AC Loss Effect of High-Order Harmonic Currents in a Single-Phase 6.5 MVA HTS Traction Transformer

Traction transformer is one of the critical electrical devices in high-speed trains. To increase transformer efficiency up to 99% and reduce weight and fire risks, we are currently building a 6.5 MVA superconducting traction transformer. Assuming 15 as a realistic cooling penalty, thermal loss from the transformer windings cannot exceed 4 kW in order to meet 99% of efficiency. There are non-ignorable high frequency harmonics in the traction side of the transformer, so it is important to investigate the effect of harmonic current on ac loss in transformer windings. We explored the dependency of ac loss in transformer on the amount of the 13th harmonic in traction side, by means of numerical calculation in a 2-D axisymmetric model using the H formulation. Simulations were also carried out to study the influence of the phase state of the 13th harmonic, either in phase or out of phase compared to the fundamental current, and multiple harmonics together in traction winding. Results show that the 13th harmonic must be limited under 1.5% of the rated current to fulfill the aim of 99% efficiency. AC loss in transformer increases when maximum amplitude of distorted currents carrying the same order in-phase harmonic but different amount increases. With various harmonic components in the LV winding, it is important to consider both the maximum current and waveforms to have a clear idea of loss behavior in the transformer.