Role of Flux Diverters in Reducing AC Loss in a Single-Phase 6.5 MVA HTS Traction Transformer for Chinese High-Speed Train Carrying High-Order Harmonic Currents

The traction transformer is a critical electrical component in high-speed-trains. We are currently building a 6.5 MVA superconducting traction transformer which will increase transformer efficiency to 99%, halve the weight, and avoid the fire risk of the conventional transformer system. AC loss from the transformer windings cannot exceed 2 kW to meet both the efficiency and weight targets. There are significant high frequency harmonics, particularly the 13^{\mathrm {th}} harmonic at 650 Hz, on the traction side of the transformer. Our previous work has shown the estimated AC loss in the transformer windings almost doubled with the addition of a 650 Hz harmonic current at the 8% level specified, which is not acceptable. Therefore, it is critical to reduce AC loss in the transformer due to high harmonics. In this study, we explored the role of flux diverters positioned at the outer ends of the high voltage (HV) and low voltage (LV) windings in restraining AC loss in the windings carrying high-order harmonic currents below 2 kW. Five different ring-shaped flux diverter designs were modelled in this work, utilizing the previously developed 2D axisymmetric model using the H formulation. Simulations were carried out to study the influence of the phase of the 13th harmonic, either in phase or out of phase with respect to the fundamental current, and of the inclusion of multiple harmonics on the loss. The simulation results show that placing flux diverters near the end of the windings leads to a magnetic flux distribution in the windings with reduced radial field, and hence significantly reduces the loss. Three of the flux diverter designs achieve the loss target by limiting the AC loss to below 2 kW, with a wide/thin design having the minimum weight and a wide/thick design achieving only 6% less loss, despite being almost four times heavier.