Guidelines for protecting three-phase power transformers of more than 5 MVA rated capacity and operating at voltages exceeding 10 kV is provided to protection engineers and other readers in this guide. In some cases, a user may apply the techniques described in this guide for protecting transformers of less than 5 MVA ratings or operating at voltages less than 10 kV. Information to assist protection engineers in properly applying relays and other devices to protect transformers used in transmission and distribution systems is also provided. General philosophy, practical applications, and economic considerations involved in power transformer protection are discussed, with an emphasis on practical applications. Types of faults in transformers are described. Technical problems with the protection systems, including the behavior of current transformers during system faults, are discussed, as well as associated problems, such as fault clearing and post trip reenergization.

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. 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 re-energization, are discussed as well.