When distribution transformers become overloaded, they stop working.
The distribution transformer is the green box sitting in the yard of a home or the grey or blue-can hanging from a utility pole down the street.
When a distribution transformer becomes overloaded, it cuts the electricity to all the homes being served by the transformer for several hours. Condos and apartments have significantly larger distribution transformers located on the property to supply all the residences.
Substation transformers supply distribution transformers and as the load on the distribution transformers increases, the load on the substation transformers increases also.
Each residential distribution transformer supplies the electricity for around four or five homes, and is usually a 25, 371/2 or 50 KVA unit, depending on the number of homes being served and the electrical load in those homes.
The issue that is hard to pin down, is, “How many PHEVs or EVs can be garaged (and recharged) in homes served by a distribution transformer before the distribution transformer becomes overloaded and causes an outage?
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 this nighttime, off- peak hours. Under this scenario, where charging is done during off-peak hours, every home could probably recharge a vehicle without causing the transformer to fail.
If, however, vehicles are recharged during other hours of the day, especially in the summer, there is a possibility the transformer will become overloaded and cause an outage – depending on how heavily the transformer was loaded before starting to recharge the battery.
How heavily existing distribution transformers are loaded is an unknown at virtually every utility. Typically, homeowners have been adding appliances, such as flat panel TVs, so the load on the transformer has gradually increased to the point where the additional recharging load could cause the transformer to become overloaded and have to be replaced with a larger unit.
Similar conditions exist at condominiums and commercial properties, where the utility doesn’t know how heavily existing transformers are loaded.
I want to thank Hal Wright, Manager of Electric Operations for the City of Geneva, Illinois, in helping me wrestle with the issue of distribution transformer loading. In short, there is no easy answer to knowing whether distribution transformers are heavily loaded before adding the load from PHEVs and EVs.
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 cause distribution transformers to have to be added or replaced with larger units. All of this also impacts the substation transformers which supply the power to the distribution transformers.
In each case cited above, the transformer will fail without warning and the utility will have to rush to restore electrical service to its customers, which will take several hours.
It’s a relatively simple matter to change out 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 of work each.
I’ll discuss the cost of replacing transformers in the next article.
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