AC loss measurements in HTS coil assemblies with hybrid coil structures

Abstract

Both AC loss and wire cost in coil windings are critical factors for HTS AC machinery applications. We present AC loss measurement results in three HTS coil assemblies at 77 K and 65 K which have hybrid coil structure comprising one central winding (CW) and two end windings (EWs) wound with ReBCO and BSCCO wires with different self-field Ic values at 77 K. All AC loss results in the coil assemblies are hysteretic and the normalized AC losses in the coil assemblies at different temperatures can be scaled with the Ic value of the coil assemblies. The normalised results show that AC loss in a coil assembly with BSCCO CW can be reduced by using EWs wound with high Ic ReBCO wires, whilst further AC loss reduction can be achieved by replacing the BSCCO CW with ReBCO CW. The results imply a flexible hybrid coil structure is possible which considers both AC loss and wire cost in coil assemblies.

Exploiting asymmetric wire critical current for the reduction of AC loss in HTS coil windings

“…The AC loss values obtained at different frequencies agree at both temperatures. This implies the AC loss in the assembly is hysteretic [22,34]. Similar results (not shown) were obtained for the other two assemblies.…”

“…4.1. I c measurement of coil assemblies Figure 8 shows the E-I curves of the three coil arrangements measured at 77 K and 65 K, after subtracting the voltage contribution resulting from the contact resistance between the double pancakes [22]. In this figure, the calculated E-I curves in the UU, UL, and LL assemblies are plotted together.…”

“…Both the overall coil I c and the AC loss of all three assemblies were measured and compared. The measurement method is described in detail in our previous work [22] and here we just recap the main points of the method. The coil assemblies were put in a foam-insulated liquid nitrogen dewar, the temperature of which is variable from 77 K to 65 K by reducing the pressure by pumping.…”

“…However, there has been no report to date on the influence of asymmetric wire critical current on coil AC loss. In this work, we explore the influence of wire I c asymmetry on overall coil I c as well as on the AC loss of hybrid coil assemblies [21][22][23].…”

“…The concept of hybrid coil assemblies arises due to the fact that conductors situated in different parts of a coil winding experience different magnetic field strengths and orientations, and therefore that using different conductors in the central and end parts of the coil can improve both the I c and the AC loss of the overall coil [22] while simultaneously minimizing cost. Here, we go one step beyond this and consider improvements in performance that can be achieved simply by being aware of the relative orientations of the different windings as they are put together to form the coil assembly.…”