Transformer Fault Analysis Using Event Oscillography

Transformer differential protection operates on Kirchhoff\´s well-known law that states, "the sum of currents entering and leaving a point is zero". Although Kirchhoff\´s law is well understood, the implementation of the law in transformer differential protection involves many practical considerations such as current transformer (CT) polarity, phase-angle correction, zero-sequence removal, and CT grounding. Still, even correctly implemented transformer differential protection misoperates occasionally, resulting from conditions such as CT saturation during heavy through faults. Whereas electromechanical and electronic relays provide no or very little fault information, numerical relays provide an abundance of information. However, the analyst must still select the correct fault information from this abundance of information to perform useful fault analysis. This paper demonstrates how to begin analysis of such events by using real-life oscillographic data and going through a step-by-step analysis of the relay algorithm using a mathematical relay model. Relay engineers can use this paper as a reference for analyzing transformer oscillography in a systematic and logical manner