Big Sky is experiencing significant growth with new housing developments, commercial developments and upgrades to all the amenities that make Big Sky what it is today. To accomodate and keep up with this high growth, the electrical infrastructure that serves the community needs to grow as well. Electricity usage in Big Sky is currently growing at about 8% per year. The existing 2 substations that serve the community have already been upgraded and will soon reach a level that they will not be able to support the electrical needs of the community during high usage times - which are during the coldest days of the year. In addition to meeting the current and expected growth needs, a third substation is also needed to enhance the reliability of service for the whole community and will provide needed backup to the existing substations in the event of an outage.

Please see PDF "Midway Substation Alternative Sites Evaluation" at the bottom of the questions

As part of our site selection and permitting process with Gallatin County, we engage professional 3rd party visual simulation experts to accurately simulate or ‘render’ what the development will look like from multiple viewpoints and perspectives. Professional visual simulation expertise is necessary to accurately model the existing landscape before and after the facility is constructed and to scale.

Northwestern has not purchased any property for the substation. As part of our normal course of business seeking properties for business needs, we regularly seek option agreements with willing sellers for select properties. Option agreements are a low risk approach to reaching a purchase agreement for suitable properties that allow time for due diligence analysis and permitting without obligating the purchase. NorthWestern has reached an option agreement with the seller of the property on Sleeping Bear Road and is in discussions with the owner of the Rainham property.

The total ‘footprint’ of the new substation is expected to be approximately 2 acres in size. The size of the parcel necessary to accommodate the 2 acre footprint is dependent on the topology, slope, shape, and location of available land parcels but NorthWestern anticipates needing a minimum 5 acre parcel to accommodate the development with access roads, setback buffers, and site screening needs.

There are many mitigation or ‘disguising’ options available and will vary dependent upon the location selected and requirements. The first and key mitigation is location and natural screening features around the development such as topology, trees and landscaping relative to various view sheds. Other options available include unique landscaping, decorative walls and other naturally conforming features and even tree planting offsite on other properties if necessary with some landowners. NorthWestern anticipates natural screening and strategically placed and planted trees will largely mitigate view shed concerns. We are also willing to entertain potential community improvement amenities such as walking paths and access depending on the amenities and ability to provide these benefits with the final property selected.

NorthWestern anticipates constructing a state of the art conventional substation with industry standard air cooled equipment that relies on the natural open air environment for safe and reliable equipment operation and cooling. This type of facility is a proven, cost-effective, and reliable solution and can be designed and constructed to fit with the landscape. Although there are many ways to mitigate the visual impact, this type of facility cannot be enclosed or built underground. There are some substation technologies that utilize gas-insulated equipment (GIS for short) that can be housed indoors in a large building for unique applications. Although a GIS substation might reduce the footprint and the presumed visual impact of the facility, GIS substations are very expensive, and cause significant operating and maintenance concerns for NorthWestern in the high elevation and cold climate of Big Sky. In addition to the significantly higher cost and operating concerns of GIS applications, the Sulfur Hexafluoride (SF6) gas used in large volumes for this application is a major greenhouse gas contributor. Consideration of an indoor enclosed GIS substation will require additional detail review and analysis of site specific designs, operating & maintenance analysis, cost estimate comparisons, consideration of who pays the additional cost burden and perhaps a cost prudency review with the Montana Public Service Commission.

The depreciable life of this substation would be approximately 40 years.

A GIS or gas insolated substation performs the same function as a traditional or air insolated substation (AIS). A GIS substation is designed so that some (the switchgear) or many (the switchgear and transformers) of the components of the substation are enclosed in metal and insolated with sulfur hexaflouride (SF6) gas instead of using the open air for temperature regulation. It's important to know that the exact differences between a GIS and AIS system will be unique to each site-specific design and application. Generally, GIS systems are significantly smaller in footprint and height than traditional or AIS substations. It is also possible to enclose much or all of the substation componentry in a building, allowing for the use of architechtural designs consistent with community expectations and the landscape to further mitigate the visual impact of the substation. The construction of a GIS substation is, however, significantly more expensive than traditional, air insulated technology and the additional costs would need to be managed responsibly and fairly. Although GIS technology has proven to be quite reliable and SF6 gas is nontoxic, it is one of the most powerful greenhouse gases, and must be handled accordingly. Several participants in the small-group meetings that took place over the summer of 2017 raised the possibility of constructing a GIS facility as a way of mitigating the potential adverse impacts of a traditional facility. NorthWestern energy has committed to assessing the feasibility of GIS for the mid-mountain substation. As the results of that assessment become available, they will be shared here.