Table of Contents

1 Overview
   1.1 Scope
   1.2 Purpose
2 Normative references
3 Definitions
4 Device numbers
5 Philosophy and economic considerations
6 Types of failures in transformers
7 Relay currents
   7.1 Minimum internal faults
   7.2 Maximum internal faults
   7.3 Performance of current transformers
   7.4 Reasons for mismatch
8 Electrical detection of faults
   8.1 Fuse protection and self-powered resettable fault
        interrupters
   8.2 Differential protection
   8.3 Overcurrent relay protection
   8.4 Ground-fault protection
   8.5 Fault detection for special-purpose transformers
   8.6 Backup and external fault protection
   8.7 Temperature relays
   8.8 Miscellaneous
9 Mechanical detection of faults
   9.1 Gas accumulator relay
   9.2 Gas detector relay
   9.3 Pressure relays
10 Thermal detection of abnormalities
   10.1 Thermal relays for winding temperature
   10.2 Other means of thermal protection
11 Fault clearing
   11.1 Relay tripping circuits
   11.2 Circuit breakers
   11.3 Tripping of remote circuit breakers
   11.4 Circuit switcher
   11.5 Fuses
   11.6 Self-powered resettable fault interrupters
12 Reenergizing practice
13 Gas analysis
   13.1 Level of TDCG and rate of change of TDCG approach
   13.2 Ratio of gases approach
14 Special protective schemes
   14.1 Overall unit generator differential
   14.2 Unit transformer of three-legged core form type
   14.3 Grounding transformer inside the main transformer
        differential zone
   14.4 Unbalanced voltage protection for wye-connected,
        three-legged, core-type transformers
   14.5 Differential protection of single-phase transformers
        connected in three-phase banks
   14.6 Differential protection of a bank of three single-phase
        autotransformers with delta tertiary
   14.7 Differential protection of single-phase transformers
        in a three-phase bank with a spare transformer protection
        of power transformers
15 Other considerations
Annex A (informative) Application of the transformer
        through-fault-current duration guide to the
Annex B (informative) Transformer failure statistics
Annex C (informative) Examples of setting transformer protection
        relays
Annex D (informative) Thermal overload protection
Annex E (informative) Phase shift and zero-sequence compensation
        in differential relays
Annex F (informative) Bibliography
Annex G (informative) Additional sources of information

Abstract

Presents general philosophy, practical applications, and economic considerations involved in power transformer protection.

General Product Information

Published
Document Type	Standard
Status	Current
Publisher	Institute of Electrical & Electronics Engineers
Pages
ISBN
Supersedes	IEEE DRAFT C37.91 : D7.0 2006