Decomposition of fault currents in power transformers into suitable sets of components for application to fault characterization and modelling: Part 1: Theory

This paper provides the theoretical background for understanding and classifying the currents in a power transformer after an internal fault inception. The approach is based on the definition of four “sets of components”, which are further decomposed into their symmetrical components. The transformer condition during a fault can be reconstructed or synthesized from the four sets. The key issue has been the definition of the sets to avoid a difficult identification and description of their components. For this purpose the four sets of components that have been defined in this paper are based on the following analysis: currents in the sound transformer, electrical contribution of the internal fault, currents due to the leakage flux distortion, and loss of power delivery capability. The selection of these four sets have proven to make available an easy interpretation of the currents during an internal fault, as well as provide insight in the distortions provoked in the differential currents by a fault inception. The paper addresses the theoretical analysis and is the basis for a subsequent characterization and modelling of the transformer behaviour, in addition to a prompt identification of internal faults.