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

Abstract

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,ef). The first is closely related to the power loss distribution within the transformer. 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,ef can give rise to errors more significant than 16% in calculating these losses, depending on imbalances in the harmonic currents.

Revisited Concept of Three-Phase Transformers’ Short-Circuit Resistances in Light of the Institute of Electric and Electronics Engineers (IEEE) Standard C57.110-2018

“…To avoid the errors that R ccN usually introduces in the calculation of load losses of three-phase transformers, two types of short-circuit resistances were developed in [45,46], based on the expressions of the load losses included in the IEEE Standard C57.110-2018 [6,7], which have been included in Section 1.1. These are the short-circuit resistances of L. Sima et al (R k ) [45] and the effective short-circuit resistances of each phase (R cc,z ) [46], and their expressions are indicated in Section 1.2 of this article.…”

“…To avoid the errors that R ccN usually introduces in the calculation of load losses of three-phase transformers, two types of short-circuit resistances were developed in [45,46], based on the expressions of the load losses included in the IEEE Standard C57.110-2018 [6,7], which have been included in Section 1.1. These are the short-circuit resistances of L. Sima et al (R k ) [45] and the effective short-circuit resistances of each phase (R cc,z ) [46], and their expressions are indicated in Section 1.2 of this article. However, the short-circuit resistance of L. Sima et al is inappropriate, since its application determines values of the load losses that are different in each transformer's phases than those obtained from the IEEE Standard C57.110-2018, as was verified in reference [46]; therefore, L. Sima et al's short-circuit resistance is not the object of study of this work.…”

“…These are the short-circuit resistances of L. Sima et al (R k ) [45] and the effective short-circuit resistances of each phase (R cc,z ) [46], and their expressions are indicated in Section 1.2 of this article. However, the short-circuit resistance of L. Sima et al is inappropriate, since its application determines values of the load losses that are different in each transformer's phases than those obtained from the IEEE Standard C57.110-2018, as was verified in reference [46]; therefore, L. Sima et al's short-circuit resistance is not the object of study of this work.…”

“…To avoid the drawbacks of the short-circuit resistance of L. Sima et al and to correctly characterize the load losses of each phase of three-phase transformers according to IEEE Standard C57.110-2018, reference [46] developed the effective short-circuit resistances of each phase of the transformer (R cc,z ), as follows:…”