High Temperature Superconducting Transformer Research Articles

The Study on the Overcurrent Pulse Testing of Bi-2223/Ag HTS Winding and HTS Transformer

This paper summarizes the experimental investigations on a small-scale air-core high-temperature superconducting (HTS) transformer operated in liquid nitrogen. The designed HTS transformer is being used as part of a current pulsed-power supply. First, the HTS winding was tested to experience a short overcurrent pulse. The results indicated that a 170-A critical current Bi-2223/Ag HTS winding can bear a short overcurrent pulse to at least 1 kA without being damaged electromechanically. The transformer consists of an air-core 48-turn primary winding, which is enclosed by a 6-turn secondary winding. The primary and secondary windings are one- and three-strand parallel conductors of Bi-2223 Ag-sheathed multifilamentary tapes, respectively. The experimental results show that the output current of the HTS transformer can reach up to 1154 A, which is several times than the critical current of the secondary HTS winding.

Computation of the Thermal Effects of Short Circuit Currents on HTS Transformer Windings

The active development of high temperature superconductor (HTS) materials has led to extensive research and development studies of superconducting transformers worldwide. Considerable benefits have been accomplished with the introduction of the HTS transformer such as reduced power loss, size, and transformer weight. However, recent advancement in the design technology has focused on the HTS transformer's ability to also perform as a fault current limiter (FCL). This paper presents the computation of the thermal effects of short circuit currents on a non-FCL HTS transformer and demonstrates how it will behave with an HTS-FCL winding conductor.

Hysteresis Loss Improvement in HTS Transformers Using Hybrid Winding Schemes

The amount of hysteresis losses is a key parameter for the design of high-temperature superconducting (HTS) transformers. Leakage field, particularly its radial component, decreases critical current and increases hysteresis losses in HTS tapes. Prior art proposed the adoption of two auxiliary windings to an HTS transformer for minimizing the leakage flux densities by generating a field that opposes the leakage field, thereby reducing the hysteresis loss in an exemplary HTS transformer. In this paper, a proposed design will become prominent by obtaining a variant of distributive ratios via an intelligent choice of the secondary-primary-secondary-primary turn ratios in the neighborhood of the auxiliary windings to produce magnetic poles canceling or diminishing the corresponding leakage fields to the maximum effect. The mentioned schemes require accurate, yet efficient, advanced numerical techniques using 3-D solver of FLUX software for a precise calculation of the leakage flux density and hysteresis losses in an exemplary HTS transformer as a precious guide.

The Effects of Short-Circuit and Inrush Currents on HTS Transformer Windings

Rapid changes and developments are being witnessed in the transformer design technologies. The phenomenal growth of power systems has put tremendous responsibilities on the industry to supply reliable and cost-effective transformers. The advent of high-temperature superconductor (HTS) materials has increased interest in research and development of superconducting transformers with major projects being carried out worldwide. The major challenges in the design and development of HTS transformers are the modeling of short-circuit and inrush currents the transformer can withstand. Even though HTS technology is claimed to be more efficient, reliable, and eco-friendly, use of HTS transformers must be appropriately verified through the proper modeling of power system network.

The Construction Progress of a High-Tc Superconducting Power Substation in China

It is expected that superconducting technologies will play an important role in the future smart grid, because the application of superconductor technologies in the power grid can decrease power losses, relieve overload, avoid higher levels of transmission voltage, increase power transmission capacity, and improve power quality and grid stability. In recent years, high temperature (high-Tc) superconducting power technologies have achieved remarkable progress. High temperature superconducting (HTS) power equipment, such as HTS power cables, HTS transformers, high-Tc superconducting fault current limiters (SFCL), and high-Tc superconducting magnetic energy storage devices (SMES) have been demonstrated in the power grids of many countries. With the development of HTS power equipment, the construction of a HTS power substation is ready. In China, a 10.5 kV HTS power substation is under construction in Baiyin city, Gansu province. The substation integrates a HTS power cable, a HTS transformer, a HTS fault current limiter, and a high-Tc SMES. All these HTS power devices, which were previously developed by the Institute of Electrical Engineering (IEE), Chinese Academy of Sciences (CAS), have been demonstrated to operate for a long time in the commercial power grid. In this paper, the design and constructing progress of the HTS substation are introduced in detail.

Mechanical Force Analysis in Heavy-Current HTS Transformers Based on Field and Current Nonuniformity Coupled Analysis

Mechanical stresses of magnetically induced origins are becoming more serious with the trend of high-temperature superconductor (HTS) heavy-current transformer (HCT) applications. Mechanical forces in HTS HCTs that act on the transformer windings are generated by the interaction between the current density and the leakage flux density. This paper presents the calculation of leakage flux and mechanical force distribution in high-current HTS transformer windings. This requires the need for advanced numerical techniques for simulation studies using finite-element method by adapting two auxiliary windings for the leakage flux mitigation and, thereby, mechanical forces within the main windings. A model is developed for the calculation of mechanical force considering nonuniformity of both field and current distributions in the windings, which is the main motivation for initiating this paper.

Partial Discharge Measurements of Cryogenic Dielectric Materials in an HTS Transformer Using HFCT

Various types of high-temperature superconducting (HTS) devices have been developed, some of which have completed field testing and are ready for use in commercial applications. In order to facilitate the commercialization of HTS electric equipment for extra high voltage levels, cryogenic dielectric technologies should be established. In this work, in order to develop partial discharge (PD) measurement techniques for HTS transformers, artificial defects were installed both in liquid nitrogen and insulating oil. Experimental investigations were carried out for the diagnosis of the HTS transformer, and various PD patterns, which occurred due to the artificial defects of immersion in both insulating oil and liquid nitrogen, were measured. Throughout this work, the PD patterns that occurred in liquid nitrogen were detected, analyzed, and compared to those in insulating oil through the use of a high frequency current transformer (HFCT) diagnostic method. The partial discharge measuring techniques we developed could be fundamental in establishing HTS transformer diagnosis technologies for power applications.

Investigation of partial discharge phenomena in HTS transformer adopting different type of sensors

In this work, we proposed the partial discharge diagnostic techniques to evaluate the cryogenic dielectric insulation of high temperature superconducting (HTS) transformers. Three kinds of different artificial defects were fabricated in order to evaluate partial discharge (PD) characteristics of HTS transformers. Various PD sensors were installed to evaluate the performance of PD detecting characteristics in liquid nitrogen.From the experiments, it was possible to analyze PD signals and to discriminate what kinds of defects exist in cryogenic liquids using various types of PD sensors. Among three PD sensors, AE sensor and HFCT sensor showed more stable performances compared to UHF sensor. But according to the characteristics of defects, three sensors showed different sensitivities according to the PD signal frequency bandwidth of artificial defects. Consequently, this PD measuring technique could be utilized to establish the diagnostic devices for HTS transformer.

Insulation design of HTS transformer with the use of continuous transposed conductor

Important key technologies of a high-temperature superconducting (HTS) transformer may include the HTS wire technology, cooling, AC loss reduction, large current technology, and cryogenic temperature insulation. Among them, the cryogenic temperature insulation technology will be specifically a core technology for ensuring reliability for the smaller size, stability, economic efficiency of a HTS transformer.In this paper, we have simulated a HTS transformer applied with continuous transposed conductor, which has been studied as a way of reducing AC loss. Also, we analyzed the insulation composition and considered various characteristics of insulation breakdown in liquid nitrogen. Insulation design was realized in a way of calculating breakdown voltage of 0.1% with Weibull statistical treatment of the obtained experimental results and calculating insulation distance meeting basic impulse insulation level (BIL) 750kV, a target voltage.

The AC loss of an HTS transformer by 2D and 3D numerical calculation

For the design of a high temperature superconductor (HTS) power transformer the AC loss is a critical issue because the energy is dissipated as heat in the low temperature. It is necessary to study the AC losses theoretically and experimentally. For the analysis of AC losses in an HTS power transformer, two-dimensional numerical analysis has been carried out until now. However, AC losses which are calculated by using two-dimensional numerical analysis differ from those using three-dimensional numerical analysis because the geometry of the HTS transformer is not symmetric. AC losses of a 1 MVA-single-phase HTS transformer were calculated using both two-dimensional and three-dimensional numerical analysis. To prove the effectiveness of the calculations, the numerical results were compared with the measured results for a transformer fabricated using Bi 2Sr 2CaCu 2O 8+ δ tapes in 2004.

Insulation Design and Experimental Results for Transmission Class HTS Transformer With Composite Winding

In the response to the demand for electrical energy, much effort aimed to develop and commercialize high temperature superconducting (HTS) power equipments has been made around the world. In Korea, companies and universities are developing a power distribution and transmission class HTS transformer that is one of the 21st century superconducting frontier projects. The composite winding of transmission class HTS transformer is concentrically arranged primary winding (High)-Secondary winding (Low)-tertiary winding (High) from center. Primary winding is continuous disk type, Secondary winding is layer type and tertiary winding is Double pancake type. For the development of transmission HTS transformer with composite winding, the cryogenic insulation technology should be established. We have been analyzed insulation composition and investigated electrical characteristics such as breakdown of , barrier, kapton films and surface flashover on Fiber Reinforced Plastic (FRP) in Liquid nitrogen . We are going to compare with measured each value and apply the value to most suitable insulating design of the HTS transformer.

Influence of High Temperature Superconducting Transformer Geometry on Leakage Magnetic Field

Leakage magnetic fields in high temperature superconducting (HTS) transformer windings reduce the critical current and increase the ac losses in HTS tapes. Moreover, because of the anisotropic properties of the HTS tapes, the influence of the radial component of the leakage field on critical current and ac losses is much stronger than that of the axial one. For these reasons, leakage magnetic fields must be carefully considered in HTS transformer design. In this paper, we report a study of the influence of the core structure and the winding configuration on the leakage field by the finite-element method (FEM), and offer some suggestions for reducing the maximal radial component of leakage field to make the HTS transformer more efficient.

A new approach for AC loss reduction in HTS transformer using auxiliary windings, casestudy: 25 kA HTS current injection transformer

AC loss is one of the important parameters in HTS (high temperature superconducting)AC devices. Among the HTS AC power devices, the transformer is an essential partin the electrical power system. The AC losses in an HTS tape depend on themagnetic field. One of the techniques usually adopted to mitigate the unwantedmagnetic field is using a system of coils that produce a magnetic field oppositeto the incident one, reducing the total magnetic field. In this paper adding twoauxiliary windings to the HTS transformer to produce this opposite magneticfield is proposed. The proper use of these auxiliary windings could reduce theleakage flux and, therefore, the AC loss. A mathematical model is used to describethe behaviour of a transformer operating with auxiliary windings, based on thetheory of electromagnetic coupled circuits. The influence of the auxiliary windingson the leakage field is studied by the finite element method (FEM) and the ACloss of an HTS transformer was calculated. Also, the simulation results showthat employing auxiliary windings will improve the HTS transformer efficiency.

Design of 154 kV class 100 MVA 3 phase HTS transformer on a common magnetic core

We have proposed a high temperature superconducting (HTS) transformer which can substitute for a conventional oil transformer for power distribution in Korea. The conventional transformer is composed of three identical single phase transformers because of the limitations on volume and weight. Now the Korean power company has started to consider the possibility of a transformer on a common magnetic core because it can be a solution for the increment of capacity without new construction of substations. In this paper, we proposed an HTS transformer on a common core. Its capacity is the same as the previous HTS transformer, which was 100MVA, 154kV/22.9kV. The former is smaller and lighter than the latter. We assumed a transformer tank which covers the common core and windings. The tank also acts as a vacuum seal for a cooling system and so the cryostats for the windings do not need to have vacuum layers. The design parameters are compared with those of both a conventional one and an HTS transformer with separate cores.

Design of the Cryogenic System for 100 MVA HTS Transformer

A 100 MVA, commercial class high temperature superconducting (HTS) transformer was designed. In this paper the cryogenic system to extract the heat generated at the winding due to the AC loss and the heat penetration through the wall and the current lead and keep the winding temperature under design temperature is presented. The operation temperature is set at 67 K to increase the critical current and reduce the amount of HTS tape usage and the volume. The HTS winding was concentrically arranged to reduce the AC loss. The system consists of main cryostat with the windings and secondary cryostat with cooler, cryopump and the heat exchanger inside. The liquid nitrogen is entering the main cryostat at 65 K and assumed to be leaving at 67 K. The winding temperature distribution is simulated with the anticipated heat load using Fluent. The effect of the liquid nitrogen flow rate, inlet location is investigated and discussed.

Comparison of Insulation Test of Mini-Models With Different Winding for a HTS Transformer

Recently, universities and companies are developing a power distribution and transmission class high temperature superconducting (HTS) transformer that is one of the 21st century superconducting frontier projects in Korea. For the development, it is necessary to establish the dielectric technology at cryogenic temperature such as insulating design, cooling system, manufacture, compact, and so on. Also, verification of insulating stability is one of important dielectric technology. Therefore, we prepared three models for a small simulated the HTS transformer, one is concentric arrangement, another is reciprocal arrangement, the other is Z continuous winding arrangement from Kapton insulated Cu tape and measured their insulation characteristics such as partial discharge (PD), AC (50 kV, 1 min) and impulse (154 kV, 1.2times50 mus) withstand test. Before manufacture each model, we have been analyzed insulation composition and investigated electrical characteristics such as breakdown of LN2 , polymer and surface flashover on fiberglass reinforced plastic (FRP) in LN2. We are going to compare with measured value and apply the value to most suitable insulating design of the HTS transformer

Numerical analysis on magnetic field of HTS transformer with different geometry

In a high-temperature superconducting (HTS) transformer leakage magnetic field, especially its radial component, decreases the critical current and increases the ac losses in the tapes. Because of zero resistance of HTS tapes a slight unbalance among the branch inductances of the windings may result in heavy circulating current, however, an accurate calculation of it must depend on the magnetic field analysis. Thus, a numerical analysis of the leakage magnetic field is especially necessary for an HTS transformer design. In this paper the leakage magnetic field of several transformer geometries is studied by finite element method, and based on it, the circulating current distribution is calculated. Some suggestions for improving the leakage field and circulating current distribution are presented to make HTS transformers more efficient

The New Generation of Superconductor Equipment for the Electric Power Grid

High-temperature superconductor (HTS) power equipment, such as power cables, synchronous condensers, fault current limiters (FCLs), and transformers, are poised to address key technical issues in the U.S. Power grid. This equipment is enabled by the successful development of robust long-length HTS wire, available commercially from a variety of manufacturers worldwide. Compared to conventional cables, HTS power cables offer higher capacity with minimal disturbance to neighboring underground infrastructure, and their low series impedance opens the opportunity for cost-effective power flow control with phase angle regulators. HTS cables are being actively prototyped worldwide. Based on technology developed for industrial and ship propulsion motors, an HTS synchronous condenser has been developed and successfully tested in the Tennessee Valley Authority (TVA) grid; this is the first HTS power equipment to be offered commercially. It addresses the growing need for dynamic reactive power compensation for grid stability and power quality. HTS FCLs and transformers are also in active development

Overview of the development of the advanced power system by the applied superconductivitytechnologies programme in Korea

At the end of July 2004, DAPAS (development of the advanced power system by appliedsuperconductivity technologies), which is a 10 year long programme, finished the first ofthree phases. As a result of the first phase, a process for a metre long coated conductor andthe core technologies of the distribution line voltage class high temperature superconductor(HTS) power systems including cables, transformers, fault current limiters (FCLs), andmotors have been developed. The ultimate goal of the HTS cable project is to develop1 GVA class HTS transmission cables. During the first phase, a three-phase 50 MVA/22.9 kVclass HTS cable of 30 m length has been developed and tested successfully; it hasbeen installed by LS Cable Ltd. In the second phase, grid-connected HTS cableswill be developed and established in the real utility line. In case of FCLs, therewere two types of FCL developed during the first phase: a reactive type and aresistive type. Both types had the same project targets with the specification of6.6 kV/200 A for three-phase, and entered the second phase with the target of a 22.9 kVclass. In the case of the HTS transformer project, a 1 MVA/22.9 kV class wasdeveloped, and after the second phase this project will be considered for practicalapplication. Technically, the optimal design and the manufacturing technologies of HTStransformers as well as the analysis tools of electromagnetic field in the transformer weredeveloped. Furthermore, a 100 hp class motor has been developed so that the keytechnologies for the utilization of superconducting motors could be obtained. Thedeveloped motor consists of Bi-2223 field coils of 100 A operating current at 30 K and aclosed-loop cooling system, the results of which will lead us to develop 1 and 5 MVAmotors in the second and third phase, respectively. In the case of the coatedconductors (CCs) programme, a batch-type co-evaporation process and a reel-to-reelpulsed laser deposition process gave us 10 m long CC tapes in February 2005. Weexpect that 100 m long or longer CC will be developed during the second phase.

Analysis of Magnetic Field and Circulating Current for HTS Transformer Windings

In a high-temperature superconducting (HTS) transformer, the leakage magnetic field decreases the critical current and increases the ac loss in the tapes. Moreover, because of nearly zero resistance of HTS tapes, a slight unbalance of the branch inductances of the windings might result in heavy circulating current. So, the numerical analysis of the leakage magnetic field and circulating current is especially necessary for an HTS transformer design. In this paper, the influence of the winding configurations on the stray field and circulating current is studied. That is, the magnetic field distribution is analyzed by finite-element method and then, based on the inductance matrix obtained after a magnetic field analysis, the circulating current is calculated by circuit analysis. Some measures for improving the leakage field and circulating current distribution are also proposed to make HTS transformers more efficient.