HTS Transformer Research Articles

Critical Current and Cooling Favored Structure Design and Electromagnetic Analysis of 1 MVA HTS Power Transformer

This paper presents a conceptual design of a 1 MVA / 10 kV / 0.4 kV high temperature superconducting power transformer by using Dynamically Innovative BSCCO (DI- BSCCO) tapes. The three-phase three-limb iron core is formed by laminated silicon steel sheets at room temperature. The high- voltage (HV) windings take the form of five solenoid groups per phase, where the five solenoids are coaxially placed and connected in series. The preliminary design of the low-voltage (LV) windings includes two winding schemes. One is with the form of five coaxial solenoids in parallel; the other is to use the ten-stacked tapes directly to wind one solenoid with 17 axial turns. The structural designs and electromagnetic analyses are presented with critical current and uniformity of current distribution discussed. The designed 1 MVA HTS transformer has the advantages of smaller volume, lighter weight and less operation loss as compared to a 1 MVA conventional transformer.

Magnetic properties measurement and discussion of an amorphous power transformer core at room and liquid nitrogen temperature

In energy generation, transmission and distribution systems, power transformers are one of the most common and important components. Consequently, the performance of these transformers is crucial to global efficiency of the systems. To optimize transformers efficiency, the selection of an adequate ferromagnetic material is very important. For example, the use of amorphous ferromagnetic materials in transformer cores, replacing crystalline electrical steels, decreases total magnetic losses of the device. Other possible solution to increase energy systems efficiency, is the installation of high temperature superconducting power transformers (HTS transformers), normally cooled by liquid nitrogen at 77 K. In order to contribute to HTS transformer efficiency improvement, a 562.5 VA transformer with an amorphous ferromagnetic core was designed and built. For this core, the most important magnetic properties are measured at room and cryogenic temperature, and then compared with those of a typical crystalline grain-oriented electrical steel. Amorphous material magnetic losses (static and dynamic) at room and 77K are also presented and discussed.

Analysis on heat loss characteristics of a 10 kV HTS power substation

A 10kV High Temperature Superconducting power substation (10kV HTS substation), supported by Chinese State 863 projects, was developed and has been running to supply power for several factories for more than two years at an industrial park of Baiyin, Gansu province in Northwest China. The system of the 10kV HTS substation compositions, including a HTS cable, a HTS transformer, a SFCL, and a SMES, are introduced. The SMES works at liquid helium temperature and the other three apparatus operates under liquid nitrogen condition. There are mainly four types of heat losses existing in each HTS apparatus of the 10kV HTS substation, including AC loss, Joule heat loss, conductive heat, and leak-in heat from cryostat. A small quantity of AC loss still exists due to the harmonic component of the current when it carries DC for HTS apparatus. The principle and basis for analysis of the heat losses are introduced and the total heat loss of each apparatus are calculated or estimated, which agree well with the test result. The analysis and result presented are of importance for the design of the refrigeration system.

Damping of inrush current in superconducting transformer

In present study we analyze the inrush current in transformers with windings made of high-temperature superconducting materials. We analyze the phenomenon of damping the inrush current in superconducting transformers. Theoretical analysis results are compared with results of measurements of one-phase HTS 8,5 kVA transformer. We discuss differences in inrush current waveform in superconducting and conventional transformers. We also explain the damping mechanism of inrush current in HTS transformer when its windings are in superconductive, and in resistive states. S owa kluczowe: transformator, nadprzewodnictwo, transformator HTS, pr d w czania

Verification Testing for a 1 MVA 3-Phase Demonstration Transformer Using 2G-HTS Roebel Cable

We present results from testing to verify the performance of major subsystem components for a 1-MVA three-phase transformer demonstration project. The transformer utilizes a 15-strand Roebel cable for the low-voltage windings and a 4-mm-wide superconductor for the high-voltage windings. Both windings use a YBCO conductor. The winding assemblies will be housed in three individual vacuum insulated glass-fiber composite cryostats and will be cooled by circulated liquid nitrogen and subcooled to a target operating temperature of 70 K. Results from thermal insulation performance tests on a sample composite cryostat are presented. AC loss test results of a low-voltage winding assembly are also presented. The Roebel cable is proven to exhibit low AC loss operation at high current. We conclude that AC loss is not a fundamental obstacle to HTS transformer commercialization.

Tests and Performance Analysis of 2G HTS Transformer

Transformers represent one of the oldest and most mature elements in a power transmission and distribution network. The new superconducting transformers are smaller and lighter than conventional ones and they have lower power losses. Also, the new 2G superconducting tapes with high resistivity in the normal state allow to build transformers with high short-circuit strength. The short-circuit current limiting feature of the superconducting transformer, which is the most important benefit of replacing conventional windings by superconducting ones, provides protection and significantly reduces the wear and tear of circuit breakers and other substation power equipment. This paper describes the design and test results of a model of a 1-phase, 8.8 kVA superconducting transformer with windings made of 2G HTS tape. The authors also present the results of a numerical analysis of the design. A special regard is given to the ability of the device's superconducting winding to limit the short-circuit current, in particular its resistivity in normal state at a temperature of 77 K.

A Quantitative Analysis on Future World Marketability of HTS Power Industry

This study estimates total world market volumes of HTS power industry, one of the main smart grid equipment in the future when the market penetration ratio reaches 100% in the future. The results of this paper are based on the GOULDEN Report providing the total market volumes of conventional power industry for the world market. This study determines the market penetration ratio using the judgment method with market penetration S-curve induced from Delphi method and Product Life Cycle from 2011 (expected to be a launching year, not the actual year) to 2050 (expected to be a final target year). For this purpose, HTS market penetration ratio of each stage, so called innovation, early adapters, early/late majority and laggard stage, is analyzed using S-curve and thus calculated the total future market volumes of HTS equipment. In conclusion, this paper shows the quantitative analysis for HTS cable, FCL, transformer and rotational machine in the world market.

Risk Mitigation in the Development of a Roebel Cable Based 1 MVA HTS Transformer

Progress is reported on the development of a 3-phase 1 MVA 11 kV/415 V HTS transformer using Roebel cable for the secondary windings. We describe efforts to address risks associated with short circuits, insulation, and heat transfer. We present modelling results for the response of the windings to a short circuit. Sample strands have been tested to demonstrate that cables immersed in liquid nitrogen can survive short circuits. The primary windings use insulated conductor to withstand computed impulse voltages. Breakdown and partial discharge testing confirms that the production insulation scheme is adequate for our design. Heat transfer results are presented for a sample Roebel cable winding

A Possible Solution to Reduce Magnetic Losses in Transformer Cores Working at Liquid Nitrogen Temperature

The efficiency of power transformers should be always taken in account when design and operation conditions parameters are chosen. In high temperature superconducting transformers (HTS transformers), normally cooled by liquid nitrogen at 77 K, cores are usually kept at room temperature in order to minimize total magnetic losses, losing the possibility to use the cooling liquid to minimize electric risks and to cool the core, and increasing the complexity of the cryostats that, in these cases, must only embrace the superconducting windings. This work try to evaluate the magnetic core losses increasing at 77 K, for different magnetic materials, and the possibility of reducing these losses under some specific manipulation of magnetic materials. For this purpose, several low temperature measurements are presented to characterize the magnetic behavior of four electrical steels usually used in transformer cores. The chosen magnetic materials are three crystalline materials, two grain-oriented and one non oriented steel, and an amorphous elaectrical steel. The most significant results show that grain oriented steels have lower losses increasing at cryogenic temperature, comparing with the other two magnetic materials, and that above a certain value of magnetic induction, B, total magnetic losses at 77 K became smaller than room temperature losses. Results interpretation is presented and some suggestions are made concerning production of magnetic materials for applications at 77 K.

The development of a Roebel cable based 1 MVA HTS transformer

This paper describes recent experimental results in a continuing program to develop a 1 MVAdemonstration transformer that employs HTS Roebel cable for the high current windings. Theelectrical design parameters of the transformer are presented, including the configuration ofthe specially developed Roebel cable. This paper discusses estimation of the cableIc frommeasured Ic(B) data for samples of the strands, AC loss minimization using flux diverters, proving thesuitability of the polyimide insulation scheme to withstand the effects of a high voltageimpulse and modelling and experimental verification of the fault current limiting behaviourexpected from the HTS conductor.

The Barrier Effect on Breakdown for Design of 154 kV Class HTS Transformer

LN2 is used as an insulating material as well as a cryogen for a 154 kV class HTS transformer. However, main insulation with LN2 is not enough, and thus, research on barrier has been performed. The most of experiments on this barrier effect are combination of paper and air (air, SF6, etc.), and also the results from these experiments on barrier effect are insufficient to be used practically for the electric insulation design of the 154 kV class HTS transformer. Moreover, there are insufficient researches regarding cryogenic liquid used for superconducting equipments. In this paper, we examined the breakdown properties of the barrier related to the position, number and materials. With the database in the electrical breakdown properties, the electric insulation design as the main insulation for the 154 kV class.

The electrical characteristics of solid insulators for 154 kV class HTS transformer

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.

The short-circuit test results of 6.9 kV/2.3 kV 400 kVA-class YBCO model transformer

We are developing an elemental technology for 66kV/6.9kV 20MVA-class power transformer with YBCO conductors. The protection of short-circuit technology is one of the elemental technologies for HTS transformer. Since short-circuit current is much higher than critical current of YBCO tape, there is a possibility that superconducting characteristics may be damaged during short-circuit period. We made a conductor to compose the YBCO tape with copper tape. We manufactured 6.9kV/2.3kV 400kVA-class YBCO model transformer using this conductor and performed short-circuit current test. The short-circuit current of primary winding was 346 A which was about six times larger than the rated current. The I–V characteristics of the winding did not change before and after the test. We may consider this conductor withstands short-circuit current.

Multicriteria Optimal Winding Scheme in HTS Transformers by Analytical Hierarchy Process

When designing high-temperature superconducting transformers, leakage magnetic field, ac loss, circulating current, and electromagnetic forces are important issues. These factors are accounted for by a proposed method providing initially different winding schemes, i.e., helical, cylindrical, and pancake. We pursue a versatile optimization scheme that is applicable to different winding types. The above requires advanced numerical techniques and hence simulation studies for calculating the leakage flux density and electromagnetic forces in each configuration scheme. Considering the optimization of all the aforementioned criteria (ac losses, circulating currents, and electromagnetic forces), an intelligent choice of optimal winding geometry is obtained using analytical hierarchy process optimization, which is the main motivation for this brief.

HTS Transformer: Construction Details, Test Results, and Noted Failure Mechanisms

An experimental high temperature superconducting transformer has been designed and built using Bi2223 HTS tape. The transformer is unique in that the magnetic circuit is comprised of air and a silicon steel partial core. Electrical tests were performed on the transformer and it was found to be 98.6% efficient at full load. The transformer failed during a full load endurance run and an investigation was carried out to determine the cause of the failure. The cause was believed to be from operating the HTS windings close to critical conditions. Presentation of the failure details will be of use to other researchers who are building HTS transformers.

Toroidal HTS transformer with cold magnetic core – analysis with FEM software

The aim of this paper is to present a thorough characterization of the toroidal HTS transformer by means of FEM analysis. The analysis was a 2D/3D harmonic electromagnetic and thermal analysis. The toroidal transformer operated in LN2 by being immersed together with the magnetic core in it, for which its power losses were acceptable. Two extreme variants of windings were analysed. The first one called parallel and the second called perpendicular. Three variants of the magnetic core were considered. In the first one the core was put outside of the windings, in the second the core was inside of the windings and in the third variant the core was outside as well as inside of the windings. The windings were made of HTS tape BiSCCO-2223/Ag while the magnetic core was made of the nanocrystalline material Finemet. The two windings, with a 1:1 turn-to-turn ratio, were uniformly distributed along the whole torus circumference. The output power, efficiency and power density are in the results of the analysis. The temperature distribution was also calculated. In summary, the performance of the transformer is better than those currently known.

The Insulation Design of 154 kV HTS Transformer and on Load Tap Changers

The HTS transformer is one of the promising HTS power applications that are expected to be commercialized in the near future. Among the auxiliary functions of a transformer, the voltage regulation function is known to be the most important one. For the voltage regulation of HTS transformers, they are normally equipped with the on-load tap changers (OLTC). It is also required that, in order to develop a HTS transformer as a complete system, not only the basic technologies for all the components including OLTC and bushing, but also the technologies for the insulation of these components at a cryogenic temperature should be well established as applicable. Taking such facts into account, the breakdown characteristics of the insulation components have been evaluated for such application areas as turn-to-turn insulation, layer-to-layer insulation, c, drive shaft-to-drive shaft insulation, tap connecting terminal insulation and others. In overall, this paper discusses the electric insulation design and experimental results with regard to HTS transformer, OLTC and bushing.

Conceptual Design of a 5 MVA Single Phase High Temperature Superconducting Transformer

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 2 G 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.

Breakdown and Voltage–Time Characteristics of Turn-to-Turn Models for an HTS Transformer

The breakdown and voltage-time characteristics of turn-to-turn models for point contact geometry and surface contact geometry using copper multiwrapped with polyimide film for a high-temperature superconducting transformer were investigated under ac and impulse voltage at 77 K. Polyimide film (Kapton) 0.025 mm thick is used for multiwrapping of the electrode. As expected, the breakdown voltages for the surface contact geometry are lower than that of the point contact geometry, because the contact area of the surface contact geometry is lager than that of the point contact geometry. The time to breakdown t 50decreases as the applied voltage is increased, and the lifetime indexes increase slightly as the number of layers is increased. The electric field amplitude at the position where breakdown occurs is about 80% of the maximum electric field value.

HTS toroidal helical transformer

The paper describes novel HTS toroidal transformer with helical windings, operating at 50 Hz. The turns of outer winding are parallel to turns of inner one. This arrangement of windings results in the same length and shape of each turn. The latter ensures homogeneous distribution of flux density. The magnetic core, in turn, improves coupling between windings, giving at the same time homogeneous magnetic flux distribution in windings. Proposed shape of windings reduces perpendicular component of flux density in HTS wires. It allows for the best possible utilisation of the HTS wire. The transformer has the magnetic core cooled with LN2. The transformer has turn-to-turn ratio 1:1 as it is designed for modular system. Its windings are made of Bi2Sr2Co2Cu3Ox wire. Material of magnetic core has accepted power losses in LN2. The magnetic core is also kept in ambient temperature. Thorough analysis of proposed HTS transformer will be carried out using Finite Element Method (FEM) and measurements resulting in critical current, rated power density and rated efficiency. The circuit model of the transformer is also given.