Forewords and guest editors

Estimating that the global electricity demand will be growing 2.1 % per year to 2040, many different interventions including operational, technological, as well as policy and investment-based decisions are needed to make modern power systems - one of the most important infrastructures in modern society - more flexible, more reliable, more resilient, more compact, more efficient, and more affordable. With this ever-increasing load demand in the modern society, energy loss values, efficiency rates, reliability, and short circuit level of the power network are major concerns. On the other hand, there are environmental issues that utilities or network operators need to address, including price of land, CO2 footprint, and concerns related to oils used in the apparatus or their recycling after the end of life. In addition, climate change and global warming concerns will make production of the traditional conductors very expensive over the next two decades, as their production is linked with mining activities, and then proper purification process which are both sources of pollution. Therefore, these issues raise a strong need for game-changing dramatical solutions to decrease losses, reduce environmental footprint, increase efficiency and reliability. Cryo-electrification of the power system that takes advantage of cryogenic and superconductivity technologies would be this game-changer. Superconductors with their much higher current density, near zero resistance, and almost loss-free performance seem to be a viable solution for the replacement of the conventional conductors such as copper and aluminium. Superconductivity is a phase of material at cryogenic temperatures - in some specific metals or alloys - which can be defined with three critical limiting factors, i.e., critical temperature, critical current density, and critical field. Superconductors carry 100 to 500 times – depending on operating condition - higher current compared with copper, almost with no loss if they work within the three aforementioned limits. Applied superconductivity offers huge opportunities for optimization and modernization of whole energy and power systems for making it more efficient, compact, smarter, reliable, lighter, sustainable, and environmentally friendly by using physical essence and attribute of superconductors and implementing them into the products. The success of superconductivity, however, depends on the overcoming challenges such as initial purchasing cost, as well as technical issues. The power transformers, power cables, and fault current limiters are the most important conventional apparatus in the power network so that their replacement with superconducting versions sound promising. Some of the technical challenges that need to be addressed in the current decade are as follows: • Superconductor manufacturing issue of producing longer length of tapes / wires, • Tape / wire final price, • Discovery and fabrication of superconductors with a higher critical temperature, • Invention of new or improvement of existing cryocoolers and cooling systems, • Overcoming technical issues in design and development of superconducting devices. I believe that working on the following topics would be part of the roadmap for superconductivity in power network or transportation applications: • Focusing on manufacturing of cheaper wires / tapes • Application of artificial intelligence for smarter manufacturing and condition monitoring of superconducting devices, • Integration of magnesium diboride into superconducting devices, • Using new coolants such as hydrogen for making the free cooling systems where possible, especially in renewable energy plants, • Integrating DC power cables in long HVDC transmission lines, • Additive manufacturing for faster, more precise, and smarter manufacturing of superconducting devices, • Recycling of superconducting components and devices. This special issue aims to provide a forum for the latest developments, future plans, and long-term roadmap for superconductivity in power grid applications. The focus was on the achievements of superconductivity-based technological developments, and applications in transmission, and distribution of electricity, fault current limitation, loss evaluation, feasibility studies. Topics ranged from an individual device to integrated systems. All the papers published in this special edition underwent stringent single-blinded peer-review process involving a minimum of two reviewers comprising internal (editorial board of the magazine) as well as external referees. This was to ensure that the quality of the papers justified the high expectation of Transformers Magazine editorial board, which is renowned as one of the most important technical magazines on the topic in the world. We thank the authors for agreeing to publish their papers in this special edition, and the guest editors and reviewers involved in the publishing process of these papers. We hope that this special edition will shed a light on the disruptive innovation in the field of applied superconductivity that is expected to change and modernize the future power networks. In addition, we aim for motivating the utility and power network stakeholders to invest in the applied superconductivity and cryo-electrification for reaching to this modernized future network. Dr. Mohammad Yazdani-Asrami, Deputy Editor-in-Chief for Special Edition Superconductivit

Artificial intelligence, machine learning, deep learning, and big data techniques for the advancements of superconducting technology: a road to smarter and intelligent superconductivity

The last 100 years of experience within the superconducting community have proven that addressing the challenges faced by this technology often requires incorporation of other disruptive techniques or technologies into superconductivity. Artificial intelligence (AI) methods including machine learning, deep learning, and big data techniques have emerged as highly effective tools in resolving challenges across various industries in recent decades. The concept of AI entails the development of computers that resemble human intelligence. The papers published in the focus issue, "Focus on Artificial Intelligence and Big Data for Superconductivity", represent the cutting-edge and forefront research activities in the field of AI for superconductivity

Heat transfer in HTS transformer and current limiter windings

Design of cryogenic systems for HTS transformers and fault current limiters (FCL) must provide for fault conditions as well as normal operation. In the course of a fault the HTS windings may be heated rapidly to a temperature of 300 K or higher. The ability of the device to return to normal operation after a fault current depends critically on the efficiency of heat transfer from the hot windings to the cooling system. The engineering of the interface of the winding with the cryogen, generally liquid nitrogen in the case of HTS transformers and FCL, can properly be considered a crucial component of cooling system design. We report measurements of heat transfer from short metal and superconductor tape samples immersed in liquid nitrogen in conditions which approximate those in an HTS transformer winding. Samples were subjected to current pulses of several hundred A/mm2 for time intervals up to 2 seconds. The average sample temperature was estimated from the resistance. The current density during cool down was varied over the range corresponding to HTS transformer operation. Heat transfer was measured on samples with UV-cured polymer coatings as well as on bare samples. Somewhat paradoxically, by thermally insulating the metal surface from the liquid nitrogen the coating can drastically improve heat transfer. It does this by avoiding film boiling - the formation of a gas sheath on the surface - and extending the range of efficient cooling by nucleate boiling where the liquid is able to continuously wet the hot surface. We find that heat transfer during conductor cool down: - Is highest in subcooled operation e.g. at 65 K at atmospheric pressure compared to operation at the boiling point e.g. 77.3 K at atmospheric pressure - Is significantly increased by a thermally insulating coating of optimised thickness applied to the conductor compared to bare conductor, and reduced by wrapped-paper electrical insulatio

Challenges for developing high temperature superconducting ring magnets for rotating electric machine applications in future electric aircrafts

One of the most promising applications for high temperature superconducting (HTS) material is magnet. HTS magnet is considered to be used in a wide range of applications including MRI, particle accelerator, electrical machine, and etc. HTS rotating machines are mainly utilized for cryo-electrification, i.e. application of superconductivity for modern electrification. One of the main aspects of cryo-electrification is electric aircraft application which is recently enabled by continuous progress in design development of superconducting magnets and HTS machines. This paper discusses the challenges facing a newly developed magnet type, i.e. HTS ring magnet, that being considered in superconducting rotating machine in future electric aircrafts. HTS ring magnet is compact, easy to develop, fault tolerant, and light in weight, and it recently reached to a high level of magnetic field

AC loss analysis in superconducting cables carrying characteristic and non-characteristic harmonic currents

Harmonic distortions - especially in current waveform - are the inherent nature of any power system such as urban grids, wind farms, electric aircraft, and other electrified transportation units that could change the AC loss value in High Temperature Superconducting (HTS) cables. The aim is to investigate the impact of non-sinusoidal currents with different integer-harmonics, inter-harmonics, and sub-harmonics on the AC loss characteristics of a 22.9 kV, 50 MVA HTS cable. This was accomplished by using an Equivalent Circuit Model (ECM). To do so, current waveforms containing different harmonic components were passed to the ECM of HTS cable. For evaluating the impact of distorted current waveforms on the AC loss of the HTS cable, the ECM was validated by means of Finite Element Method (FEM) in tape level. The results of validation phase have shown a good agreement between the AC loss value derived by ECM and those calculated by FEM published in literature. The results showed that when current waveform was distorted by harmonics, the value of AC loss was changed significantly with respect to variations of harmonic phase angle, order, and amplitude. Results also indicated that 5 th harmonic order has the highest impact on the AC loss value and could increase 6% to 80% of AC loss in comparison to pure sinusoidal current. Sub-harmonics and inter-harmonics could also increase the AC loss value to maximum 88% and 64% higher than that of sinusoidal condition

AC Loss Characterization of HTS Pancake and Solenoid Coils Carrying Nonsinusoidal Currents

Application of high-temperature superconducting devices become promising in power networks, and transportation, including ship, train, and electric aircraft propulsion systems, with the advantages of light weight, compact size, and high efficiency, compared to conventional devices. In reality, electric networks—either in grid or transportation propulsion system—are polluted with harmonics due to the widespread use of power electronic devices and nonlinear loads. It is essential to explore the dependency of harmonic ac losses of different coil configurations carrying nonsinusoidal current. We modeled and compared harmonic ac loss behaviors in three coil configurations, single pancake coil (SPC), double pancake coil (DPC), and solenoid coil (SNC), where SPC and SNC are wound by identical wire length and DPC has twice conductor number compared to SPC. The research work has been carried out by means of H-formulation finite element method in a 2-D axisymmetric modeling environment of COMSOL Multiphysics. We explored and reported ac losses in these three coil structures carrying nonsinusoidal current with the third and the fifth harmonic orders, respectively, under different total harmonic distortion (THD) and fundamental current levels. It has been concluded that ac loss in these coils first decreases with the increase of the third harmonic content, when THD of the third harmonic 0.2. AC loss in coils monotonically increases with the increase of the fifth harmonic, drastically. We found that ac loss in SPC carrying the third harmonic and the fifth harmonic at different THD are more than 3.8 times of that in DPC; ac loss in SPC carrying either third or fifth harmonics at different THD are around 4.5 times of that in SNC

Comparative study of a new structure of HTS-bulk axial flux-switching machine

A high-temperature superconducting (HTS) axial flux permanent magnet (AFPM) machine was designed, using superconducting bulks over the rotor surface and rare-earth magnets in the middle of the stator teeth. Because of diamagnetic behavior of the HTS bulks and zero field cooling, leakage flux significantly reduces in the proposed machine compared to the existing machine with mounting rare-earth magnets. Three-dimensional finite element (FE) modelling was used to validate the design performance. The magnetic flux distribution, induced electromotive force (EMF), inductance, PM flux, losses, total harmonic distribution and cogging torque are computed and compared in two structures. The results show that the proposed machine structure is more efficient than the existing one

AC Transport Loss in Superconductors Carrying Harmonic Current with Different Phase Angles for Large Scale Power Components

It is of great industrial interest and academic importance to investigate current harmonics impacts on AC losses of superconductors, especially in large-scale power devices. However, only effect of amplitude of in-phase current harmonics on AC loss has been studied in the works of literature. We numerically characterized nonsinusoidal AC loss of superconducting tape carrying harmonic currents with orders below 20th versus phase angles. A drastic AC loss variation was found when phase angle was considered for harmonic components. We observed that different harmonic orders show different AC loss profile versus phase angle

A new intelligent estimation method based on the Cascade Forward Neural Network for the Electric and Magnetic fields in the vicinity of the High Voltage Overhead Transmission Lines

The evaluation and estimation of the electric and magnetic field (EMF) intensity in the vicinity of overhead transmission lines (OHTL) is of paramount importance for residents’ healthcare and industrial monitoring purposes. Using artificial intelligence (AI) techniques makes researchers able to estimate EMF with extremely high accuracy in a significantly short time. In this paper, two models based on the Artificial Neural Network (ANN) have been developed for estimating electric and magnetic fields, i.e., feed-forward neural network (FFNN) and cascade-forward neural network (CFNN). By performing the sensitivity analysis on controlling/hyper-parameters of these two ANN models, the best setup resulting in the highest possible accuracy considering their response time has been chosen. Overall, the CFNN achieved a significant 56% reduction in Root Mean Squared Error (RMSE) for the electric field and a 5% reduction for the magnetic field, compared to the FFNN. This indicates that the CFNN model provided more accurate predictions, particularly for the electric field than the proposed methods in other recent works, making it a promising choice for this application. When the model is trained, it will be tested by a different dataset. Then, the accuracy and response time of the model for new data points of that layout will be evaluated through this process. The model can predict the fields with an accuracy near 99.999% of the actual values in times under 10 ms. Also, the results of sensitivity analysis indicated that the CFNN models with triple and double hidden layers are the best options for the electric and magnetic field estimation, respectively

A fast numerical modelling approach based on boundary field method for calculating AC losses in superconducting motors

To improve the computational efficiency of the numerical methods for calculating AC loss in HTS motor, a new method called ‘Boundary field method’ is proposed in this paper. This method combines the strength of ANSYS and COMSOL, which can reduce the complexity of modeling and computation time. In this paper, three finite element method (FEM) based models, as case studies, were developed for a 100-kW superconducting induction motor. Model A models entire motor in COMSOL using A-H formulation, considering non-linear E-J relationship of HTS tapes, which can calculate AC loss of motor directly; Combining another two models: Model B (the same entire superconducting motor in ANSYS) and Model C (only superconducting stator windings in COMSOL) can achieve new method for calculating AC loss. The AC loss calculation error between two methods for two-layer windings in one slot is 2.5% and 7%, respectively. These results conclude the effectiveness and accuracy of ‘Boundary field method’. More importantly, the computation time reduces dramatically from 48 hours in Model A to only 2 hours for proposed approach. This accurate and computationally efficient new numerical modelling approach could be used to calculate AC loss in various types of superconducting applications in future