Impact of Plug-in Hybrid Vehicles on the Electric Grid (2006)

Plug-in hybrid vehicles (PHEVs) are being developed around the world; much work is going on to optimize engine and battery operations for efficient operation, both during discharge and when grid electricity is available for recharging. However, there has generally been the expectation that the grid will not be greatly affected by the use of the vehicles, because the recharging would only occur during offpeak hours, or the number of vehicles will grow slowly enough that capacity planning will respond adequately. But this expectation does not incorporate that endusers will have control of the time of recharging and the inclination for people will be to plug in when convenient for them, rather than when utilities would prefer. It is important to understand the ramifications of introducing a number of plug-in hybrid vehicles onto the grid. Depending on when and where the vehicles are plugged in, they could cause local or regional constraints on the grid. They could require both the addition of new electric capacity along with an increase in the utilization of existing capacity. Local distribution grids will see a change in their utilization pattern, and some lines or substations may become overloaded sooner than expected. Furthermore, the type of generation used to recharge the vehicles will be different depending on the region of the country and timing when the PHEVs recharge. We conducted an analysis of what the grid impact may be in 2018 with one million PHEVs added to the VACAR sub-region of the Southeast Electric Reliability Council, a region that includes South Carolina, North Carolina, and much of Virginia. To do this, we used the Oak Ridge Competitive Electricity Dispatch model, which simulates the hourly dispatch of power generators to meet demand for a region over a given year. Depending on the vehicle, its battery, the charger voltage level, amperage, and duration, the impact on regional electricity demand varied from 1,400 to 6,000 MW. If recharging occurred in the early evening, then peak loads were raised and demands were met largely by combustion turbines and combined cycle plants. Nighttime recharging had less impact on peak loads and generation adequacy, but the increased use of coal-fired generation changed the relative amounts of air emissions. Costs of generation also fluctuated greatly depending on the timing. However, initial analysis shows that even charging at peak times may be less costly than using gasoline to operate the vehicles. Even if the overall region may have sufficient generating power, the region's transmission system or distribution lines to different areas may not be large enough to handle this new type of load. A largely residential feeder circuit may not be sized to have a significant proportion of its customers adding 1.4 to 6 kW loads that would operate continuously for two to six hours beginning in the early evening. On a broader scale, the transmission lines feeding the local substations may be similarly constrained if they are not sized to respond to this extra growth in demand. This initial analysis identifies some of the complexities in analyzing the integrated system of PHEVs and the grid. Depending on the power level, timing, and duration of the PHEV connection to the grid, there could be a wide variety of impacts on grid constraints, capacity needs, fuel types used, and emissions generated. This paper provides a brief description of plug-in hybrid vehicle characteristics in Chapter 2. Various charging strategies for vehicles are discussed, with a consequent impact on the grid. In Chapter 3 we describe the future electrical demand for a region of the country and the impact on this demand with a number of plug-in hybrids. We apply that demand to an inventory of power plants for the region using the Oak Ridge Competitive Electricity Dispatch (ORCED) model to evaluate the change in power production and emissions. In Chapter 4 we discuss the impact of demand increases on local distribution systems. In Chapter 5 we conclude and provide insights into the impacts of plug-ins. Future tasks will be proposed to better define the interaction electricity and transportation, and how society can better prepare for their confluence.