IEEE C37.91-2008

The scope of this guide includes general philosophy, practical applications, and economic considerations involved in power transformer protection. Emphasis is placed on practical applications. General philosophy and economic considerations in protecting transformers are reviewed. Types of faults in transformers are described. Technical problems with the protection systems, including the behavior of current transformers (CTs) during system faults, are discussed. Associated problems, such as fault clearing and reenergization, are discussed as well.