Determination of zero impedance of 3-phase-current transformers by measurements

Special difficulties arise in ascertaining zero impedances of 3-phase-current oil power transformers. Basic conditions for a zero-current flow and thus the acting of a zero impedance in a power transformer are at least a star- (Y-) connected winding and an earthed neutral. Systems with low-resistance and short-time earthing require exact knowledge of the transformers' zero impedance for the calculation of the fault current. Due to the complex electrical and magnetic interactions in the transformer, especially in polyphase transformers with several counter-current conducting windings, ascertaining of the zero impedances is practically only possible by measurement. Proceeding from a classification of transformers according to their design properties, a possibility to assume zero impedances of power transformers of similar designs is created. The mathematical modelling of the transformer, transformer classification, and measurement and calculation of the zero impedance are discussed.