Temporary transformer utilized at Basin Electric's substation

The 345/230-kilovolt (kV) transformer at the Sidney, Nebraska substation, part of the Missouri Basin Power Project (MBPP), has been deemed unrepairable.

After the transformer failed and was taken out of service, an internal winding fault was identified as the cause, signaling the end of its 46-year lifespan.

“We sample oil annually from all our transformers and perform a dissolved gas analysis on them. This test looks for combustible gases that, depending on their types and ratios, can indicate various internal problems in a transformer,” said Derik Johnson, manager of Transmission System Maintenance (TSM). “During the annual oil test, the sample from the Sidney transformer revealed a severe high gassing event, prompting us to take the transformer out of service.”

The gassing event indicated severe internal arcing, leading the TSM team to conduct a series of internal electrical tests and a thorough inspection.

“Despite extensive testing, inspections, and consultations with industry experts and peers, we were unable to locate the source of the arcing, pointing to an internal fault within the transformer’s windings,” said Johnson.

Once the failed transformer is removed mid-summer, the spare MBPP 345/230-kV transformer will be relocated from Laramie River 345-kV substation in Wheatland, Wyoming to the Sidney 345-kV substation near Sidney, Nebraska. While the spare transformer, which was purchased in 2012, is not an exact match for the failed one, several departments at Basin Electric have been working diligently to develop and execute a plan to have it energized by August.

In the future, an evaluation will be done to determine the need for a new transformer to replace the spare transformer.