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Massachusetts state regulators, in a Nov. 4 order, granted NSTAR Electric’s petition seeking approval to build and operate two new underground 115-kV transmission lines in South Boston to improve service reliability from the company’s Andrew Square substation in South Boston and its Dewar Street substation in Dorchester, according to TransmissionHub.
The two new lines, which would parallel the route of two existing underground transmission lines, would run from the company’s K Street substation on East First Street to the median of Columbia Road near its intersection with G Street in South Boston, a distance of just under one mile, the state Department of Public Utilities (DPU) said.
Currently, all load served by the Andrew Square and Dewar Street substations is fed by the two existing 115-kV underground lines from the K Street substation. The two existing lines connect to separate wye joints beneath the Columbia Road median near G Street. Each wye joint splits the incoming line into two segments; one segment of each incoming line proceeds to the Andrew Square substation and the other segment to the Dewar Street substation.
However, the DPU added, the capacity of the four line segments is limited to the capacity of the two lines entering the wye joints. Thus, if either of the two incoming lines was out of service, the Andrew Square and Dewar Street substations would share the capacity of the one remaining line to serve a total of 86,000 customers. The company asserts that the combined peak summer load of the Andrew Square and Dewar Street substations exceeds both the summer normal and summer long term emergency ratings of a single 115-kV line, the DPU added.
Under the company’s proposal, the two existing lines from the K Street substation would be connected to two new straight joints, creating two dedicated lines to the Dewar Street substation. The two new lines would be connected to two other straight joints, creating two new dedicated lines to the Andrew Square substation.
The project would result in a doubling of the capacity of the lines serving both the Andrew Square and Dewar Street substations. The project would also involve the installation of a new 115-kV breaker at the K Street substation to isolate one of the new lines and the existing autotransformer.
The company estimated that the project would cost $20.5 million.
Among the potential alternatives that the company considered were whether a program of targeted, accelerated energy efficiency measures could defer or avoid the need for the project. The company estimated that 1.8 MVA of incremental energy efficiency could be obtained each year in the future, but that load in the area will grow at an even higher rate. The company concluded that energy efficiency could not resolve the current or future capacity and reliability needs in the area served by the Andrew Square and Dewar Street substations.
NSTAR Electric has explored a reasonable set of alternatives, the DPU said, noting that the non-transmission alternatives offer only limited load relief throughout the forecast period and would not be adequate to improve the company’s ability to meet load during either N-1 or N-1-1 contingencies. Nonetheless, NSTAR should continue to encourage its customers to take full advantage of its energy efficiency programs and the company should, in general, also continue to explore creative ways to use non-transmission alternatives to avoid or delay the need for new transmission infrastructure, the DPU said.
The company hopes to begin project construction this fall, or within about two months of receiving DPU approval, and expects construction to last about nine months, assuming a six-day workweek, the DPU said.
The DPU said its review of the record in the case has identified numerous potential environmental impacts associated with the project. For instance, regarding construction schedule impacts, the record shows that the City of Boston Municipal Code limits construction to weekdays only. While the company is interested in seeking approval form the city to begin construction work at 7 a.m., on Saturdays, the DPU approved construction from 7 a.m., to 5 p.m., Monday through Friday and, with approval from the city, from 9 a.m., to 5 p.m., on Saturdays, excepting public holidays.
Among other things, the DPU said that because the issues addressed in the order relative to the project are subject to change over time, construction must begin within three years of the order’s date.
NSTAR Electric is a Northeast Utilities company.