One of the important questions we face as we navigate towards an electrified future is the need to upgrade local power distribution systems. The need to upgrade regional transmission systems has gotten a lot of attention. The costs of upgrading local substations, wiring, and transformers have gotten less attention.
Recently, I sat down with Craig Spinale, the General Manager of Belmont Light — the municipal utility that distributes power in Belmont. He gave me a very rough sense of the kinds of investment that would be needed to support full electrification of heating in the town. He was kind enough to allow me to publish these notes from our conversation
Belmont’s Power Distribution System 101
Belmont Light serves 10,949 residential meters and some commercial, industrial, and municipal meters.
Belmont Light receives power through a 115KV line coming in from Cambridge to the substation at Blair Pond. At that substation, two 60MVA transformers step the voltage down to 13KV. The highest summer peak load experienced has been approximately 36MVA. While there are two transformers that could theoretically handle a load of 2 x 60MVA or 120 MVA together, that would not be an ideal condition because if one went down, a significant part of the town could be out for an extended period — the idea is to keep one transformer in reserve.
From that substation, ten 13KV lines emanate across the town. They can carry 8MVA each. Five of the lines go out across the town and directly serve local transformers. Another five of those lines feed older substations that step down the voltage further to 5KV and deliver power to areas of the town are still on the 5KV service. It is a work in progress to eliminate the old substations and directly serve the whole town with 13KV lines.
Homes in the town need 240/120 volt service. So, there are additional transformers that tap into the main lines (at either 13KV or 5KV) and step the voltage down to 240/120 service. There are approximately 900 of these transformers serving the 10,949 residential units in the town. The current standard size transformer is 50KVA and with traditional load expectations of up to 3KVA per home, they can handle 10 or 20 homes. However, many of the transformers in town are older and smaller at 25KVA and can handle only 5 or 10 homes. The traditional assumption was that homes would not simultaneously draw more than an average of 3KVA. With electrification, expected load my run much higher and with battery and solar power discharge available, the load may go in either direction.
Roughly 2/3 of the feeder lines (13KV or 5KV) distributing power around the town are in underground conduits.
Upgrades required for full electrification.
- Approximately $5,000 or 10,000 per home for transformer upgrades. At the level of the individual home, with a combination of heat pumps, batteries, solar and electric vehicles, many homes are choosing to upgrade to 200 or 400Amp services because their home could draw several dozen KVA or could send several dozen KVA back to the grid. Especially for a 400Amp circuit, the home may need a dedicated transformer; the home will certainly need to be on a transformer serving fewer homes. The cost of installing a dedicated transformer may be up to $10,000 or even more (excluding the roughly $5,000 panel upgrade that might be required inside the home). Depending on where the existing transformer is located, it may be necessary to reconfigure the connections from the 5 or 13KV feeder lines along the block. A new transformer could also force the replacement of the pole that would carry it. Replacements of transformers in areas where power is underground may involve additional costs to create appropriate manholes to install the switches between the transformer and the feeder lines. Currently, BMLD usually bears some of the cost of the transformer upgrade but charges the customer for much of it.
- Approximately $100,000,000 for an additional substation. It is expected that in a fully electrified home, the winter heating load will peak at roughly 3x the current max load for air conditioning. During a cold snap, all heat pumps will need to be running at simultaneously for an extended period with limited potential for demand management after the first day or so of the snap. That would suggest a peak load approaching 100 MVA. The current substation cannot handle more than 60MVA with full reliability. Additionally, the ten lines coming out of the substation can only handle approximately 80MVA (10x8MVA). To meet the new peaks and to provide appropriate reliability and redundancy it will likely be necessary to bring in a new 115KV transmission line to a substation at the west end of town. The recent new east-end substation cost approximately $100,000,000. The need for an additional substation is a decade or two away.
- Other potential costs. The 13KVA feeder lines running around the town may need upgrades where they are underground. Lines on poles can dissipate heat easily, but underground lines need to be sized to stay cooler.
With 10,949 homes in town, this cost of upgrade could work out to $20,000 per home — roughly $1000 per year per household for the next couple of decades if the costs were socialized across the rate base. All of these numbers are very approximate, and could be too low or too high.