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Foreword

(This Foreword is not a part of American National Standard Requirements for Ventilated Dry-Type Power Transformers, 501 kVA and Larger, Three-Phase, with High-Voltage 601 to 34 500 Volts, Low-Voltage 208Y/120 to 4160 Volts, ANSI C57.12.51-1981.)

This new dry-type transformer standard describes ventilated dry-type transformers with self-cooled kilovolt-ampere ratings 501 to 7500 and high voltages 601 to 34 500 volts; forced-air-cooled ratings for certain sizes are listed in Part II. (American National Standards Committee on Specialty Transformers, C89, has responsibility for transformers with high voltages 600 volts and below.) This standard is part of a new series of dry-type standards that also includes American National Standard General Requirements for Dry-Type Distribution and Power Transformers, ANSI/IEEE C57.12.01-1979; American National Standard Test Code for Dry-Type Distribution and Power Transformers, ANSI/ IEE C57.12.91-1979; American National Standard Requirements for Ventilated Dry-Type Distribution Transformers, 1 to 500 kVA, Single-Phase, and 15 to 500 kVA, Three-Phase, with High-Voltage 601 to 34 500 Volts, Low-Voltage 120 to 600 Volts, ANSI C57.12.50-1981; and American National Standard Requirements for Sealed Dry-Type Power Transformers, 501 kVA and Larger, Three-Phase, with High-Voltage 601 to 34 500 Volts, Low-Voltage 208Y/120 to 4160 Volts, ANSI C57.12.52-1981.

Dry-type transformers have been served in the past by a variety of documents, including American National Standard General Requirements for Distribution, Power, and Regulating Transformers, ANSI C57.12.00-1973; American National Standard Test Code for Distribution, Power, and Regulating Transformers, ANSI C57.12.90-1973; American National Standard Dry-Type Transformers for General Applications, ANSI/NEMA ST 20-1972; and NEMA Standards Publication for Commercial, Institutional, and Industrial Dry-Type Transformers, NEMA TR 27-1965 (R1976). The new standards are the result of an effort encompassing the interests of users, manufacturers, and others dedicated to developing voluntary consensus standards primarily for dry-type transformers. They have various significant changes, including higher BILs (for most voltages), more stringent and meaningful short-circuit requirements, improved descriptions of "usual" and "unusual" service conditions, and numerous other improvements. These new standards should be easier and more efficient to use since they are arranged according to the sequence in which information is normally needed. ANSI C57.12.51-1981 includes customary units and metric (SI) units in accordance with ANSI policy. Sound levels were discussed in the preparation of this standard, but are not included at present. As a matter of information, NEMA TR 1-1980, Transformers, Regulators, and Reactors, includes data concerning dry-type transformer sound levels.

This standard is a voluntary consensus standard. Its use is mandatory only when required by a duly constituted legal authority or when specified in a contractual relationship. To meet specialized needs and to allow innovation, specific changes are permissible when mutually determined by the user and the producer, provided such changes do not violate existing laws and are considered technically adequate for the function intended.

When this document is used on a mandatory basis, the word "shall" indicates mandatory requirements, and the words "should" and "may" refer to matters which are recommended and permissive, respectively, but not mandatory.

The applicable ANSI rules and procedures for the preparation and approval of voluntary consensus standards have been followed. These specify procedures for voting, review and attempted reconciliation of dissenting viewpoints, a 60-day public review period, and a final review and approval by the ANSI Board of Standards Review.

Suggestions for improvement of this standard will be welcome. They should be sent to the American National Standards Institute, Inc, 1430 Broadway, New York, N.Y. 10018.

This standard is intended to set forth characteristics relating to performance, limited electrical and mechanical interchangeability, and safety of the equipment described, and to assist in the proper selection of such equipment.

This standard describes certain electrical and mechanical characteristics and takes into consideration certain safety features of 60-Hz, two-winding, three-phase, ventilated dry-type transformers with self-cooled ratings 501 kVA and larger, generally used for step-down purposes. Forced-air-cooled ratings for certain sizes are listed in Part II.

Specific rating combinations are described in the range from 750/1000 to 7500/10 000 kVA inclusive, with highvoltage 2400 to 34 500 volts inclusive and low-voltage 208Y/120 to 4160 volts inclusive.

This standard does not apply to other types of transformers such as specialty, ventilated dry-type 500 kVA and smaller, sealed dry-type, pad-mounted dry-type, liquid immersed, instrument, regulating, furnace, mine, and rectifier transformers. Transformers with high voltage 600 volts or less are excluded.

When this standard is used on a mandatory basis, the word "shall" indicates mandatory requirements, and the words "should" and "may" refer to matters that are recommended and permissive, respectively, but not mandatory.

NOTE - The Foreword of this standard describes the circumstances under which the document may be used on a mandatory basis.

Part I of this standard describes electrical and mechanical requirements. Part Il describes other requirements or alternatives which may be specified for some applications.