Time-domain elements optimize the security and performance of transformer protection

Transformers experience magnetizing inrush that creates an operating current in the transformer differential protection. Early transformer differential relays had to address the limits of analog technology. Analog filter circuits could extract the second-and fourth-harmonic components of the differential current, and these were used as a surrogate measure for determining if the operating current was due to inrush. Today, we can design algorithms that distinguish directly between inrush characteristics and fault current characteristics. By using the new time-domain algorithms, we can improve sensitivity and speed of transformer differential protection. We can also maintain protection security for transformers built using improved core steels.