If PHEVs or EVs are recharged at night between the hours of 10 pm and 6 am, it’s unlikely the transformer serving the homes will fail. Homes generally have very low loads during nighttime, off- peak hours.
If, however, vehicles are recharged during the day, especially in the summer, there is a possibility the transformer will become overloaded and fail – depending on how heavily the transformer was loaded before starting to recharge the battery.
There isn’t a single utility that has any idea how heavily existing distribution transformers are loaded, unless their units have overload lights, a practice that fell out of favor decades ago.
Typically, homeowners have been adding appliances, such as flat panel TVs, so the load on the transformer has been gradually increasing, possibly to the point where adding the recharging load could cause the transformer to fail.
Adding charging stations at condos or commercial buildings, or in downtown commercial districts, will result in people charging their vehicles during the day, when loads are heavy, which will result in the need to increase the size of distribution transformers. This also impacts the substation transformers.
When a transformer becomes heavily overloaded it will fail without warning and the utility will have to rush to restore electrical service to its customers. And this usually takes several hours.
Most distribution transformers are not changed until they fail, as this pole type unit can attest to.
It’s a relatively simple matter to replace a smaller distribution transformer with a larger unit. There are few weight limitations for pole-type units, and the pads for pad-mounted units are usually large enough to accommodate up to a 100-KVA pad-mounted transformer.
Larger substation transformers which can easily cost as much as a million dollars will require advance planning and several weeks to replace … so a failure can have far greater consequences.
The cost of changing out one 50 KVA distribution transformer, and replacing it with a 75 KVA unit, is well over $3,000. The cost of replacing a substation transformer can exceed $1 million.
At some point in time, if the population of PHEVs and EVs increases, a large number of distribution and substation transformers are going to have to be replaced with larger units. Whether this happens when they fail or under a planned regimen will depend on whether utilities know where vehicles are being recharged.
The problem is exacerbated if vehicles are recharged during the day when transformers are already heavily loaded.
Previously, the estimated cost was over $25 billion to replace distribution transformers when the population of PHEVs or EVs was forecast to be 87 million vehicles.
But with PHEV and EV sales falling way below forecast, the more likely danger will be where clusters of vehicles are sold, such as might be the case in California where these vehicles are being heavily promoted.
Similarly, the weakness of PHEV and EV sales has reduced the risk of expensive replacement of million dollar substation transformers.
Ideally, PHEVs and EVs should be charged during nighttime, off-peak hours.
Unless utilities know where PHEVs and EVs are being recharged, they will always have to react in an emergency mode when a transformer fails.
If, however, utilities are notified by the electrical contractor whenever a charging station is installed, utilities could develop a plan for replacing distribution transformers before they fail.
This isn’t an entirely satisfactory solution since many vehicles will be charged using an existing 120-volt outlet in the garage, but it could help reduce the number of times a utility has to respond to an outage where an overloaded transformer has caused several homeowners to be without electricity.
The complexity and cost associated with distribution transformers becoming overloaded as the result of recharging the batteries of PHEVs and EVs is significant, and should become part of the dialog surrounding these vehicles, especially in states where these vehicles are being heavily promoted.
When discussing PHEVs and EVS, we shouldn’t overlook their hidden costs, even though sales have been anemic and the hidden costs may not be incurred until sometime in the future.
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