Analytical Description of the Load-Loss Asymmetry Phenomenon in Three-Phase Three-Limb Transformers

Abstract

Abstract-The asymmetry phenomenon observed during the load-loss test of three-phase transformers is rigorously analyzed. The transformer is represented as a three-port impedance network despite its six-port network nature, making any formal study more tractable. The analytical restrictions required for such a representation are derived. As a result, it is possible to mathematically prove (and understand) the asymmetrical behavior of a three-phase transformer subjected to the load-loss test. Only two circuit-field simulations or tests (single-phase load-loss tests) are required to obtain the impedance system of the six-winding transformer. A 3-D finite-element (FE) model of a transformer is used here to obtain its short-circuit impedances, exemplifying the use and extent of the analytical results. The eddy currents generated at the transformer tank and frame are properly taken into consideration. The short-circuit impedance model is validated with the 3-D FE model and with a six-port network approach that has been previously verified.Index Terms-Asymmetry phenomenon, finite-element models (FEMs), network models, short-circuit impedances.

Comparison of four methods for computing positive‐sequence reactances of three‐phase core‐type transformers

“…The 3D geometry of the three‐phase core‐type transformers is asymmetric. This fact has an influence on the load‐loss measurements [10, 11], which are measured in the same tests than the transformer impedances. On the other hand, the results from the 2D models are different than the results from the 3D models in some cases of the zero‐sequence impedances [12].…”