Evaluation of the calculation method for stray losses in structural parts of HVDC converter transformers

Stray losses in structural parts are calculated separately during determining load losses of HVDC converter transformers, but the calculation method proposed by IEC 61378-2 standard shows significant difference from the experimental results. The nonlinearity, hysteresis losses, and frequency characteristic are thought as the sources of errors in this paper, and then an electromagnetic-circuit coupling FEM analysis is performed to investigate them. It is shown that the frequency characteristic of the stray losses is the major factor that causes the errors, and the material characteristic should be considered. The magnetic and nonmagnetic materials are suggested to be calculated separately.