Accurate Method of Excitation Current and Relay Current Prediction during an Offset Fault at the Terminals of Power Transformer and Derivation of Saturated Relay Current

This article describes a method of accurately predicting CT excitation current and relay current from first principles during a terminal fault of a power transformer including AC and DC components which are required for over design values of CT flux as well as the knee point values of the CT depending on the relay applications desired. It is known that the CT secondary current is an accurate replica of the primary fault current but the relay current depends on the CT excitation current drawn. In case the relay current is to be analytically predicted the excitation current also has to be predicted accurately whose value has to be subtracted from the total secondary current. This excitation current depends on the CT saturation level and can form a considerable portion of the CT secondary current depending on the CT saturation level. Therefore an accurate method of analysis of CT excitation and relay currents becomes important. This article recommends a method of accurately deriving the excitation and relay current from first principles as a function of time taking into account both the AC and DC components for purposes of CT design and relay applications.