Substation Transformer Replacement Project

Substation Transformer Replacement Project

Project Manager

Electric Utility / Curt Schmidt, Generation and Transmission Superintendent

What We're Doing

Electric substations take high voltage power from the transmission grid and reduce it to a distribution voltage level for delivery to homes and businesses.  In Stillwater, electricity is delivered to the City via high voltage transmission lines at 138,000 and 69,000 volts.  The electricity is fed into substation transformers where it is reduced to 12,470 volts, then delivered via the City’s electric distribution system to a distribution transformer near the customer which steps down the voltage to 120 or 240 volts for individual consumer or business use.

The Stillwater Electric Utility (SEU) currently operates eight electric substations.

This project will focus on the transformers located at three of those substations which have reached the end of their reliable service life and are in need of replacement.  These transformers are the oldest in the fleet having been installed between 1967 and 1975. Parts availability and reliability are becoming problematic and prompting the need for replacement.   Substation transformers take approximately 8-12 months to manufacture and deliver.  Rather than waiting for the transformers to fail, SEU is taking a proactive approach by replacing the transformers to ensure reliability and grid stability.  

While each of these substations have been modernized and rebuilt in the last 20 years, the transformers were not replaced as part of those projects.  Due to budget constraints it was determined that these large transformers would be changed out at a later date.  Now it is the time to create a project to replace them.  These transformers are the heart of the substation.  They are about the size of a small school bus and weigh approximately 100,000 pounds.


Increase in capacity: The three substations each house two transformers which have ratings ranging from 14 Million Volt-Amps (MVA) to 20 MVA.  The MVA rating can simply be explained as how much power the transformer can supply to the distribution system for use by the customers.

When Ferguson Substation was rebuilt as a part of the Stillwater Energy Center project, the transformers were upsized to 28 MVA to add more capacity to the system.  The replacement at Ferguson marked the establishment of a new standard transformer size.  The new higher capacity transformers will enable each substation to meet the projected future load demands as well as increase reliability of the substation.

It is not uncommon for a transformer to go offline when heavily loaded during the summer peak season.  In the past these situations typically resulted in outages while a portion of the load had to be manually diverted to other substations in the system because the second transformer in the substation would not have the capacity to meet the full load of the entire substation.

Standardization: With the transformer replacement project staff is standardizing on a single transformer manufacturer.  Standardization of this large equipment is an essential part of an efficiently run system, especially with the very unique equipment associated with each transformer.  As described in previous reports, SEU has been standardizing substation equipment because it is the key to increasing reliability and minimizing repair time.  Smaller parts inventories and fewer special service tools mean lower lifecycle costs when a utility standardizes their substation transformer fleet.   


Under Construction Timeline Graphic

Project Timeline: Taking into consideration funding constraints, scheduling substation outages, and the long lead times for delivery of equipment, staff anticipates that the project will be completed in late spring of 2020.  The project will be executed in three phases:

  • Phase One: Boomer Lake Substation. Began construction March 12, 2019 with expected completion in the late May of 2019. 

  • Phase Two: Hospital Substation. Scheduled to begin in the early fall of 2019 with expected completion in the early spring of 2020.

  • Phase Three: Central Substation. Scheduled to begin in the late fall of 2019 and expected to be completed in the late spring of 2020.


March 12, 2018: Stillwater Utilities Authority Trustee’s approved the substation transformer replacement project (SUA-18-10).

August 27, 2018: Stillwater Utilities Authority Trustee’s approved the bid recommendation for 6 transformers from Pennsylvania Transformer (SUA-18-34).

December 17, 2018: Stillwater Utilities Authority Trustee’s approved the bid recommendation for the construction contractor services to Sayers Construction, LLC at Boomer Lake Substation (SUA-18-41).

March 11, 2019: Sayers mobilized to Boomer Lake Substation site and began work. 

March 27, 2019:

  • Sayers has completely removed the two 20MVA power transformers from the project site.
  • Sayers has removed the underground feeder cables and duct bank from the risers in the substation to the adjacent switchgear.
  • Sayers has removed all foundations with the exception of one circuit breaker pad.
  • Remainder of the contractor furnished materials will be delivered to site this upcoming week.
  • Sayers is currently forming transformer and circuit breaker foundations in preparation to pour concrete along with the installation of underground conduit routes.

May 13, 2019:

  • Sayers has extended the transformer foundation pad for the West transformer foundation.

  • Sayers has installed new conduit for the transformers and their associated Circuit Breakers.

  • Sayers has constructed new Cable duct banks where multiple conduit runs are required.

  • A new oil spill containment structure has been constructed around the foundations

  • Site cleanup is ongoing in preparation to receive new transformers.

  • Sayers has installed new equipment grounding leads, and have connected these to the existing substation grounding grid.


May 14, 2019: Stillwater Electric Utility received one of the two substation transformers replacing older versions at the Boomer Lake substation Tuesday. The second transformer will arrive later this week. This transformer is the first of six substation transformers ordered to replace aging transformers at other distribution substations.