Analysis and Harmonic Mitigation in Distribution Transformers for Renewable Energy Applications

As renewable energy generation in South Africa continues to take a lion share of the republic's energy generation, the design of distribution transformers for renewable energy applications has introduced several design and performance challenges that have not previously been taken into consideration during the design stage. Transformers in RE application have failed as a result of challenges associated with harmonics and distortion, distorted load cycle, over-voltages and oil-gassing issues. This study is used to give an analysis of various transformers ranging from 100 kVA to 1500 kVA that have failed within RE plants. In the study, the harmonic spectrum is considered to adequately specify the harmonic loading of the transformers during service. Three results from analytical method, Finite Element Method (FEM) and practical measurements are considered to ascertain the transformer service losses under the supplied harmonic spectrum. The improvement in computational accessibility of designing the transformers in FEM has proven to produce reliable results yielding about 98 % in comparison to measured results. In the end, the study reveals that the use of FEM in the transformer design for RE application has high prediction accuracy, and that these transformers failed on account of the loading exceeding the transformer ratings due to underestimation of the harmonic-induced losses during the design stage.