IEEE Standard Requirements for Liquid-Immersed Distribution Substation Transformers

Small power transformers have become a significant element in distribution systems supplying large commercial customers like major resort hotels and site-specific industrial customers that desire the local utility to own, operate, and maintain the serving transformer. These transformers can range in sizes from 112.5 kVA to 10 000 kVA with primary voltages at 69 000 V and below and secondary voltages from 34 500 V to 120 V. Transformers in this standard are generally for larger distribution customers often with special voltages or installation requirements like convention centers with large chiller plants and extensive exhibit space. There is often a desire to serve these transformers from underground systems using side-mounted bushings on the primary. This standard seeks to define the small power transformer that is applied as more than just a limited-scope version of the power transformers covered by IEEE Std C57.12.10(TM) and as more than a large distribution-class transformer covered by IEEE Std C57.12.34(TM).