In this technical guide we will discuss the principles of transformer differential protection, walk you through detailed relay setting calculations, explore discrimination techniques that distinguish inrush current from real faults, and provide practical testing procedures. 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. criteria for protection schemes. Transformer failure can have severe consequences: Transformer. Primary protection (e., overcurrent, zero-sequence) provides redundancy for extended coverage.