Will EVs Overwhelm Our Distribution Grid?
by Bob Shively, Enerdynamics President and Lead Facilitator
The number of registered electric vehicles (EVs) in the U.S. is growing quickly despite barriers such as upfront purchase costs, limited range, and concerns about lack of charging infrastructure. Both government policy and car makers’ product plans suggest exploding growth rates. According to International Energy Agency (IEA) projections, the current number of about 3 million EVs in the U.S. will more than triple in the next two years and will increase more than tenfold to 39.6 million vehicles by 2030.
Source: International Energy Agency (IEA)
Given that each EV has an electric charging load of about 7 kW (compared to a typical house with a peak demand of 4 kW), the projections raise the question – can our grid manage this rapid load growth?
Impacts of load growth
Perhaps a better question, given that the electric industry has continued to serve customers through nearly 100 years of escalating growth, is not whether we can serve the growth but how we’ll do it. First, we need to consider how EVs impact the grid. While EVs increase both peak load (kW) and electric usage (kWh), many of the impacts will come through increased peak load unless utilities can influence or control what time of day this new demand occurs. Various studies of the California electric market project that, due to EVs, peak loads in 2035 will be 112 to 150% higher than in 2025.
Key components of the distribution grid to consider include the distribution substation, conductors in the primary and secondary feeders, and the service transformer.
Experience to date suggests that the most concerning components are conductors and transformers. Since conductors are often routinely oversized to allow for load growth, transformers are perhaps the most critical link. A typical transformer may be sized so that peak loads are expected to be about 60% of the transformer capacity. Let’s look at a suburban circuit with a 37.5 kVA transformer serving six homes:
In this case, the peak loading on the transformer has the potential to be 24/37.5 = 64% if all the homes hit their peak at the same time. If one customer adds a 7 kW EV and it is charging at the peak, the loading on the transformer has now increased to 31/37.5 = 83%, and if two get added the transformer may now be loaded to 102% of rated capacity. And if all the neighbors want to keep up with each other, it’s clear the transformer will be dramatically undersized. Depending on the conductor sizing, reconductoring may also be required.
Even with only a few EVs on a circuit we must consider other impacts as well. Transformers fail because of overheating. Loading a transformer for night charging may result in the transformer not having the opportunity to cool down at night. And new harmonics from charging stations may also cause transformer heating. Lastly, the impact of EV loads during circuit restoration following an outage may not be well understood, making it harder for utilities to restore critical loads such as air conditioners or heating.
What distribution utilities can do
The standard utility response to load growth is to simply install bigger transformers and bigger conductors. But this approach is capital intensive. If the increased revenue from the load growth pays for the cost of upgrades, then everyone comes out okay. But since load growth due to EVs is uncertain, there is the risk that capital will be spent without assurance that the costs will be covered. Another potential difficulty with this approach is that utilities are currently struggling to acquire sufficient transformers due to supply chain issues. So, what else might utilities do? Here are some options utilities are exploring:
- Non-wire alternatives: Implement time-of-use rates, specific EV rates, incentives, or demand-side management programs that encourage customers to charge EVs at non-peak times or to shift other loads to offset EV charging. Install batteries on the distribution grid or contract for use of third-party batteries which can reduce circuit loads to avoid peak loading.
- Direct control of EV charging: Implement programs that allow the utility to curtail EV charging during peak load times or when restoring the system after outages.
- Smart transformers: Add communications and monitoring to service transformers to provide real-time monitoring. Couple monitoring with software providing for analytics and alerts based on actual data. This will allow distribution operators to rapidly identify transformers that require attention and selectively address overloaded transformers rather than implementing wholesale transformer replacement.
Clearly the expected growth of EVs will put significant demands on utility distribution systems. This will challenge our industry’s traditional way of dealing with new loads. But by developing and implementing new tools, innovating solutions will allow utilities to efficiently manage new EV loads with similar effectiveness as the industry has handled transformational load developments throughout the history of electric utilities.
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