Studies and Reports > 2009 MN Biennial Report > NE Planning Zone > Needs: 6.3.9 Duluth Area 230 kV Project

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6.3.9 Duluth Area 230 kV Project

Tracking Number. 2007-NE-N1

Utility. Minnesota Power

Inadequacy. During 2007, Minnesota Power experienced some difficulty scheduling maintenance outages in the Duluth area due to concerns associated with overloading of the Arrowhead and Hilltop Transformers as well as 230 kV line overloads. This has been and continues to be managed by scheduling maintenance during light load periods. The addition of the Arrowhead-Weston 345 kV line (energized in January of 2008) and associated Stinson Phase Shifter operating changes has reduced the number of hours this is an issue. As Duluth area load continues to grow, this concern will reach critical levels and additional transmission will be required.

A map of the area is shown following the discussion.

Alternatives. The only alternative under consideration is the rebuild of an existing 115 kV line to the Hilltop Substation to 230 kV capability.

Analysis. In 1993, Minnesota Power constructed a new 230 kV substation (the Hilltop Substation) in Duluth. This project involved the rebuilding of existing 115 kV lines for 230 kV operation in order to provide a single 230 kV source to the Hilltop Substation and upgrades of several unshielded 115 kV lines to improve reliability. As part of the application for the Hilltop project MP laid out long range plans which identified the future need for a second 230 kV source to the Hilltop substation once Duluth load dictated its need. The Commission recognized this future need and approved rebuilding of portions of the unshielded 115 kV lines as part of the Hilltop Project for future 230 kV operation.

Because Minnesota Power anticipated this future need, only approximately three miles of line construction will be required to provide a second 230 kV source to the Hilltop Substation. The majority of this construction will involve rebuilding an existing 115 kV line. Due to the configuration of the existing Duluth area transmission system and need to provide a second 230 kV source to the Hilltop Substation, no other alternative to this project will provide a cost effective or reasonable solution to this pending inadequacy. Other transmission alternatives would require longer 230 kV line construction and increase both social and economic impacts associated with construction of such a line, and distributed generation is not preferable from either a cost or operational standpoint to the preferred project.

Minnesota Power is continuing to monitor line loading, voltage support and load growth in the Duluth area.

Schedule. Minnesota Power is in the process of updating previous studies of the Duluth area transmission system. The addition of the Arrowhead to Weston 345 kV line allowed the flows through the Stinson Phase shifter to be reduced, which increased load serving capability in the Duluth, Minnesota, and Superior, Wisconsin, areas. This increased load serving capability has delayed the need for new 230 kV transmission and 230/115 kV transformer capacity in the Duluth/Superior area. Based on the preliminary results of this updated analysis, a Certificate of Need application is not expected to be submitted until the 2015-2019 timeframe. However, this is subject to change if loads increase at rates greater than expected.

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