IEEE C37.121-1989

1.1 This standard covers the requirements for three-phase unit substations for step-down operation in the range of 112.5 kVA through 10 000 kVA at primary voltages of 601 volts through 38 kilovolts. 1.2 The standard is intended for use as the basis for the coordination of equipment 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 (throat) sections. Since the transformer section determines the kVA and voltage capabilities of a unit substation, the ratings of the various types of unit substations are described in terms of transformer capability. 1.3 This standard does not cover the following installations: 1) Substations in which the transformer section includes load-tap-changing equipment 2) Substations in which the transformer section is described and defined as "network," "subway," "vault," or "underground" in American National Standards C57.12.24-1989, C57.12.40-1982, and C57.12.57-19871 3) Substations in which the transformer section is described and defined as "pad-mounted" in American National Standards C57.12.22-1989, C57.12.26-1987, and C57.12.271 4) Substations with gas-insulated equipment, such as substations insulated with SF6, described in ANSI/IEEE C37.122-19891 5) Rectifier-type substations 6) Mobile substations 7) Installations in ships, watercraft, railway rolling stock, aircraft, or automotive vehicles other than mobile homes and recreational vehicles 8) Installations for mines 9) 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. 10) Installations of communication equipment that is under the exclusive control of communication utilities, located outdoors or in building spaces used exclusively for such installations. 11) 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. 1.4 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, these sections shall meet any additional requirements imposed by this standard when they are used as part of a unit substation.