Transformer Differential Settings: Transformers are critical substation components that need sensitive protection. Relay protection for transformers involves calculations for differential current thresholds, through-fault stability, inrush restraint, and harmonic. This guide focuses primarily on application of protective relays for the protection of power transformers, with an emphasis on the most prevalent protection schemes and transformers. Setting procedures are only discussed in a general nature in the material to follow. Abstract: 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. An assessment of sensitivity of the measuring elements of relay protection was performed. The type of protection used should quickly isolate the transformer for internal faults to reduce the risk of catastrophic failure, and to simplify eventual repair.