The Western Electricity Coordinating Council (WECC) has notified NorthWestern Energy that the Mountain States Transmission Intertie (MSTI) has been approved to carry up to 1,500 MW of electricity north-to-south and up to 1,100 MW south-to-north.

WECC's Three Phase Path Rating Process is a comprehensive technical planning study that defines the transfer limit that a new transmission line (or lines) can achieve without interfering with the reliability or stability of other lines in the western interconnection. In addition to technical modeling, the path rating process requires a peer review by all interested transmission providers and other stakeholders.

The company was recently notified that the project has received "Phase 3" status, which means that the study has been concluded, the path rating established and MSTI can be constructed with the approved rating. Phase 3 concludes when the project is placed into service.

"This is an important project milestone that has been several years in the making," said Mike Cashell, vice president – Transmission. "We still have much work to do before MSTI is in operation, but this and other recent studies affirm the regional benefit of this project to spur new energy development and alleviate congestion on the existing system."

The WECC recently published a 10-year transmission planning study and the Northern Tier Transmission Group (NTTG) recently published its biennial planning report, both of which look at what the grid may look like in 2020 under different operating conditions. Both reports found that new transmission lines are needed throughout the west to accommodate demand for proposed renewable energy development. Both studies also found benefit (mostly in the form of congestion relief) should transmission be built to accommodate renewable energy development in Montana.

MSTI is still working its way through the siting process with the Bureau of Land Management and Montana Department of Environmental Quality. While the path rating is independent of the siting process, the path rating study did evaluate the "common corridor" approach to what has been termed the "northern route alterative" that may allow MSTI to more closely parallel the existing BPA double-circuited 500kv towers from Townsend to Deer Lodge. The rating was affirmed for all of the potential alternative routes, including the scenario that places MSTI in a common corridor with the BPA line.