Mate

Transformers:
Basics, Maintenance,
and Diagnostics

U.S. Department of the Interior
Bureau of Reclamation

April 2005

Transformers:
Basics, Maintenance,
and Diagnostics

U.S. Department of the Interior
Bureau of Reclamation
Technical Service Center
Infrastructure Services Division
Hydroelectric Research and Technical Services Group
Denver, Colorado

April 2005

PREFACETransformers have been used at powerplants since the inception of
alternating-current generation, a century ago. While operating
principles of transformers remain the same, the challenges of
maintaining and testing transformers have evolved along with
transformer design and construction. Modern transformers are
designed to closer tolerances than transformers in the past. Thus,
effective,regular maintenance and testing is even more essential to
continued operation when traditional “overdesign” cannot be relied on
to overcome abnormal conditions. The utility engineer must be
familiar with all aspects of maintenance and testing and make use of
state-of-the-art tools and techniques for evaluating transformer
condition. While on-line diagnostic systems and computerized testingmethods are very helpful, they are not a substitute for sound
engineering judgment and expertise.
This volume provides timely, practical advice to those seeking to
better understand how transformers work, how they are best
maintained, and how to test and evaluate their condition. It has been
developed with the assistance of Bureau of Reclamation engineers
responsible for operating and maintainingtransformers at important
powerplants in the Western States. Support and funding was provided
through the Reclamation Power Resources Office in Denver and via
the Manuals and Standards development program.
The authors gratefully acknowledge the assistance of all who
contributed.
Hydroelectric Research and
Technical Services Group
Denver, Colorado
April 2005

iii

Transformers:Basics, Maintenance, and Diagnostics

Contents
Page

1. Introduction.........................................................................

1

2. Introduction to Transformers ..............................................
2.1 Principle of Operation................................................
2.2 Transformer Action....................................................
2.3 TransformerVoltage and Current..............................
2.4 The Magnetic Circuit .................................................
2.5 Core Losses................................................................
2.6 Copper Losses............................................................
2.7 Transformer Rating....................................................
2.8 Percent Impedance.....................................................
2.9 Internal Forces ...........................................................
2.10 Autotransformers .......................................................
2.11 Instrument Transformers............................................
2.12 Potential Transformers...............................................
2.13 CurrentTransformers.................................................
2.14 Transformer Taps.......................................................
2.15 Transformer Bushings................................................
2.16 Transformer Polarity..................................................
2.17 Single-Phase Transformer Connections for
Typical Service to Buildings...................................
2.18 Parallel Operation ofSingle-Phase Transformers
for Additional Capacity...........................................
2.19 Three-Phase Transformer Connections......................
2.20 Wye and Delta Connections.......................................
2.21 Three-Phase Connections Using
Single-Phase Transformers .....................................
2.22 Paralleling Three-Phase Transformers.......................
2.23...