Load Losses and Short-Circuit Resistances of Distribution Transformers According to IEEE Standard C57.110

Abstract

[EN] Load losses determine transformers¿ efficiency and life, which are limited by overheating and deterioration of their elements. Since these losses can be characterized by short-circuit resistances, in this article, we have developed expressions for the short-circuit resistances of three-phase transformers according to IEEE Standard C57.110. Imposing the condition that these resistances must cause load losses of the transformer, two types of short-circuit resistance have been established: (1) the effective resistance of each phase (Rcc,z) and (2) the effective short-circuit resistance of the transformer (Rcc,e f ). The first is closely related to the power loss distribution within the transformer; (2) the second is just a mathematical parameter. Applying these resistances to the 630 kVA oil-immersed distribution transformer of a residential network, we have concluded that both types of resistances determine the total load losses of the transformer. However, only Rcc,z accurately provides the load losses in each phase. Rcc,e f can give rise to errors more significant than 16% in calculating these losses, depending on imbalances in the harmonic currents.