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 2D 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.

Bifilar coil structure is promising for reducing ac loss in resistive-type superconducting fault current limiters (FCLs). However, the electric power networks could be contaminated with harmonic components due to nonlinear loads and switch converters. We built a numerical model to calculate transport ac loss in a bifilar stack composed of two coated conductors, carrying distorted current, by means of H-formulation. We compared the calculated losses in the bifilar stack exposed to nonsinusoidal current with the third and fifth harmonics plotted as a function of total harmonic distortion (THD), where THD is defined as the ratio of current magnitudes for the harmonic frequency and fundamental frequency, I _hf,peak /I _ff,peak . AC loss results in the bifilar stack carrying current with the third harmonic in each conductor were compared with those in an inductive stack and single tape carrying the same current as the conductors in the bifilar stack. Results show that harmonic components have substantial influence on ac loss in the bifilar stack. AC loss values in the stack with fifth harmonic increased monotonically with increasing THD, while ac loss value in the stack with the third harmonic had minimum value when THD is approximately 0.2.

Fiber Bragg grating (FBG) temperature sensors are now re-garded as a relatively mature sensing technology for tempera-ture measurement in ambient conditions, and may offer poten-tial as a cryogenic temperature sensor for quench detection. In this work, the temperature response of three different coated FBG sensors from 77 K to 293 K was studied; bare (silica) FBG, acrylic coated, and polyimide coated FBG sensors. For compar-ison measurements were repeated with sensors from two FBG suppliers. The measured temperature response was then com-pared and contrasted with the predicted sensor response. In or-der to assess the sensor sensitivity, repeatability and any hyste-resis, these sensors were then subjected to repeated cool-down / warm-up cycles (77 $\times$ 293 K). Finally, the measurement of quench induced temperature change in an epoxy potted HTS coil was simulated through fabrication of bare FBG sensor embed-ded in a 10 mmμ2 mmν4 mm epoxy block. The effect of epoxy resin impregnation on the temperature response characteristics was analyzed by comparing the sensor response characteristics before and after immersion. From these results, we propose the most cost-effective and practical FBG type for HTS coil quench detection.

High-temperature superconducting traction transformers (HTSTTs) have the merits of small size and lightweight in comparison with their conventional counterparts. This article reports the development progress of a 6.6 MVA HTSTT operating at 65 K, including the design, testing, and system cooling. The introduction of flux diverters and an optimized winding design realized a short-circuit impedance higher than 43% and AC loss less than 3 kW. The insulation structure was designed to pass insulation tests specified in standard in China GB/T 25120-2010. An open cooling system with reduced pressure was developed, which realized the efficiency of the 6.6 MVA HTSTT above 99%. Before assembling the prototype transformer, we conducted tests for critical current and dielectric performance of the HTS double pancake coils (DPCs) used in high-voltage (HV) and low-voltage (LV) windings to verify the current-carrying and insulation performances of each DPC. Finally, we measured the critical current and no-load loss of the HTSTT prototype at 77 K. Test results showed that the mass of the transformer is 33% less than conventional transformers. At 77 K, the critical current of the LV winding and HV winding is higher than 700 A and 50 A, respectively. Moreover, the HTSTT on a no-load test reached the test voltage of 25 000 V and loss of 6 kW. In the next step, we will continue to conduct experimental research, and verify the feasibility of the HTSTT on the train, and develop a circulating cooling system, all meeting the commercial requirements of the HTSTT.

Large electric superconducting machines are being sought to solve emissions challenges in aircraft and provide a solution to >12 MW wind turbine electricity generation. Superconducting motors with only high temperature superconducting (HTS) armatures can offer advantages of high reliability, high power density, and high efficiency compared with conventional superconducting motors. In this paper, a novel HTS squirrel-cage induction motor is proposed for high speed operation, which adopts the structure of HTS windings with a certain inclination angle in the stator slots. Due to the limitation of curvature radius of superconducting tapes, the pitch of HTS windings can be only set to 1 and adopt the short pitch arrangement structure, which such design details of the HTS motor would ensure the superconducting coils can undertake larger current and reduce AC losses. In order to keep the HTS windings in superconducting state and larger current density, a special fixed cryogenic cooling system below 70 K with the method of gas extraction and decompression, which is made of aramid fiber, has been fabricated and the whole structure of the stator is placed in liquid nitrogen. According to the motor control principles and electromagnetic field theory, the electrical performances of the novel designed stator-HTS motor, which driven by the variable-voltage variable-frequency (VVVF) inverter, are analyzed including the flux density distributions, the torque, the induced electromotive force, losses and efficiency by using the finite element method. Finally, the components of HTS squirrel-cage induction motor have been manufactured according to the designed parameters. Next step, the motor will be assembled and tested.

The dynamic resistance which occurs when a superconductor carrying DC current is exposed to alternating magnetic field plays an important role in HTS applications such as flux pumps and rotating machines. We report experimental results on dynamic resistance in a four-tape coated conductor stack when exposed to AC magnetic fields with different magnetic field angles (the angles between the magnetic field and normal vector component of the tape surface, θ) at 77 K. The conductors for the stack are 4-mm-wide SuperPower SC4050 wires. The field angle was varied from 0° to 120° at a resolution of 15° to study the field angle dependence of dynamic resistance on field angle as well as wire I _c (B, θ). We also varied the field frequency, the magnetic field amplitude, and the DC current level to study the dependence of dynamic resistance on these parameters. Finally, we compared the measured dynamic resistance results at perpendicular magnetic field with the analytical models for single wires. Our results show that the dynamic resistance of the stack was mainly, but not solely, determined by the perpendicular magnetic component. I _c (B, θ) influences dynamic resistance in the stack due to tilting of the crystal lattice of the superconductor layer with regard to buffer layers.