Search

Inrush current in high-temperature superconducting (HTS) transformers is a little-studied phenomenon. After connecting to the power grid, a current flow through the windings which exceeding the critical current value of the superconducting tape. It may cause significant overheating and thermal damage of winding. The purpose of the study is to develop a mathematical model for calculating inrush current pulses in a HTS transformer and its verification by physical experiments. To achieve the goal of the study, a mathematical model has been developed that accurately represents the electromagnetic and thermal transient processes after HTS transformer is turned on at idling or under load. The model considers the critical parameters of the HTS tapes, the process of heating and cooling of the windings, quench characteristics, and the electrical and magnetic parameters of the transformer. Good compliance of the experimental results and mathematical modeling with a deviation of 1.99 % allowed us to verify the model. The most important result is the creation of a mathematical model of the HTS transformer at the moment of connecting to power grid. This model represents the temperature changing of the windings during the loss of superconductivity. The developed model can be used in the analysis and modeling of inrush current in designed and operating HTS transformers for any power. The obtained results are significant for determine the optimal starting characteristics, geometric and electrical parameters of HTS transformers. The proposed methods for reducing the inrush current ensure safe and reliable operation of the HTS transformer when switched on at idling or under load.

The study's aim is to substantiate and employ materials with high technical characteristics for the active elements of a transformer, particularly in the context of increased operating frequencies. To accomplish this objective, the following tasks were addressed: An analysis and synthesis of the properties of a magnetic core composed of amorphous iron and superconducting windings in a high-temperature superconducting transformer (HST) were conducted. Dependencies and graphs illustrating the impact of frequency increase on heat losses related to hysteresis and eddy currents were derived. These losses occur within the magnetic circuit. The study also demonstrates the relationship between reducing size and weight parameters and material consumption of windings in high-temperature superconducting (HST) tapes. Current densities in these tapes can reach up to 500 A/mm². The most noteworthy outcome, taking advantage of superconductivity in the HST, is that HST acts as an ideal diamagnetic material. Additionally, the windings exhibit high electrical conductivity. This resolves issues related to current displacement to the conductor's surface, commonly known as the "skin effect," and effectively addresses concerns about increased resistance in transformers and electrical machines. The absence of the skin effect in HST conductors has been rigorously demonstrated using Bessel functions. The significance of these results is in enhancing the efficiency of transformers operating at high frequencies. This is achieved by synthesizing the properties of the transformer's active elements, which include an amorphous iron magnetic core, HST windings, and a dielectric medium utilizing liquid nitrogen.

Today, high-temperature superconducting (HTS) current limiters and transformers allow to limit the surge short circuit current during failure without negatively affecting on the power grid com-plex at the normal operation mode. However, the transition of a superconductor to a resistive state at the moment of current limitation can cause significant heat generation, which can destroy the transformer windings. The research goal is to provide optimal technical characteristics of the HTS transformer to achieve effective short circuit current limitation and prevent thermal breakdown of its windings. To achieve this goal, a mathematical model of a HTS transformer was developed. The presented method considers the material type and geometry of the superconducting tape, the critical parameters of the superconductor (current and temperature), the parameters of the cryogenic liquid, dependence of the resistance and heat capacity of the HTS tape layers on temperature. The simulation model was created in the Matlab/Simulink software. The most important result is the possibility of obtaining optimal elec-trical and thermal parameters of the HTS transformer windings during the short circuit current limitation, as well as ensuring the thermal stability of the superconducting tape at the quench moment. The obtained results are significant in the design and operation of HTS transformers. For efficient and safe operation in the current-limiting mode, it is necessary to take into account heat generation on the transformer windings. It is important for the superconductor returning to the superconducting state without causing significant overheating of the windings.

High temperature superconducting (HTS) transformers have many advantages over conventional transformers in terms of volume, weight, and total efficiency. However, fault conditions limit the electro-thermal performance of an HTS transformer. Based on this, we investigate the thermal modeling of the HTS transformer in the fault-current state. For thermal modeling, heat transfer and parameters affecting it such as surface roughness and bubble behavior have been considered. At first, an accurate thermal model for a 10 kVA HTS transformer has been prepared. Experimental results show that the penalty of this model is about 2.5 K in the hot-spot point (HSP) temperature in fault-current conditions. In a second step, the impact of a roughness on the heat transfer and mass transfer rate in nucleated boiling mode is investigated. Results show that surface roughness decreases the HSP temperature by more than 10 K in fault-current conditions. These results have significant impacts on protecting HTS windings against quenching. [ABSTRACT FROM AUTHOR]

In order to improve the ability of superconducting transformer on limiting fault current, the parallel stainless steel & REBCO tapes are used as the winding materials. Firstly, the critical current of an individual REBCO tape and the parallel stainless steel & REBCO tapes under fault current are measured experimentally to compare their ability of limiting fault current. Then the current distribution in the parallel stainless steel & REBCO tapes under different fault currents are simulated. Lastly, a 6 kV/400 V (120kVA) HTS transformer is designed, and the fault current characteristics of wingdings are simulated based on finite element method (FEM). Experimental and simulation results show that the parallel stainless steel tape can improve the current limiting performance of the windings, which is of important referential value for the design of HTS transformer. [ABSTRACT FROM AUTHOR]

The paper provides the results of the experimental and computational study of the processes occurring in high temperature superconducting transformer windings while secondary winding is short-circuited. The obtained mathematical simulation matches closely with the experimental results. The temperature variation curves for superconducting windings were analysed, and conclusions were made on the necessity of changes in HTS transformer design, namely the necessity of windings heat-insulation from each other and adding a high-resistance coating material for HTS wire in HTS transformer primary winding. [ABSTRACT FROM AUTHOR]

In Offshore Substations (OSs), using High Temperature Superconducting (HTS) transformers instead of conventional power transformers to reduce operational and construction limitations such as weight, capacity, marine pollution, and future planning can be considered as desirable. HTS transformers can reduce operating system costs in wind power plant development programs. However, OSs have specific operating conditions in fault mode that should be considered in the design HTS transformers. In this paper, a 250 MVA 33/220 KV HTS transformer is designed, and electromagnetic analysis is done in normal conditions to calculate AC loss and thence to design the cooling system. The second step, thermo-flow analysis is done by Finite Element Method (FEM) to calculate the temperature distribution of the windings. Increasing turbulence of inlet flow and creating a porous coating on the tapes are used in this transformer to increase the heat-transfer rate. lastly, the specifications of the HTS transformer are compared with the conventional transformer to give useful information to OS designers to investigate using novel construction ideas such as floating OS by HTS transformer. [ABSTRACT FROM AUTHOR]

HTS transformers can provide lower loss and higher efficiency with smaller size and lighter weight compared with traditional transformers. In this paper, we designed a 500 kVA HTS transformer and developed its simulation using finite element software based on the H -formulation, and the simulation incorporates a heat model and E-J power law. The simulation shows that there is a large radial flux leakage at the end of the low-voltage (LV) windings (superconducting windings), leading to a reduction in critical current density and an associated increase in joule loss in the LV winding. In order to reduce joule loss, various LV winding structures were designed. The results demonstrate that either increasing the axial distance between the coils at the LV winding end or installing a flux diverter outside the LV windings can effectively diminish the radial flux leakage at the LV winding end, consequently reducing joule loss in the LV winding. The impact of the size, position and relative permeability of the flux diverter on radial flux leakage and joule loss are also studied. These structure optimizations and the corresponding effects have important significance to the design of HTS transformers. [ABSTRACT FROM AUTHOR]

Manufacturers and consumers of transformers rather conservatively concern all innovations in transformer manufacture since in power distributive transformers are inherent long term of the operation, essential cost and a purchasing cycle. Progress of perfection of a design of the transformer is substantially defined now by creation new and perfection used conduction, magnetic and insulating materials.Regarding creation new conduction the materials applied by manufacture of mains transformers, it is possible to allocate high-temperature superconducting materials and, accordingly, the HTS transformers created with their application. Pre-production models of HTS transformers with use HTS of a tape of 2nd generation on the basis of superconducting ceramics GdBCO are already made and are tested. Development of manufacturing techniques of the hightension cable has allowed to create a cable with isolation from cross linked polyethylene (XLPE) and to apply it in the field of transformer technologies by working out of the new dryformer.Regarding creation of new magnetic materials the special role is taken away so-called, to the nano-crystal alloy which features have laid down in a basis of creation of new type of amorphous core transformers. Now in the world works on working out of tapes from the amorphous alloys possessing improved magnetic properties and cheapness, in particular Fe(Co)SiBPCCu and 2605SA1 are conducted. As tendencies in change of type of the core and its design are observed. More and more wide application at manufacturing of power three-phase transformers small and a mean power is found by the strip-wound cores which design is executed on technology UNICORE. As interest to a design of cores of the electrotechnical steels executed from different types and received the name “Sandwich” is shown.Regarding perfection of used insulations materials, it is possible to note application of electrotechnical gas (SF6) in transformers in circuit-breaker oil replacements. Such transformers have received the name gas-insulated transformers, and their obvious advantage are ecological compatibility and fire safety.Now it is much spoken about creation of so-called Smart Grid, and their transition in the future to the most effective systems of a highest level called now Microgrid. Certain demands are made to the specified electric systems: maintenance optimum capacity factor and high electrical power factor. The resulted requirements, at present, can be realised only new semiconductor transformers, such as the “Sen” transformer.

One of critical issues for HTS transformers is achieving sufficiently low AC loss in the windings. Therefore, accurate prediction of AC loss is critical for the HTS transformer applications. In this work, we present AC loss simulation results employing the H -formulation for a 1 MVA 3-Phase HTS transformer. The high voltage (HV) windings are composed of 24 double pancakes per phase wound with 4 mm – wide YBCO wire. Each double pancake coil has 38 ¼ turns. The low voltage (LV) windings are 20 turn single-layer solenoid windings wound with 15/5 (15 strands of 5 mm width) Roebel cable per phase. The numerical method was first verified by comparing the numerical and experimental AC loss results for two coil assemblies composed of two and six double pancake coils (DPCs). The numerical AC loss calculated for the transformer was compared with the measured AC loss as well as the numerical result obtained using the minimum magnetic energy variation (MMEV) method. The numerical AC loss result in this work and experimental result as well as the numerical result using MMEV at the rated current agree to within 20%. Further simulations were carried out to explore the dependence of the AC loss on the gap between the turns of the LV winding. The minimum AC loss at rated current in the 1 MVA HTS transformer appears when the gap between turns is approximately 2.1 mm turn gap in the LV winding. This is due to the change of relative heights between the HV and LV windings which results in optimal radial magnetic field cancellation. The same numerical method can be applied to calculate AC loss in larger rating HTS transformers. [ABSTRACT FROM AUTHOR]

With the mature of manufacture process and technology of high temperature superconductors, the critical current and stability are gradually increased. High temperature superconductors could be used in transformer, current limiter, generator, magnet and etc. This paper focuses on development and characteristic tests of a single-phase high temperature superconducting (HTS) power transformer with capacity of 330 kVA by using GdBCO tapes, which is 1/3 model of a 1 MVA / 10 kV / 0.4 kV HTS transformer. The specifications of iron core, HTS windings and cryostat are described in detail. The iron core is made of silicon steel plate. The arrangement of HTS windings are based on experimental and simulated results. The cryostat with a room temperature bore is manufactured using nonmetallic materials. Several characteristic tests and insulation tests are performed in liquid nitrogen of 77 K. The efficiency and AC loss at rated load is 99.90% and 243.7 W, respectively. In addition, an overload test was also performed. [ABSTRACT FROM AUTHOR]

This paper focuses on conceptual design and electromagnetic analysis of a single-phase high-temperature superconducting (HTS) power transformer with a capacity of 300 kVA by using GdBCO tapes, which is one-third model of a 1 MVA/10 kV/0.4 kV HTS transformer. The conceptual design consists four parts: iron core, HTS windings, current lead, and cryostat. In the electromagnetic optimization of the 330 kVA HTS transformer windings via finite element method, the influence of winding configurations on the radial magnetic field and the current nonuniformity are considered. The details about the design and optimization of radial magnetic field will be discussed in this study. [ABSTRACT FROM PUBLISHER]

This paper focuses on the comparison of the characteristics among three different types of high temperature superconducting (HTS) transformers. The iron core structures of the three types of HTS transformers are air core, partial core, and full core. The iron cores are made of oriented silicon steel sheets. The primary windings are four layers connected in series, and the secondary windings are nine double pancake coils connected in parallel. Fundamental tests, such as critical test, no-load test and load test, are performed in the environment of liquid nitrogen. The current nonuniformity of HTS partial core and full core transformers in secondary windings are also confirmed by the load test. Finally, we also investigate the characteristic of fault current limiting by sudden short-circuit tests. The experiment results show that the structure of the iron core have significant effects on the characteristics of HTS transformers. The results of this paper could support with useful data for the design and selection of HTS transformers. [ABSTRACT FROM PUBLISHER]

In recent years, using high-temperature superconductor (HTS) transformers in transportation and electrical power systems has attracted much interest. The major advantages of HTS transformers compared with conventional ones are much smaller size and volume and lighter weight in confined spaces of urban areas or transportation devices. One of the most critical parameters to design, fabricate, and commercialize HTS transformers is hysteresis loss in HTS tapes. Using numerical methods, the accurate value of hysteresis loss and efficiency can be calculated or estimated in HTS transformers. In this paper, two innovative techniques to reduce the leakage flux and thereby hysteresis loss in HTS transformers have been proposed. In the first technique, the concentric windings of HTS transformer are apportioned into a certain number of sub-windings. In the second one, with optimizing the distributive ratios of sub-windings, the windings become unsymmetrical. In order to demonstrate the effectiveness of proposed techniques, an accurate and advanced numerical method based on finite element analysis has been used to design four symmetrical and unsymmetrical primary (P)-secondary (S)-P-S-P and P-S-P-S-P-S-P winding schemes. The simulation results prove the effectiveness of both techniques in reducing the radial and axial leakage flux magnitudes and thereby hysteresis loss in HTS windings. [ABSTRACT FROM AUTHOR]

Abstract: HTS transformer, without any loss of insulation lifetime due to the reduction in terms of size and weight, can increase the overload capacity, and have some benefits such as the improvement in efficiency, minimization of environmental pollution, and convenient spatial arrangement, which contribute a lot to electric power system operation. However, for practical insulation design of the HTS transformer, it is necessary to establish the research on electrical properties LN2 as well as solid insulators. These solid insulators have been used as main insulations for HTS transformer. In this paper, we discussed breakdown and V–t characteristics of glass fiber reinforced plastics (GFRP) and pressboard in LN2. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

The magnetic losses of steels used in high temperature superconducting (HTS) transformers are very important for the transformer's efficient design and for a correct choice of operating conditions. This introduced the necessity to analyze electrical steels behaviour at low temperature in order to characterize their magnetic properties and, in particular, the magnetic losses. Three steels were chosen for this analysis: one non-oriented silicon steel and two different grain-oriented silicon steels. Several measurements of total magnetic losses were made at 77 K and 298 K and for different values of maximum magnetic induction, B, in order to compare the dependence between them at both temperatures. These results are presented in Pronto et al. (European Conference on Applied Superconductivity, ). Subsequently, measurements of hysteresis losses (DC regime) at both temperatures were also made in order to obtain all terms of magnetic losses (hysteresis, classical eddy-current losses, and excess losses). The results are presented in this paper. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

The technologies to be required to commercialization a HTS transformer are of a reduction in AC loss, an improvement of conductor characteristics, cooling technology, high voltage technology, bushing technology and so on. Amongst the technologies, the development of bushing is the important technology which should be accomplished to apply high voltages into the winding of the HTS transformer. However, the studies and developments have done on bushing for HTS transformer are not enough. The bushing is energized with the line-to-ground voltage between the coaxial center and outer surrounding conductors, in the axial direction there is a temperature difference between room temperature and 77 K. So it has to endure for electrical insulation as well as the thermal contraction as well. Therefore, in order to insulating design of a 60 kV class bushing for HTS transformer this paper will report on the experimental investigations in impulse breakdown and surface flashover characteristics of the insulation materials. Insulation design was realized in a way of calculating breakdown voltage of 0.1% through Weibull statistical treatment of the obtained experimental results and calculating insulation thickness and separation distance meeting BIL 325 kV, a target voltage. [ABSTRACT FROM AUTHOR]

Abstract: 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 Bi2Sr2CaCu2O8+ δ tapes in 2004. [Copyright &y& Elsevier]

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 LN2, barrier, kapton films and surface flashover on Fiber Reinforced Plastic (FRP) in Liquid nitrogen (LN2). We are going to compare with measured each value and apply the value to most suitable insulating design of the HTS transformer. [ABSTRACT FROM AUTHOR]

HTS transformers are being developed by one of 21st Century Frontier R&D Programs in Korea. The final target of the program for the HTS transformer is to develop a 3 phase 100 MVA HTS power transformer. Before then, we proposed a single phase 5 MVA HTS transformer using 2G wires. It is not for commercialization but for the preliminary research for fabricating the 100 MVA HTS transformer. We supposed volt per turn of 100. It has primary, secondary and tertiary windings. Its size is almost same with that of a 33 MVA single phase HTS transformer in previous work. AC losses due to the winding methods and the variations of other parameters are analysed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The cryogenic high voltage bushing for a 154kV/100MVA high temperatures superconducting (HTS) transformer is described. The bushing is energized with the line-to-ground voltage between the coaxial center and outer surrounding conductors, in the axial direction there is a temperature difference between ambient and 77K. Therefore, it has to endure for electrical insulation as well as the thermal contraction as well. In this research, the electrical insulation characteristics of GFRP to achieve high durability in the cryogenic environment were surveyed in the air and LN2. Moreover, the insulation constructions of the commercial condenser type bushing were studied. Based on these data, the electrical insulation design of the cryogenic condenser type bushing for a 154kV class HTS transformer was performed. [Copyright &y& Elsevier]

Abstract: In the response to the demands for electrical energy, much effort aimed to develop and commercialize HTS power equipments have been made around the world. Among them HTS transformer is one of very promising one. 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 continuous disk winding of transmission class HTS transformer is concentrically arranged H1–L–H2 from center. For the development of transmission HTS transformer with continuous disk winding, the cryogenic insulation technology should be established. We have been analyzed insulation composition and investigated electrical characteristics such as the breakdown of LN2, barrier, kapton films, flashover on FRP in LN2. We are going to compare with measured each value and apply the value to most suitable insulating design of the HTS transformer. [Copyright &y& Elsevier]

HTS transformers which have been developed until now had only fundamental structures. Among the auxiliary functions of conventional transformers, voltage regulating is the most important one. For the voltage regulation, conventional transformers are equipped with on load tap changers (OLTCs). In this paper, we investigate the possibility of the HTS transformer with OLTC. For the case study, we designed a single phase 33 MVA HTS transformer with OLTC. It is one of three individual HTS transformers which composes a 3 phase, 100 MVA transformer. It is expected to substitute for a 3 phase, 60 MVA conventional transformer in Korea. The parameters of an HTS transformer are varied due to the gap length between primary and secondary windings. The length was decided for the transformer to have the impedance of 12%. Its size was limited to the one of the conventional transformer. The characteristics of the HTS transformer were analysed in both case of having OLTC and not. [ABSTRACT FROM AUTHOR]

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. This work was supported by a grant from the Center for Applied Superconductivity Technology of the 21st Century Frontier R&D Program funded by the Ministry of Science and Technology, Republic of Korea. [ABSTRACT FROM AUTHOR]

Abstract: Current leads of HTS transformer are a part that leads to heat leakage. Its design and fabrication influence greatly the loss of nitrogen during operation. It is important to calculate and identify the heat leakage of current leads. According to the requirements of the 300kVA HTS transformer used for electrical locomotives, based on finite element method and heat transfer theory, this paper designed unitary current leads, which are winded just like solenoid shape. And, as the low voltage current lead was not feed DC current or feed different DC currents, a series of experiments were carried out on measuring the gas flux respectively. Then, the calorimetry principle about heat leak is obtained. The experimental results show a good according with the simulation, which show that the unitary helical current lead meets the requirements of design. [Copyright &y& Elsevier]

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, 1.2 × 50 μs) 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. [ABSTRACT FROM AUTHOR]

Abstract: For the development of electrical insulation design of a HTS transformer with Z continuous winding, we have discussed insulation composition and investigated breakdown characteristics such as breakdown of LN2, polymer and surface flashover on FRP and breakdown-surface combination in LN2. Also we have designed and manufactured a bobbin that has spiral slot for the Z continuous winding. A Z continuous winding mini-model from Kapton film insulated Cu tape for simulated 22.9kV class HTS transformer has been constructed using 0.1% breakdown strength obtained by Weibull distribution. The winding model was measured with respect to its insulation characteristics such as ac (50kV, 1min) and impulse (154kV, 1.2×50μs full wave, 3 times) withstand test and its excellent performance was confirmed. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Abstract: In a HTS transformer a slight unbalance of the branch inductances of the windings might result in heavy circulating current because of zero resistance of HTS tapes. It is well-known that the circulating current worsens the distribution of leakage magnetic field and consequently does harm to the ac loss and critical current in the tapes. In this paper, the influence of the secondary winding configurations on the circulating current is studied by combining magnetic field calculation and circuit analysis, and some suggestions for improving the distribution of circulating current are also presented to make HTS transformers more efficient. [Copyright &y& Elsevier]

A 1 MVA single phase high temperature superconducting (HTS) transformer was manufactured and tested. The rated voltage of primary and secondary of HTS transformer are 22.9 kV and 6.6 kV respectively. BSCCO-2223 HTS tape was used for HTS windings of 1 MVA HTS transformer. In order to reduce AC loss generated in the HTS winding, the concentrically arranged winding was adopted to 1 MVA HTS transformer. Single HTS tape for primary windings and 4 parallel HTS tapes for secondary windings were used considering the each rated current of the HTS transformer. A core of HTS transformer was fabricated as a shell type core made of laminated silicon steel plate. And a GFRP cryostat with a room temperature bore was also manufactured. The characteristic tests such as no load test, short circuit test and several insulation tests were performed at 65 K using sub-cooled liquid nitrogen. Through these test results, the validity of design of HIS transformer was ascertained. [ABSTRACT FROM AUTHOR]

In this paper, a 3 phase 60 MVA high temperature superconducting (HTS) transformer was designed with YRCO coated conductor windings and its characteristics were analyzed. First, an automatic calculation program for HTS transformer design was developed, in which the Ic-B characteristics of HTS tape was considered. Using the calculation program, basic dimensions of windings and iron core were determined and electromagnetic field analysis was performed for the loss computation. Finally, efficiency, weight and wire length of HIS transformer with coated conductor winding were calculated. The efficiency was over 99.9%, which is higher than that of any conventional transformer. The weight of the HIS transformer was about 16.6 tons and the length of HIS wire was about 47 km when the voltage per turn is 89.7234. Moreover, the magnetization AC loss of coated conductor winding was very small compared with that of BSCCO-2223 winding. To reduce the AC losses, we introduced the low current density winding method in all windings and the magnetization loss was reduced to about 66% of that of general winding. [ABSTRACT FROM AUTHOR]

In Korea, the Hyosung Industrial, Korea Polytechnic University and Gyeongsang National University are developing a power distribution and transmission class HTS transformer that is one of the 21st century superconducting frontier projects. 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 an important dielectric technology. Therefore, we prepared two models, one is concentric arrangement, the other is reciprocal arrangement, from Kapton insulated Cu tape for a small simulated HTS transformer and measured their insulation characteristics such as PD, ac (50 kV, 1 mm) and impulse (154 kV, 1.2 × 50 μs) withstand test. Before manufacture of each model, we have analyzed insulation composition and investigated electrical characteristics such as breakdown of LN2, polymer and surface flashover on FRP in LN2. We are going to compare with measured each value and apply the value to most suitable insulating design of the HTS transformer. [ABSTRACT FROM AUTHOR]

AC loss is one of the important parameters in HIS (High Temperature Superconducting) AC devices. Among the HTS AC power devices, the transformer is an essential part in electrical power system. But, AC loss is one of the most serious problems of the HIS transformer, especially with pancake windings, because high alternating magnetic field is applied perpendicularly to the surface of BSCCO wire in HIS windings of that, comparing with the other HTS AC power devices. For the reason above the calculation of AC loss generated in the HTS windings should be carried out in advance when designing the HTS transformer. In the paper we performed study for optimization of winding design to minimize the magnetization loss of HTS winding such as the spaces between pancake windings and operating temperature of HIS wire. The calculation of the AC loss was accomplished by 2-dimensional Finite Element Method. [ABSTRACT FROM AUTHOR]

Abstract: 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. Especially, HTS transformer is one of the most promising devices. For the development of HTS transformer, the cryogenic insulation technology should be established. In this paper V−t characteristics of polyimide (Kapton) tape and GFRP used as turn-to-turn and structural insulations, respectively were studied. Moreover, breakdown hole site of GFRP after breakdown was also discussed. The experimental results show that the time to breakdown is conditioned on applied electric stress and the lifetime indices n of Kapton tape decrease slightly as the number of tape increases while the lifetime indices n of GFRP decrease strongly with increasing thickness. Furthermore, the breakdown holes of GFRP were not at the contact point, at which the electric field is maximum value, between sphere electrode and GFRP sample and its location depends on applied voltage as well as sphere diameter. [Copyright &y& Elsevier]

In the response to an increasing demand for electrical energy, much effort aimed to develop and commercialise HTS power equipments is going on around the world. For the development, it is necessary to establish the dielectric technology in LN2. Hence many types of dielectric tests should be carried out to understand the dielectric phenomena at cryogenic temperature and to gather various dielectric data. Among the many types dielectric tests, the characteristic of surface flashover and the barrier effect were conducted with the simulated electrode after analysing the insulating configuration of the pancake-coil-type HTS transformer. The influence of a barrier on the dielectric strength was measured according to the size of the barrier, the position of the barrier and the effect of the back-electrode. It was shown that the effectiveness, namely the ratio of the breakdown voltage in presence of barrier to the voltage without barrier, is highest when the barrier is placed at the needle electrode side. The effect increased up to 1.8 times when collar length is 10 mm. The flashover characteristic with back-electrode was remarkably lower than the characteristic without one in the case the electrodes located at the same surface of dielectric plate. On the contrary, in the case the barrier was placed between the electrodes, the characteristic was even improved slightly. [Copyright &y& Elsevier]

A 22 kV/6.9 kV–1 MVA high-Tc superconducting (HTS) power transformer has been developed as a prototype with single-phase part of a 3 MVA HTS power transformer. The prototype unit is cooled by a continuous subcooled liquid nitrogen (LN2) supply system with cryocoolers. During the field tests, the HTS transformer was connected to a distribution line at Imajuku substation (Kyushu Electric Power Co., Inc.) in Fukuoka, and inrush-current test and a long-term operation test were implemented. The test results demonstrated the HTS power transformer''s applicability to a power grid. [Copyright &y& Elsevier]

We studied the technical design targets of high temperature superconducting (HTS) large capacity transformers through the design of 3-phase 66 kV–100 MVA transformers, expecting further performance progress of HTS tapes. It is required for large capacity transformers installed in a power system to have enough percent impedance %IX in order to limit a fault current in the power system at the time of a short circuit accident. It is not easy at the present technical level for HTS transformers wound with Bi-2223/Ag tapes to secure enough percent impedance due to low air gap fields. The design targets are a percent impedance of 7.5%, an air gap flux density of 0.3 T, tape current density of about 40 A/mm2, winding strain of about 0.5% and AC losses of about 0.3 W/km A. [Copyright &y& Elsevier]

• Hysteresis losses calculation in a 3-phase HTS transformer has been developed for harmonic load. • A comprehensive accurate fast analytical algorithm has been proposed for the first time. • Finite Element Method has verified the results obtained by the proposed algorithm. • Hysteresis losses in a 3-phase HTS distribution transformer have been analyzed. • AC losses have been increased in the presence of the multiple-harmonic loads. Nowadays, the application of non-linear loads such as, rectifiers, electronic phase control, PWM drives, gives rise to a vastly increased level of harmonics in the power network. As such, the transport currents of transformers are non-sinusoidal alternating currents. On the other hand, AC losses are one of the most critical characteristics for the design and operation of high-temperature superconducting (HTS) transformers. The amount of hysteresis losses is a key parameter for the design of HTS windings affecting equipment, cooling cost, efficiency, and transformer economic feasibility. The computation of AC losses by traditional techniques, is suitable for the sinusoidal current condition. While, non-linear loads brings about different number of peaks and amplitudes in the current waveform passing through the HTS transformer windings. So that, in addition to the major hysteresis loop, a number of minor hysteresis loops are also formed changing the hysterical nature of the AC losses. Hence, this paper proposes an accurate, yet efficient analytical algorithm, for calculating hysteresis losses in a three-phase HTS transformer's windings in the presence of the harmonic loads accounting for the AC losses estimation. This is accomplished by performing a numerical Finite Element Method to verify the analytical results. The analysis of the hysteresis losses in an exemplary 315 kVA three-phase HTS distribution transformer for different harmonic loads, is another object of this paper. [ABSTRACT FROM AUTHOR]