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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

Resistive type fault current limiters (SFCLs) have attracted lots of attention from research interests and engineering applications, due to their self-triggering, fast and effective fault current limitation ability. Fast recovery of SFCL after quench is highly attractive, yet still remains challenge when recover with load compared to recovering without load. We investigated the dependence of recovery time of a helical SFCL coil on the ratio of prospective current over critical current I pros/ I c. A multilayer simulation model was built in MATLAB/Simulink to investigate the electro-thermal behavior of SFCL coil during a fault and recovery period, and the model was validated by experimental results. Quench simulations were carried out under different I pros/ I c, ranging from 15 to 119 and in different recovery conditions including recovery with load condition and recovery without load condition, while keeping the fault duration of 100 ms. We observed that the SFCL coil reached 85 K and 195 K at the time when the fault was cleared, in the quench tests with I pros/ I c = 15 and 119, respectively. SFCL coil recovered without load in 0.5 s to 10 s, depending on the ratio of I pros/ I c.

Previous studies on magnetization AC loss of high temperature superconducting (HTS) coated conductors (CCs) taken into account the ideally sinusoidal external magnetic field, or non-sinusoidal by considering only the harmonics in phase with the fundamental component. However, realistically magnetic field often contains harmonics with different phase angles and amplitudes. In this paper, magnetization AC loss in HTS CC was studied, considering the phase angle $\varphi $ of each harmonic; assuming superconductor is subjected to external magnetic field which is distorted by the $3^{\mathrm {rd}}$ and the $5^{\mathrm {th}}$ harmonics. Phase angle of each harmonic was considered in the range of 0 to $\pi $ and amplitude of fundamental magnetic field, was assumed as 10, 20, 50, and 100 mT. Different distortion level of magnetic field was considered too. It is concluded that phase angle of field harmonic has a significant effect on magnetization loss of HTS CCs.

Current is no longer sinusoidal in modern electric networks because of widespread use of power electronic-based equipments and nonlinear loads. Usually AC loss is calculated for pure sinusoidal current, while it is no longer accurate when current is nonsinusoidal. On the other hand, efficiency of cooling system in large scale power devices is dependent on accurate estimation and prediction of the heat load caused by AC loss in design stage. Therefore, estimation of nonsinusoidal AC loss of high temperature superconducting (HTS) material would be of great interest for designers of large-scale superconducting devices. In this paper, at first nonsinusoidal AC loss of a typical HTS tape was calculated under distorted currents using H-formulation finite element method. Then, a range of artificial intelligence (AI) models were implemented to predict AC loss of a typical HTS tape. In order to find the best and more adaptive AI model for nonsinusoidal AC loss prediction, different regression models are evaluated using Support Vector Machine regression model, Generalized Linear regression model, Decision Tree regression model, Feed Forward Neural Network based model, Adaptive Neuro Fuzzy Inference System based model, and Radial Basis Function Neural Network (RBFNN) based model. In order to evaluate robustness of developed models cross-validation technique is implemented on experimental data. To compare the performance of different AI models, four prediction measures were used: Theil's U coefficients (U_Accuracy and U_Quality), Root Mean Square Error (RMSE) and Regression value (R-value). Obtained results show that best performance belongs to RBFNN based model and then ANFIS based model. U coefficients and RMSE values are obtained less than 0.005 and R-Value is become close to one by using RBFNN based model for testing data, which indicates high accuracy prediction performance.

The high axial and radial forces caused by short circuits can be very detrimental to high-temperature superconductor (HTS) windings of HTS transformers. Axial forces above the HTS winding critical value cause irreversible effects on the critical current reduction of an HTS tape. Such forces can disrupt the normal performance of HTS transformers and lead to quench phenomenon and insulation breakdown. Accordingly, any optimization in the design of HTS transformers that decreases the leakage flux and electromagnetic forces is very significant. This paper presents a novel method for reduction of leakage flux and thereby electromagnetic force in the windings of an HTS power transformer. First, concentric primary and secondary windings are subdivided into certain number of segments. In the next step, the proposed method, which is based on selecting the optimum distributive ratios for the created subwindings, is applied to the windings. In order to verify the effectiveness of the proposed method, an advanced finite-element method has been used. The method uses accurate and nonlinear $E{-}J$ power law for defining the superconductivity characteristic. The results showed the efficiency of the applied method to mitigate the radial and axial leakage flux and thereby electromagnetic forces in the windings of the HTS transformer.