IEEE C37.121-2012

This guide covers three-phase unit substations for step-down operation in the range of 112.5 kVA or greater at primary voltages of 601 V through 38 kV. This guide does not cover the following installations: a) Substations in which the transformer section includes load-tap-changing equipment. b) Substations in which the transformer section is described and defined as “network,” “subway,” “vault,” or “underground” in IEEE Std C57.12.24™ [B14] and IEEE Std C57.12.40™ [B18]. c) Substations in which the transformer section is described and defined as “pad-mounted” in ANSI C57.12.22 and IEEE Std C57.12.27™ [B16]. d) Gas-insulated substations as described in IEEE Std C37.122™ [B13]. e) Rectifier-type substations. f) Mobile unit substations. g) Installations in ships, watercraft, railway rolling stock, aircraft, or automotive vehicles. h) Installations for mines. i) Installations of railways for generation, transformation, transmission, or distribution of power used exclusively for operation of rolling stock, or for installations used exclusively for signaling and railway communication purposes. j) Installations of communication equipment that is under the exclusive control of communication utilities, located outdoors or in building spaces used exclusively for such installations. k) Installations under the exclusive control of electric utilities for the purpose of communication, or metering; or for the generation, control, transformation, transmission, and distribution of electric energy located in buildings used exclusively by utilities for such purposes or located outdoors on property owned or leased by the utility or on public highways, streets, roads, etc; or outdoors by established rights on private property.

The guide is intended for use as the basis for the coordination of equipment in unit substations by assisting in the selection of components. A variety of designs for unit substations are possible using various combinations of incoming sections, transformer sections, outgoing sections, and transition sections. It is intended that the incoming, outgoing, transformer, and transition sections included in a unit substation shall meet the basic requirements of applicable industry standards for those sections. In addition, this guide provides suggested requirements when used as part of a unit substation.

Revision Standard - Superseded. The basis for the coordination of equipment in unit substations by assisting in the selection of components is intended as the use of this guide. A variety of designs for unit substations are possible using various combinations of incoming sections, transformer sections, outgoing sections, and transition sections. It is intended that the incoming, outgoing, transformer, and transition sections included in a unit substation meet the basic requirements of applicable industry standards for those sections. This guide covers three-phase unit substations for stepdown operation in the range of 112.5 kVA or greater at primary voltages of 601 V through 38 kV.