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

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.