AC Loss Research on HTS Coil Windings in Ferromagnetic Environments

Employing high-temperature superconductors in power devices provides an alternative approach to achieving high efficiency, low energy loss, and compact size. AC loss is one of the critical issues for high-temperature superconducting (HTS) power devices, as it is the main heat load that must be dissipated from the cryogenic environment, significantly influencing the performance and efficiency of the power devices. To enable efficient energy transfer between coil windings in practical HTS equipment, a strong mutual magnetic flux is desired. Iron cores with high magnetic permeability are typically used as conduits to carry the flux between coils. However, the presence of iron cores changes the surrounding magnetic field around HTS coil windings and greatly affects their AC loss behaviours. To reduce AC loss in HTS coil windings, exploiting magnetic flux diverters to suppress the perpendicular component of the magnetic field has been recognized as a proven method. Up to now, the influence of these ferromagnetic components on the AC loss characteristics of HTS coil windings has not been systematically investigated at the coil level and remains unclear for real HTS applications.

This thesis aims to conduct numerical and experimental research on AC loss in HTS coil windings within ferromagnetic environments, focusing on HTS transformers and rapid-cycling synchrotrons, both of which have iron cores and HTS coil windings. In order to obtain a better understanding of AC loss behaviours in HTS coil windings within ferromagnetic environments and to provide insights for the design of practical HTS applications, the following research questions are addressed: how does the iron core influence the AC loss of HTS coil windings carrying AC current with and without DC offsets; how to accurately measure the transport AC loss of HTS coil windings coupled with an iron core; what is the role of flux diverters in reducing the AC loss of HTS coil windings coupled with an iron core; how the iron core affects the AC loss of transformer windings; and what kind of methods can effectively reduce AC loss in transformer windings?

At first, systematic three-dimensional (3D) AC loss simulations of HTS coil assemblies coupled with a two-limb rectangular iron core are presented to explore the influence of the iron core on the AC loss of HTS coil windings. The simulated results show that the AC loss of a REBCO eight-double-pancake coil (8DPC) assembly coupled with an iron core is more than an order of magnitude larger than the loss value without the iron core. This is because the presence of the iron core causes strong perpendicular magnetic field penetration not only in the end DPCs but also in the middle DPCs of the 8DPC assembly. Increasing the distance between the iron core and HTS coil assemblies can effectively weaken the influence of the iron core, thereby reducing the coil AC losses. Compared to an iron core with a high saturation magnetic field, HTS coil assemblies coupled with an iron core that has a low saturation magnetic field exhibit lower AC loss. When a DC offset is applied, the AC loss of a 1DPC assembly without the iron core significantly increases as long as the total transport current exceeds the critical current of the coil, which is due to the flux flow. With the inclusion of the iron core, the AC loss of the 1DPC assembly further increases compared to the corresponding case without the iron core at a given AC current and DC offset.

Then, a simulation-guided experimental method is proposed to measure the net transport AC loss of HTS coil assemblies when coupled with an iron core. The simulations are conducted to make a comparison of the local magnetic flux density distributions in the iron core generated by the HTS coil assembly and a copper test coil. It is necessary to introduce a copper coil to provide a field which closely matches the field of the HTS coil and in which the intra coil losses can be simply calculated. By matching the distributions between these two configurations, the error in the indirect iron loss estimation caused by different local magnetic flux density distributions within the iron core can be avoided, thereby ensuring an accurate transport AC loss measurement for the HTS coil assembly coupled with an iron core. The experimental results prove that the iron core greatly increases the transport AC loss of HTS coil assemblies. In addition, an obvious frequency dependence is observed in the transport loss results of HTS coil assemblies coupled with the iron core, which is attributed to the eddy current generated in the iron core.

Next, AC loss reduction methods for HTS coil assemblies coupled with a two-limb rectangular iron core using flux diverters are investigated. Employing flux diverters can provide substantial AC loss reduction for HTS coil assemblies coupled with the iron core by aligning the magnetic flux lines more parallel to the end discs of the coil assemblies. The combined method of increasing the distance between the iron core and the coil assemblies, along with applying flux diverters, can lead to further loss reduction. Moreover, decreasing the vertical gap between the flux diverters and the coil assemblies can also achieve a pronounced reduction in loss. In fact, enlarging the distance between the iron core and the coil assemblies leads to increased wire consumption. Therefore, the aforementioned methods for reducing AC loss in HTS coil assemblies coupled with an iron core are evaluated with careful consideration of the actual wire usage.

Finally, 3D AC loss simulations of a 3-phase 1 MVA HTS transformer coupled with a three-limb iron core are presented to assess the impact of the iron core on transformer windings. Unexpectedly, the iron core only leads to a 1 W (1.2%) AC loss increase for a single phase of the transformer windings at the rated current. This slight difference is attributed to the non-inductive winding structure of the transformer, where a strong magnetic field generated in the space between the low-voltage (LV) and high-voltage (HV) windings effectively shields the influence of the iron core. Based on this conclusion, the combined impact of asymmetric critical current and flux diverters on AC loss of a 6.5 MVA/25 kV HTS traction transformer is discussed without the consideration of the iron core. At the rated current and 65 K, employing the flux diverters with a square-shape cross-section, the total AC loss is decreased by 73.7% and an extra 150 W loss reduction is also obtained. Moreover, an additional reduction of 37 W is realized upon utilizing the asymmetric Ic(B, θ) characteristic. The reduced 187 W in AC loss at 65 K corresponds to a reduction in ambient power requirement of over 5.6 kW.

The simulation and experimental results presented in this thesis provide useful information to help understand the AC loss generation mechanisms of HTS coil windings in ferromagnetic environments. Furthermore, the influence of the iron core on transformer windings, as revealed in this thesis, contributes to the design of HTS transformers. The AC loss reduction methods summarized and proposed in this thesis also provide valuable insights for minimizing AC loss in HTS power devices.