Influence of tertiary stabilizing windings on zero-sequence performance of three-phase three-legged YNynd transformers. Part I: Equivalent circuit models

The presence of a stabilizing winding (or tertiary stabilizing winding when is used to for auxiliary applications) in three-phase three-legged YNynd transformers remarkably affects the zero-sequence performance of both the transformer and the network. This paper presents a detailed analysis of the influence of the stabilizing winding on the zero-sequence behavior of three-phase three-legged YNynd transformers. Based on a complete set of onsite zero-sequence measurements taken in three power transformers, transformer zero-sequence performance is analyzed in relation to internal design features such as stabilizing winding position relative to high-voltage and low-voltage windings or the presence of magnetic shunts in the tank. Based on these measurements, this paper assesses the ability of various equivalent circuit models to reproduce zero-sequence performance accurately. A companion paper that complements this study evaluates the influence of stabilizing windings on tank overheating hazard and short-circuit duty in the event of asymmetrical faults.