| Summary: | 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 (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>R</mi><mrow><mi>c</mi><mi>c</mi><mo>,</mo><mi>z</mi></mrow></msub></mrow></semantics></math></inline-formula>) and (2) the effective short-circuit resistance of the transformer (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>R</mi><mrow><mi>c</mi><mi>c</mi><mo>,</mo><mi>e</mi><mi>f</mi></mrow></msub></mrow></semantics></math></inline-formula>). 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 <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>R</mi><mrow><mi>c</mi><mi>c</mi><mo>,</mo><mi>z</mi></mrow></msub></mrow></semantics></math></inline-formula> accurately provides the load losses in each phase. <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>R</mi><mrow><mi>c</mi><mi>c</mi><mo>,</mo><mi>e</mi><mi>f</mi></mrow></msub></mrow></semantics></math></inline-formula> can give rise to errors more significant than 16% in calculating these losses, depending on imbalances in the harmonic currents.
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