Our Electric Transmission and Distribution System

We serve a vast, rural area where we have more poles than customers.

What's the difference between transmission lines and distribution lines?

  • Transmission lines move electricity across long distances at high voltages.
  • Distribution lines take electricity to individual homes, businesses or neighborhoods at lower voltages.

Understanding the grid

A graphic showing transmission lines, substations, distribution lines and service lines.

Understanding the grid

You can think of the electric grid like roads. Our transmission lines carry lots of electricity long distances and are like interstates. Substations are like off ramps, connecting interstates to smaller roads. Distribution lines are like secondary highways, and service lines are like neighborhood roads or driveways.

Our 500kV Transmission System

Crews work on the 500kv transmission lines.

Our 500kV Transmission System

NorthWestern Energy is a 31% owner in the 500-kilovoltt transmission line that runs 250 miles from Colstrip to markets in Montana and further west. This line is critical for delivering reliable energy to  industrial, retail and residential customers.

A 2018 study found the line is especially important as additional Montana generation is slated to be shuttered. The study confirms that if the transmission line is decommissioned, NorthWestern Energy will have to make significant investments to handle peak customer energy needs, and to import enough energy from the West to provide reliable service to Montana customers.

Open Access Same-Time Information System

Transmission reservations for service over the Montana system occur on NorthWestern Energy's Open Access Same-Time Information System (OASIS). Transmission service for both wholesale and deregulated retail access are provided under the FERC electric Open Access Transmission Tariff (OATT) located on the NorthWestern Energy OASIS site under the "Tariffs / FERC Filings" folder located on the left hand side of the OASIS page. Formula Rate postings for transmission service under the Montana OATT are also available in the “Tariffs / FERC Filings” folder on OASIS.

For more information about our Electric Transmission Services, you may contact us via our Customer Service page or call (406) 497-4500.

FERC Order 890 requires transmission providers to post on OASIS all of their business rules, practices, and standards that relate to transmission services provided under the pro forma OATT.

NorthWestern Energy's OASIS site

South Dakota and the Southwest Power Pool

On October 1, 2015, NorthWestern Energy joined Southwest Power Pool, Inc. (SPP), and transferred functional control of eligible 69-kV and 115-kV South Dakota transmission facilities.

SPP is a Regional Transmission Organization that serves as a single Balancing Authority for interconnected electric utilities in 14 states. Our South Dakota qualifying facilities are part of the Upper Missouri Zone (UMZ or Zone 19) within SPP, and transmission service for wholesale is being provided under SPP's Open Access Transmission Tariff (OATT). Formula Rate postings for transmission service under the SPP OATT are available on the SPP Member Related Postings page.


Western Energy Imbalance Market

In June 2021, NorthWestern Energy joined the Western Energy Imbalance Market, operated by California Independent System Operator. The Western EIM is a real-time, within-hour energy market serving 10 states.

Since its launch in 2014, the Western EIM has been using advanced technology to find and deliver the lowest-cost energy to utilities. NorthWestern Energy’s participation in the Western Energy Imbalance Market will provide the company’s Montana customers with economically efficient energy to resolve imbalances and variations in load and generation on our Montana system.