By Philip Reed
In September 2021, Rocky Mountain employees placed a mobile transformer into service at Kimball Substation in Kimball County, Nebraska. This month, they plan to take it out of service. During that seven-month period, the substation underwent a significant construction stage addition, with the mobile transformer ensuring minimal interruption of service.
“A mobile transformer was required because we needed to keep providing High West Energy customers with power," said Electrical Engineer Greg Johnson, who managed the project. “There were upgrades that we needed to make to Kimball Substation and other upgrades that we wanted to make. But that's not something you can just do when you have customers relying on the service that the substation provides."
WAPA has access to several mobile transformers, which the organization retains for this exact purpose. Upgrades can only go so far while a substation is energized, and de-energizing a substation can leave customers without power. Mobile transformers can be placed into service to bridge that gap.
“WAPA prides itself on providing reliable service," Johnson continued. “Mobile transformers allow us to do that while also increasing the quality of that service."
Kimball serves as a tap substation on the Archer-to-Sidney 115-kilovolt transmission line, which provides power to High West Energy and the City of Kimball. A new connection to Kimball Wind necessitated the upgrades.
“Adding a 30-megawatt windfarm to the end of the six-mile-long City of Kimball 115-kV feed required the sectionalization of the Archer-to-Sidney line," explained Johnson. “Thus, the upgrade project involved replacing the 115-kV taps with a four-breaker ring bus, providing greatly improved protection for the customers on the transmission line."
Placing the mobile transformer into service allowed the power to stay flowing while replacing the taps and realigning the existing transformer to its own breaker bay in the substation's new 115-kV yard. It also allowed for some additional upgrades to happen concurrently.
“The existing control building could only hold three control panels, and it was already full, so we included a new control building in the scope of the work," said Johnson. “This required rewiring and recommissioning the existing transformer and 34.5-kV breakers."
Maintenance employees took the opportunity to replace breakers that were nearing the end of their expected service life, something that was made both easier and less disruptive thanks to the mobile transformer.
“It involved several customers and quite a bit of coordination by our dispatchers in getting all the pieces together," said Electrical Engineer Brian Bucks.
The team made sure that the coordination would be as thorough and the project as non-disruptive as possible.
“We've held meetings every three weeks with High West Energy for the past year," Johnson explained. “We worked with the City of Kimball and Kimball Wind through the typical outage request process, as well."
The mobile transformer seemed like it could have been a perfect solution, but there was one complication.
“We moved the mobile transformer to the site, but during initial testing, one 115-kV bushing tested bad," said Bucks. “That's why we test things ahead of time, though. It's all part of the process."
The mobile transformer was moved to the field office in Gering to replace the bushing and then a reclaimer was used to vacuum fill the transformer. Upon its return to Kimball Substation, the crew performed another complete battery of tests to ensure that it was ready. Electricians made sure to test all other relevant equipment as well.
“We then energized the mobile transformer a week before load was placed on it," Bucks explained. “That allows it to 'soak,' allowing any problems to be identified and corrected before placing customer load on the mobile transformer."
Connecting load to the mobile transformer required an eight-hour outage to High West Energy, the City of Kimball and Kimball Windfarm. This was necessary for the upgrade to proceed safely, and the significant communication of the outage beforehand helped to minimize disruption.
“Thanks to some excellent coordination, we had what can be called a 'mutually beneficial outage,'" said Johnson. “Service was interrupted only briefly, and we were able to implement a number of necessary and helpful upgrades."
The overall upgrade work took place across several years – design began in 2019 – and the mobile transformer allowed the vast majority of it to be completed without significant service interruption.
Note: The author is a public affairs specialist.