overveiw of vehicle, plug-in and other infrastructure technologies, and suggest ways that utilities can prepare for the widespread use of PEVs in the US.

At that time there were serious concerns that the additional load of PEVs on the grid, and in particular about the risk of spikes caused by many people plugging in at the same time.

Southern California Edison (SCE) has just published a fascinating report outlining its experiences with electric vehicles over the past several years.  SCE was an early adopter of electric powered vehicles - at one time it had the largest private fleet of more than 300 plug-in electric vehicles (PEVs) that had traveled more than 1 million miles. 

Southern California is ahead of most jurisdictions in terms of the total number of plug-in electric vehicles.  SCE has about 5 million customers in its Los Angeles service area.  At the present time SCE customers own or lease more than 12,000 plug-in vehicles.  SCE estimates that by 2020, there will be about 350,000 PEVs in their service territory.

Low impact on grid

SCE has found that since 2010, of the nearly 400 upgrades that were made to the circuits that serve PEV customers, only 1 percent of the work was due to additional power demands from PEVs.  SCE has several thousand circuits and to upgrade all of them would have been cost-prohibitive.  SCE decided to only upgrade circuits when they were notified of an actual PEV charging location on the circuit.

About 65 % of SCE customers with PEVs drive a plug-in hybrid (PHEV), and about 35 % drive a battery electric vehicle (BEV), powered solely by electricity.  SCE has found that about 50 % of PHEV owners charge at Level I (120 volts).  This means a much lower impact on SCE grid distribution circuits than if most customers charged at Level 2 (240 volts).

Randomizing charge starting times

Many PEV owners charge at night at home and during off-peak hours at Level 1. That means low impact on the grid, low installation costs and the low electricity costs for customers who are on a time-of-use rate.  But if everyone started charging when off-peak rates begin, typically midnight, the result could be a power spike which can create problems for the power distribution network.

PEVs have settings for either a “start charge” time or an “end charge” time.  SCE recommends setting the end charge time because it creates a random start time pattern.   Batteries have different states of discharge and charge at different levels. When customers set an “end charge” time for charging to be complete, the effect is to randomize the start times for charging, which prevents a large number of vehicles from coming online at the same time thus avoiding the power-load spikes that would happen if everyone set the same start time, for example. when off-peak rates begin.

Typical driving patterns

SCE conducted a study of 92 Nissan LEAF (a BEV) owners and found some interesting patterns.

• Average miles driven daily: 35.
• The car was driven more during the weekdays than on weekends.
• The car was typically charged only once a day, at home and overnight.
• Over a week the typical way the car was driven did not differ from those of non-electric vehicles except for long road trips.

Apparently LEAF owners got over any “range anxiety” they may have experienced before buying one.  Most away-from-home charging takes place at work and at Level 2 (240 volts), but only a few LEAF owners have access to a charging station at work.  Apparently most LEAF owners have access to free Level 2 charging other than at work, but less than 40 percent took advantage of it during the period of the study and those that did use it, used it rarely.  At-home-charging start times also corresponded to the time-of-use rate plan which confirmed for SCE that their rate plans are effective in encouraging off-peak charging which is what they were designed to do.