Introduction

POWER Engineers was retained to prepare a Final Siting Study and Preliminary Engineering Report in 2006. The Report was undertaken to evaluate the options of 230kV, 345kV, and 500kV alternatives. In addition, a direct current (DC) alternative was evaluated. End points were identified by NorthWestern Energy for each of these options, and a siting study was conducted to determine preferred and alternative routes for each. Likewise, alternative sites were identified for a new substation at Townsend and at Ringling, and preferred sites were identified for each.

This stage of the process is referred to as a Facility Study, and includes further analysis of alternative projects, including identifying alternative routes, a preferred route, various voltage options, AC vs. DC, design criteria, conductor and structure selection, and estimated costs for the various options. Transmission line cost estimates were prepared for each of the preferred routes for each voltage option.

Siting and Routing

The Project study area was defined to include feasible alternatives for the location of a 500kV transmission line, or alternatively a 230kV or 345kV line. The size of the study area is approximately 25,000 square miles. Major physiographic features, jurisdictional boundaries, sensitive land uses and existing utility corridors helped to define the study area boundaries, representing the limits of reasonable or feasible transmission line alternatives for the Project. The study contains portions of southwestern Montana and eastern Idaho. It includes parts of seven National Forests and 9 Field Offices of the Bureau of Land Management (BLM). Approximately 20% of the study area is National Forest System lands, 25% are public lands managed by the BLM, 27% are managed by the states of Montana and Idaho, and 28% of the lands are privately-held.

Existing land use and environmental data was collected and mapped within the study area, and constraints and opportunities were subsequently determined and mapped to assist with the identification of possible alternative routes and substation sites for the various voltage options. Corridors were first identified for each of the voltage options that would avoid or minimize the crossing of sensitive land uses and environmental constraints to the extent possible. Within these corridors the study team used aerial photography to help identify assumed route centerlines. The resulting network of alternative routes were given identifiers and a preliminary analysis of impacts was determined by using the collected and mapped data. Routes were then compared for each of the voltage options, and preferred routes were recommended.

Environmental Review

The Selected Route would cross 11 miles of National Forest lands, and as such, would require a Special Use Permit from the USDA Forest Service. Forest Service would likely be a Federal Cooperating Agency for the environmental review under the National Environmental Policy Act (NEPA), the Federal law requiring environmental review for Federal decision making. This route would also cross 110 miles of public lands managed by the BLM, and establishing the route on public lands would require a grant of right of way and environmental review under NEPA. Crossing of the INL would similarly require a grant of right of way from the BLM, but would be subject to review by INL for compliance with their mission and resource plans. INL, an agency of the Department of Energy, would be a Federal Cooperating Agency.

The project in Montana would require a Certificate of Compatibility from the State of Montana and compliance with the substantive requirements of the Major Facility Siting Act. An application, once completed, requires environmental review under the MEPA, and Montana DEQ would be the State Lead Agency for such review. The Montana DNRC would be a State Cooperating Agency for crossings of State Trust Lands they manage for the benefit of schools in Montana.

Townsend to Borah
Alternative Routes
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Joint environmental review (i.e., NEPA and MEPA) between the Federal agencies and the State of Montana will be done with a single Environmental Impact Statement (EIS), one that will meet the standards of both. The State of Idaho has no corresponding environmental law, and will rely on the Federal environmental review and normal permitting requirements. It is possible that the FS and the BLM will be Federal Co-Lead Agencies, but a more likely scenario is for the BLM to take the lead.

The MFSA Application must be prepared and accepted as complete prior to the MEPA process starting, a process that essentially is environmental review of the project proponent’s application. MFSA requires that the application be prepared to substantive standards outlined in the Administrative Rules of Montana. Some public scoping is normally required to meet the MFSA requirements, and this could include public meetings and/or agency consultations and elected official briefings. MEPA and NEPA requirements are procedural, which means that issues to be addressed in the environmental review process are defined during the public and agency scoping process.

Design and Construction

Four different alternative voltage lines were investigated for a transmission line from north central Montana near to an interconnection point on Idaho Power’s system in the south:

• 230kV line capable of 450MW of power transfer
• 345kV line capable of 650MW of power transfer
• 500kV AC and DC alternatives capable of 875MW of power transfer

For the three high voltage alternatives, a new Townsend substation would be created that would cut the existing Colstrip transmission lines in and out of the station as well as the new line south. The location of this proposed station is near the ownership boundary of the Colstrip lines between BPA and Northwest Energy.

The 230kV alternative would terminate at Northwest Energy’s existing Mill Creek Substation and traverse south parallel to existing Northwest Energy and Idaho Power transmission lines to Idaho Power’s Borah Substation, a distance of approximately 270 miles. Both stations currently have 230kV service and would require additions to the existing bus. A phase shifting transformer at Townsend and series compensation stations at Townsend and a middle point on the line are assumed within the estimate.

The 345kV alternative would terminate at the new Townsend substation and traverse southwest to a location near the Mill Creek Substation and south parallel to existing Northwest Energy transmission lines to Idaho Power’s Borah Substation, a distance of approximately 301 miles. Borah Substation currently has 345kV service while the new Townsend Substation would require transformation between the Colstrip lines and the new line south. The 345/500kV transformer, a phase shifting transformer, and series compensation stations at Townsend and a middle point on the line are assumed within the estimate.

The 500kV AC alternative would terminate at the new Townsend substation and traverse along the same path as the 345kV estimate to Idaho Power’s Borah Substation, a distance of approximately 301 miles. Borah Substation currently has no 500kV service. It is assumed that Idaho Power would construct a 500kV yard at Borah for receipt of this new line. The new Townsend location would be a switching station with five terminals for the two Colstrip lines in and out and the new Borah line south. A phase shifting transformer and a series compensation station at Townsend and a middle point on the line are assumed within the estimate.

The 500kV DC alternative would terminate at the new Townsend substation and traverse along the same path as the other lines but turn west at a location north of the Borah Station proceed to Idaho Power’s Midpoint Substation, a distance of approximately 390 miles. Midpoint Substation currently has a 500kV section of the station. DC to AC converter stations are assumed between the new line and the AC bus at both ends of the project. The new Townsend location would be a switching station with five terminals for the two Colstrip lines in and out and the new Borah line south.

The voltage analysis study conculded that the 230 kV/345 kV/500 kV DC alternatives were not viable alternatives. The 500kV alternative was selected as the most viable line voltage route.