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SCE Study finds Setting PEV End-of-Charge Time More Important than Start Time

August 13, 2013

Many PEVs have settings that can determine a “start charge” time or “end charge” time. According to a recent study by Southern California Edison (SCE) titled, “Charged Up,” it’s better for grid reliability and neighborhood circuits when drivers program their charging to be complete by a specific time. Setting an end time creates a more random start time pattern because batteries have different “states of discharge” and charge at different levels, thus requiring various amounts of electricity. When customers set an “end charge” time for charging to be complete, they randomize the start time of their charging, which prevents a large number of vehicles from coming online at the same time – avoiding power-load spikes that potentially could affect the local distribution system.

For example, it takes three to seven hours to charge a PHEV at Level 1, depending on the vehicle energy storage capacity and battery state of charge. At a Level 2 charge, that same car could be charged in one to three hours. So staggered start times at night minimize grid impacts and support system reliability as cars charge at different times. It helps offset local peaks that can occur when many customers return home at a similar time at the end of a work day and immediately plug in.

Today, SCE customers own or lease more than 12,000 plug-in vehicles. That represents about 10 percent of national PEV sales. SCE estimates that by 2020, there will be about 350,000 PEVs in its service territory. This current report is one part of SCE’s efforts to understand the needs of PEV drivers and prepare for an anticipated surge in future demand.

When SCE first started its PEV readiness efforts in 2009, it quantified the anticipated impact of PEVs on the electric grid before consumers began buying and leasing them. Because “early adopters” tend to cluster in the same neighborhoods and because PEVs can draw as much as another household’s electricity load, SCE developed an operational strategy to upgrade distribution circuits. Over time, circuits need to be resized based on standards that match the changing needs of customers.

SCE has integrated the expected load from PEVs into standards applicable to its “grid modernization” efforts. Since 2010, of all the nearly 400 upgrades made to (or identified for) circuits that serve PEV customers, only 1 percent of that work was required due to additional power demands from PEVs. The rest of the work was required under the regular infrastructure upgrade and maintenance schedule.

Upgrading all of SCE’s several thousand circuits would have been cost-ineffective and practically impossible. Rather, the company upgraded circuits as needed, wherever SCE was notified of an actual PEV charging location. And if a transformer needs to be replaced as part of routine maintenance (such as the transformer being near the end of its lifespan), it will replace it with a transformer that meets the new sizing standards, including the potential PEV load.

This approach is consistent with the PEV market’s trends. About 65 percent of SCE customers who own PEVs drive a plug-in hybrid (PHEV), which runs on electricity and gasoline, and about 35 percent drive a battery electric vehicle (BEV), powered solely by electricity. Current data shows that about 50 percent of PHEV drivers charge at Level I (120 volts), so there is a much lower impact on our grid distribution circuits than if more customers charged at the higher Level 2 (240 volts). While transformers are upgraded every day as part of routine infrastructure maintenance, less than 1 percent of these upgrades is directly attributable to PEVs (in all other cases, the grid needed reinforcement regardless of the PEV charging addition).

Current driving patterns combined with PEV features also seem to minimize the impact of PEV charging. Studies show that about 70 percent of PEV owners commute 40 miles or less daily. Most BEVs can travel at least 60-80 miles of all-electric range on a full charge and PHEVs will generally run 20-40 miles all-electric range on a full charge before going into hybrid mode. As a result, many PEV owners can fuel at night at home and fully recharge their battery during off-peak hours at Level 1. That means low impact on the grid, low installation costs and the lowest electricity costs if customers are on a time-of-use rate. This is why we urge our customers who drive PEVs to charge up every night at home.

There’s one caveat from the study: SCE is starting to see an increasing market share of BEVs and these come with on-board chargers with higher capabilities (from 3.3 kilowatts to 6.6 kilowatts or even higher). This could create new implications for grid reliability, which SCE is monitoring.

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