CITY COUNCIL AGENDA ITEM
ACTION REQUESTED:
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Accept the record of Emergency Procurement 24-255, Tollway Substation Power Transformer (30xa) Evaluation, to Power Substation Services for an amount of $58,697
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DEPARTMENT: Electric Utility
SUBMITTED BY: Brian Groth, Director
BACKGROUND:
The Electric Utility (Utility) is responsible for maintaining electric service for customers throughout the City of Naperville. A key component of the substations are transformers that step the 138kV transmission voltage down to lower voltages for distribution. At the Tollway Substation, the City owns a three-winding transformer, known as 30XA. The unit is filled with 14,000 gallons of mineral oil that serves as both electrical insulation and cooling. This transformer serves distribution circuits and customers, but also serves the 34kV Sub-Transmission network that feeds smaller substations (Indian Hill and Washington).
The transformer tripped offline during a 12kV bus fault on September 7, 2024. After clearing the bus and disconnecting the transformer from both 12kV and 34kV loads, an attempt to re-energize the transformer resulted in another trip after three seconds. Subsequent analysis of the oil through laboratory testing showed an alarming increase in combustible gases, including acetylene, suggesting internal arcing within the tank of the transformer. Further electrical testing of the unit resulted in specific evidence of a secondary winding to ground failure on one of the 12kV windings. A winding to ground failure would be fatal to the transformer.
Due to redundancy in design of the system, the loss of this one transformer has not created any extended ongoing outages; however, the Utility is now in a backup contingency configuration. This means that a critical system element is out of service, and staff must put stress on the backup/alternate equipment to maintain service to customers.
Under peak loading conditions, or if another critical component of the sys...
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