Loading characteristics analysis of specially connected transformers using various power factor definitions

This investigation examines and compares the loading characteristics and power factor values of V-V, Scott, and Le Blanc-connected transformers by a novel approach. Four definitions of power factor are compared-three considering harmonics and unbalances and one based only on fundamental components. A proper power factor value is needed to truly reflect the loading characteristics of these specially connected transformers, considering the influences of harmonics and unbalances. The computation results of the five given loading cases and two simulation cases demonstrate that the power factor values of V-V, Scott, and Le Blanc-connected transformers are different, even with the same loading on the load side. Since the V-V connection scheme has an inherent unbalanced structure, its performance in reducing the three-phase unbalance is substantially worse than that of the Scott and Le Blanc connection schemes, which can more effectively transform three-phase powers to two-phase powers. Additionally, a field measurement of the Le Blanc transformer in a substation of the Taiwan Railway System is also used to identify the contents of power factor definitions. The computation results reveal that the effective power factor could be recommended for use in fairly evaluating the load characteristics for specially connected transformers.

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