Thermal behaviour of medium-voltage underground cables under high-load operating conditions

The dynamic management of electric power distribution lines has become a topic of great interest at present. Knowledge of the ampacity of cables is fundamental to carrying out dynamic management. In this study, the ampacity of buried cables in different soil resistivities and depths was calculated. A small-scale model was built in the laboratory to simulate the operating conditions of a buried cable. With the experimental results, a numerical model based on the finite element method was validated to evaluate the ampacities calculated by two standards. A comparison was made between the ampacities calculated from the IEC 60287-1 and UNE 211435 standards and those obtained from the simulated model. In addition, a comparison was made regarding the steady-state temperatures obtained at each calculated ampacity. The results obtained from the simulated model design show that the ampacity calculation method of the IEC 60287-1 standard where drying-out of the soil occurs is the most accurate, and has the least risk of exceeding the maximum permissible cable temperature.This work was financed by the EU Regional Development Fund (FEDER) and the Spanish Government under ENE-2013-42720-R, RETOS-COLABORACION RTC-2015-3795-3 and SODERCAN/FEDER Proyectos Puente 2017 and by the University of Cantabria Industrial Doctorate 19.DI12.649. The authors also acknowledge support received from Viesgo

Modelling of high temperature superconductors for AC power applications

This Ph.D. thesis is focused on the development of novel models for calculation of AC losses, current and magnetic field profiles in high-temperature superconductors (HTS). The thesis is concentrated on the modelling of Bi-2223 conductors at 77 K, which for the moment have the most advanced manufacturing technology and will be primarily used in the first large-scale power applications of high-temperature superconductors. The analysis of AC losses in Bi-2223 conductors is the leading thread in the structure of the thesis. An introduction to high-temperature superconductivity is made with special emphasis on the mechanisms of AC losses in HTS. Presented are some of the most promising power applications of HTS materials together with a discussion on the required improvements in their performance. A model for dissociating the hysteresis, eddy-current and resistive flux-creep loss contributions, based on relatively broad-range frequency measurements on Bi-2223 tapes has been used for analysis of the frequency behaviour of the transport-current loss in self-field. The hysteresis, eddy-current and flux-creep loss components have been separated due to their different frequency dependence. The study of eddy current loss in the silver sheath and matrix has been complimented by numerical simulations using a simple electromagnetic model of HTS tapes. Described is an original method for estimating the performance of Bi-2223 tapes in typical power grid perturbations by experiments and analysis of over-critical current excursions of various waveform, frequency and current amplitude up to 20×Ic. For precise calculation of the AC losses and studying the electromagnetic properties of HTS with smooth current-voltage characteristics and complex geometry, the finite element method (FEM) has been used. The implementation of the power-law model of the E-J characteristic of HTS into the FEM software package Flux2D is presented. The Flux2D implementation has been validated by means of comparison with results from theoretical predictions, electrical measurements, and other numerical methods. The significance of the power index n and the lateral distribution of the critical current density Jc in multifilamentary HTS tapes has been evaluated by FEM simulations. New anisotropic models of Jc(B) and n(B) for textured Bi-2223 materials have been developed. The models are based on experimental data; they are fairly simple and take into account the orientation of the local magnetic field. These models have been used in FEM simulations on monofilamentary and multifilamentary Bi-2223 conductors with different geometry and filament arrangement. In conductors with given Jc(B) dependence, the notion of effective AC critical current has been defined. The AC losses in Bi-2223 multifilamentary flat tapes and wires of round and square geometry in various operating conditions have been calculated and compared. The AC loss analysis has been supported and complimented by current density and magnetic flux distributions in the different conductors. The influence of the shape factor of the geometry and the filament orientation with respect to the local magnetic field has been thoroughly investigated. The optimal geometry and filament arrangement have been determined for each application

A Comparative Study of AC Transport and Eddy Current Losses for Coil Made of HTS Tapes Coated with Copper Stabilizer

WOS: 000412085400040In this work, the evaluation of AC loss of the pancake coils wound by HTS-coated conductors, by employing the finite element method in 2D, is presented. The transport current loss of the superconducting tape and the eddy current loss of the copper stabilizer as a function of the amplitude for four frequencies of the applied current are examined numerically by utilizing a newly developed calculation method based on the A - V formulation embedded in COMSOL Multiphysics software with an AC/DC module. The number of the coil turn is 10, and the radius is about 60 mm. The superconducting layer width and height in the simulations are 12 and 1 mm, respectively. The width and height of the copper layers are 12 and 80 mm, respectively. The critical current density of tapes is taken as 300 A.Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [114F424]This study is supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under the grant number 114F424

Numerical modelling of high temperature superconducting tapes and cables

This Ph.D. thesis is focused on the numerical modelling of high-Tc superconductors (HTS) at the operating temperature of 77 K (liquid nitrogen). The purpose of numerical modelling is to precisely calculate the current and field distributions inside HTS devices (tapes, cables) and the corresponding AC losses, which are one of the most important limiting factors for a large-scale application of such materials. From the electrical point of view, superconductors are characterized by a strongly non-linear voltage-current relation, which defines the transition from superconducting to normal state. In the case of HTS, the steepness of this transition is smoother than for low-Tc superconductors (LTS), so that the commonly used critical state model (CSM) gives a too simplified representation of their electromagnetic behaviour and can be used for a qualitative description only. In this thesis the finite element method (FEM) has been used for precisely computing the current and field distributions as well as for evaluating the AC losses in HTS devices. The superconducting transition is modelled with a power-law relation, E(J) = Ec(J/Jc)n, which is derived from the fit of transport measurements. Firstly, the results obtained with the software package FLUX3D on multi-filamentary tapes have been validated by means of a comparison with the ones obtained by another software package (FLUX2D). The results from FLUX2D, having already been successfully compared with experimental measurements within the framework of two previous Ph.D. theses at LANOS, have been used as reference. Secondly, two power-law models, which take into account the spatial variation of the critical current density inside HTS tapes and its strongly anisotropic dependence on the magnetic field, have been implemented in FLUX3D. In most cases, this latter dependence is extremely important, since the transport capacity of the superconductor is considerably reduced (and its power loss sensibly increased) by the presence of a magnetic field. Afterwards, FEM modelling of HTS tapes has been extended to cables. HTS cables are in general composed by different layers of several tapes and have a quite complex three-dimensional structure: in fact, the layers are wound around a central cylindrical support with different pitch lengths and relative winding orientations, in order to obtain a uniform repartition of the transport current among the layers, which minimizes the total AC losses. For overcoming the difficulties of a direct 3D FEM simulation, a simple electrical model, which allows to find the optimal pitch lengths and whose results are the input data for a 2D FEM evaluation of the AC losses, has been developed. FEM computations have also been used to investigate the influence of the non-uniformity of the tape properties (contact resistance, critical current, power index) on the global loss performance of a single-layer HTS cable. As an alternative to FEM computations, an equivalent circuit model of HTS cables has been utilized. It describes the cable from the macroscopic point of view and allows to compute the current repartition among the layers and the corresponding AC losses, without the necessity of having detailed information about individual tapes. In the framework of the European project BIG-POWA, I have collaborated to the assembling process of a HTS power-link at the Pirelli Labs, in the Milan region, Italy. Finally, 3D simulations have been performed to extensively study the coupling effect between two superconducting filaments via the resistive matrix, which is a typical case where analytical solutions exist for very peculiar geometries and physical conditions only. FEM simulations have been utilized to study the dependence of the filament coupling on the physical and geometrical parameters of the conductors

Alternating current loss of superconductors applied to superconducting electrical machines

Superconductor technology has recently attracted increasing attention in power-generation- and electrical-propulsion-related domains, as it provides a solution to the limited power density seen by the core component, electrical machines. Superconducting machines, characterized by both high power density and high efficiency, can effectively reduce the size and mass compared to conventional machine designs. This opens the way to large-scale purely electrical applications, e.g., all-electrical aircrafts. The alternating current (AC) loss of superconductors caused by time-varying transport currents or magnetic fields (or both) has impaired the efficiency and reliability of superconducting machines, bringing severe challenges to the cryogenic systems, too. Although much research has been conducted in terms of the qualitative and quantitative analysis of AC loss and its reduction methods, AC loss remains a crucial problem for the design of highly efficient superconducting machines, especially for those operating at high speeds for future aviation. Given that a critical review on the research advancement regarding the AC loss of superconductors has not been reported during the last dozen years, especially combined with electrical machines, this paper aims to clarify its research status and provide a useful reference for researchers working on superconducting machines. The adopted superconducting materials, analytical formulae, modelling methods, measurement approaches, as well as reduction techniques for AC loss of low-temperature superconductors (LTSs) and high-temperature superconductors (HTSs) in both low- and high-frequency fields have been systematically analyzed and summarized. Based on the authors’ previous research on the AC loss characteristics of HTS coated conductors (CCs), stacks, and coils at high frequencies, the challenges for the existing AC loss quantification methods have been elucidated, and multiple suggestions with respect to the AC loss reduction in superconducting machines have been put forward. This article systematically reviews the qualitative and quantitative analysis methods of AC loss as well as its reduction techniques in superconductors applied to electrical machines for the first time. It is believed to help deepen the understanding of AC loss and deliver a helpful guideline for the future development of superconducting machines and applied superconductivity

Experimental validation of ultra-shortened 3D finite element models for frequency-domain analyses of three-core armored cables

Recently, large offshore wind power plants have been installed far from the shore, using long HVAC three-core armored cables to export power. Its high capacitance may contribute to the appearance of unwanted phenomena, such as overvoltages or resonances at low frequencies. To adequately assess these problems, detailed and reliable cable models are required to develop time-domain/frequency-domain analyses on this type of cables. This paper presents, for the first time in the literature, an assessment on the performance of 3D finite element method-based (3D-FEM) models for developing frequency-domain analyses on three-core armored cables, confronting simulation results with experimental measurements found in the literature for three real cables. To this aim, a simplified ultra-shortened 3D-FEM model is proposed to reduce the simulation time during frequency sweeps, through which relevant aspects never analyzed before with frequency-domain 3D-FEM simulations are addressed, such as total losses, induced sheath current, magnetic field around the power cable, positive and zero sequence harmonic impedances, as well as resonant frequencies. Also, a time-domain example derived from the frequency-domain analysis is provided. Remarkable results are obtained when comparing computed values and measurements, presenting the simplified ultra-shortened 3DFEM model as a valuable tool for the frequency-domain analysis of these cables

Experimental validation of ultra-shortened 3D finite element models for frequency-domain analyses of three-core armored cables

Recently, large offshore wind power plants have been installed far from the shore, using long HVAC three-core armored cables to export power. Its high capacitance may contribute to the appearance of unwanted phenomena, such as overvoltages or resonances at low frequencies. To adequately assess these problems, detailed and reliable cable models are required to develop time-domain/frequency-domain analyses on this type of cables. This paper presents, for the first time in the literature, an assessment on the performance of 3D finite element method-based (3D-FEM) models for developing frequency-domain analyses on three-core armored cables, confronting simulation results with experimental measurements found in the literature for three real cables. To this aim, a simplified ultra-shortened 3D-FEM model is proposed to reduce the simulation time during frequency sweeps, through which relevant aspects never analyzed before with frequency-domain 3D-FEM simulations are addressed, such as total losses, induced sheath current, magnetic field around the power cable, positive and zero sequence harmonic impedances, as well as resonant frequencies. Also, a time-domain example derived from the frequency-domain analysis is provided. Remarkable results are obtained when comparing computed values and measurements, presenting the simplified ultra-shortened 3DFEM model as a valuable tool for the frequency-domain analysis of these cables

Optimisation of hysteretic losses in high-temperature superconducting wires

Hysteretic loss optimisations through numerical simulation and subsequent experimental confirmation in transport current and background field measurements: ferromagnetic shielding and topological geometry optimisation is used to reduce energy dissipation in HTS coated conductor geometries. Single tapes and coil geometries are investigated. A 3D model capable of taking into account contact resistances is also presented for the Twisted Stacked Tape Conductor cable