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IEEE C57.105

78th Edition, 1999

Complete Document

Guide for Application of Transformer Connections in Three-Phase Distribution Systems

Includes all amendments and changes through Reaffirmation Notice , March 27, 2008

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(This Foreword is not a part of ANSI/IEEE C57.105-1978, American National Standard Guide for Application of Transformer Connections in Three-Phase Distribution Systems.)

In the fall of 1968, a Working Group was formed by the Performance Characteristics Subcommittee of the IEEE Transformers Committee to prepare a user's guide for application of Y-Y transformers considering user systems, manufacturing capabilities, tank heating, ferroresonance and telephone interference. The scope of the proposed user's guide was subsequently expanded to cover the full range of transformer connections commonly used in 3-phase distribution systems with emphasis on those characteristics of connections which distinguish one from another, appear most prominently with asymmetrical system conditions, and affect the selection of connections.

The guide assumes the reader has an educational or practical background equivalent to that of a graduate electrical engineer with some knowledge of transformers and distribution practices. Recognizing from the start, however, that the guide would be most needed by and most useful to the engineer with relatively little experience, the Working Group has incorporated explanatory material where considered to be helpful.

The Institute wishes to acknowledge its indebtedness to those who have given so freely of their time and knowledge to the development of this guide.  


This guide concerns transformer connections in 3-phase distribution systems. Distribution systems are characterized by primary voltages up to and including 34.5 kV, usually have a preponderance of connected transformers with lowvoltage windings below 1000 V and furnish electric service to consumers. The characteristics of the various transformer connections and possible operating problems under normal or abnormal conditions are treated.

All combinations of Δ and Y, grounded and ungrounded, T connected, zigzag, and certain special connections are considered. Only two-winding transformers are included.

Phasing procedures and loading practices are not covered. Guides for loading of oil-immersed and dry-type transformers are available in publications of the American National Standards Institute: American National Standard C57.91-1974, Appendix to C57.12.00-1973, Guide for Loading Mineral Oil-Immersed Overhead-Type Distribution Transformers with 55°C or 65°C Average Winding Rise; American National Standard C57.92-1962, Appendix to C57.12 Standards, Guide for Loading Oil-Immersed Distribution and Power Transformers; and American National Standard C57.96-1959, Appendix to C57.12 Standards, Guide for Loading Dry-Type Distribution and Power Transformers, respectively.