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Potential Impacts of Vehicles on the Grid

By: Arindam Maitra
August 2011

The rapidly approaching commercialization of plug-in hybrid electric vehicles (PHEV) and battery electric vehicles (BEV) has created an urgent need for utilities to support customer adoption of these plug-in electric vehicles (PEV), prepare for the installation of charging infrastructure in their service territories, and manage the impact of these new loads on the electric distribution system. In response to the overall need, EPRI initiated a multi-year project with 20 utilities to understand PEV system impacts with several utilities in the United States and Europe.

As part of Phase I of the Distribution Impact Project, EPRI conducted a comprehensive study assessing PEV charging effects on specific circuits within a utility's distribution system — typically one or two representative feeders per utility. This Phase I effort used very detailed simulations of each distribution system, customer load characteristics, and potential electric vehicle penetration and charging profiles to assess impacts across the distribution systems (assets, overall system loading, etc.). The results of the simulations across different distribution systems were combined to develop:

  • summaries of general concerns
  • assets that are likely to be at most risk
  • conditions that could require additional monitoring to avoid problems
  • impacts of different charging profiles (including controlled charging) on these results.

Because most charging is expected to occur at the residential level, the primary focus of this initial phase was residential feeders.

Although the residential charging standard can potentially reach power levels up to 19.2 kW (80 amps at 240 volts), most vehicles are expected to charge at power levels below 7kW. Charging levels of 120V/1.2kW, 240V/3.3kW, and 240V/6.6kW are expected to be most common. The Phase 1 effort concluded that the overall distribution system capacity is expected to be adequate for PEV charging based on near-term projections (one-to-five years) of PEV market penetration. Due to diversity in the load across a large number of customers, the maximum expected aggregate PEV demand is ~800 Watts/PEV and the average vehicle consumption during a charge period is projected to be 5-8 kWh for a midsize sedan.

Estimated Maximum PHEV/EV Charging Demand

The short-term impacts for most utilities should be minimal and localized. There is a possibility, however, of isolated impacts on some distribution transformers and secondary drops, particularly in neighborhoods with older distribution systems including underground systems. These assets near the load are most susceptible to PEV clusters as the potential benefit of spatial diversity decreases. Because older distribution systems (including underground systems), initially were designed for much lower per-customer load than their current operation, it is likely that the PEV impacts on these systems will be more severe than to newer systems. Transformers with low capacity per customer ratios are the most likely to be impacted by PEV adoption. Furthermore, transformers with capacities of lower than 25 kVA are expected to be the most susceptible to becoming overloaded. These transformers typically have lower amounts of existing capacity headroom, which can be quickly consumed by one or more PEVs.

Potential Impacts

Given past precedent, many utility experts expect PEV purchases will be geographically clustered within neighborhoods, even when overall PEV penetration is low. Figure 1 depicts the results of modeling conducted by EPRI of a distribution circuit from a Midwestern electric utility.

The model describes a utility that services 1,770 customers from 302 distribution transformers and assumes 4% PEV penetration.  Sporadic PEV clustering is evident. Some feeders, laterals, and distribution transformers see little to no PEV clustering while other feeders and laterals service clusters of several PEVs.  Also, some distribution transformers serve more than three charging PEVs.  Additional modeling conducted by EPRI suggests that PEV clustering is to be expected even at 2% PEV penetration.  Note that the only customers depicted in the circuit diagram in Figure 1 are those with charging PEVs.

Potential Impacts

EPRI believes that potential stresses on power delivery systems can be mitigated through asset management, system design practices, controlled charging of PEVs, or some combination of the three. But again, given the likely variability in customers' PEV choices, car types, varied charging patterns, varied charging speed preferences, and variable participation in utility-centric time-of-use (TOU) charging options, we believe that the utility will not be able to manage this risk in an ex-post fashion.  In many cases, the utility will likely not be notified or aware of a PEV addition, or a unique charging pattern.  In these cases, a proactive risk mitigation strategy is recommended to remove localized risk to the distribution system. Controlled charging can significantly reduce PEV loading impacts on the distribution system, but is not likely to be universally adopted. Tariffs and rates that encourage nighttime charging (e.g., load management, valley-filling, etc.) can also help to avoid or postpone system upgrades.  All of these factors should be taken into account in the analysis of potential risk as a function of distribution system conditions and geographic considerations.

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