Superconducting Power Transformer Research Articles

Methodology for determining the parameters of high-temperature superconducting power transformers with current limiting function

<span lang="EN-US">This paper substantiates a new adaptive method for determining parameters of high-temperature superconducting power transformers with current limiting function. The main focus is the design of current-limiting superconducting windings in the light of new restrictions on current density, magnetic induction, critical current and critical temperature. The presented method considers the nature of alternating current (AC) losses in a superconductor under nominal operating conditions, features of the dielectric medium (liquid nitrogen), as well as the reduced values of the short-circuit voltage (0.5 to 1.5%). The main design features of high-temperature superconducting (HTS) transformers are specified, and a prototype of a three-phase HTS transformer of 63 kVA with a short-circuit current limiting function is developed. It is shown that HTS units have some advantages over conventional transformers: a 90 to 95% active losses reduction, short-circuit current limitation function, explosion and fire safety, a 60% reduction in weight and size, and increased efficiency (up to 99.8%). Experimental studies confirm that the short-circuit current limitation function is safe and efficient. It is demonstrated that during the short-circuit current limitation, significant heat flows occur on the windings, which should not exceed the critical value above which the superconductor could not return to the superconducting state by itself.</span>

Modeling of the Rail Type Electromagnetic Launch System With Superconducting Pulsed Power Supply Considering Resistance

In the rail type electromagnetic launch system, when the projectile accelerates in orbit, it is affected by not only the electromagnetic force, but also various resistances, such as sliding friction, air resistance and so on. These resistances affect the characteristics of the pulsed power supply (PS). Nowadays, in order to analyze and design the pulsed PS conveniently, researchers usually ignore the influence of various resistors, and use fixed loads including resistors and inductors to simulate the rail load module, which will cause large errors. So, the mathematical model of the orbital electromagnetic launch system with superconducting is established, including the pulsed PS model with the multiple high-temperature superconducting pulsed power transformer (HTSPPT) and the dynamic rail load model considering various forces. The simulation results indicate that the dynamic rail load model considering various forces can reduce the error of capacitor voltage, load current and other parameters, the features of pulsed PS can be studied more precisely.

Optimization Design of a 40-kJ HTSPPT Module for Inductive Pulsed Power Supply

The topology category based on high-temperature superconducting pulse power transformer (HTSPPT) is an important branch of inductive pulse power supply (IPPS) for rail guns. Optimization design of the HTSPPT can improve the overall performance of the IPPS system. The purpose of this article is to design a 40-kJ air-core HTSPPT module with a high-temperature superconducting (HTS) primary winding and a normally conducting secondary winding. According to the application characteristics of the HTSPPT, under the constraints of the three main performance indicators of critical current, stored energy, and coupling coefficient, the amount of HTS tapes and the volume of the entire HTSPPT are reduced as much as possible. The design of the secondary winding combines the electromagnetic launch requirements for the current pulse in the approximate flat waveform and steep pulse rising edge. With this optimized design, a 43-kJ HTSPPT module with a primary inductance of 79.4 mH, a secondary inductance of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$255.4~\mu \text{H}$ </tex-math></inline-formula> , and a coupling coefficient of 0.97 was obtained. Finally, the maximum stress and strain of the designed HTSPPT in the energy storage state are analyzed through the electromagnetic (EM) structure coupling field.

Optimal Design of Flux Diverter Using Genetic Algorithm for Axial Short Circuit Force Reduction in HTS Transformers

The appealing advantages of high-temperature superconducting (HTS) power transformers over conventional ones have attracted transformer manufacturing companies, power companies, research institutes, and universities worldwide to conduct research and development in this field. Unfortunately, HTS transformers are more vulnerable to mechanical stresses than conventional transformers. The results of the interaction between current carrying windings and leakage magnetic fluxes are the electromagnetic forces, which act on transformer windings. Under short circuit events, these forces are remarkable, and, therefore, catastrophic failure of transformer may arise. Flux-diverter applications have been reported in earlier literatures for increasing critical current or decreasing ac losses in HTS transformers. In this paper, a genetic algorithm based method is employed for the optimal design of flux diverter to minimize the axial short circuit force, in design stage of a sample 132/13.8 kV, 50 MVA three-phase core type HTS transformer. In this paper, the optimal dimensions, placement, and permeability of a flux diverter have been determined. It has been shown that utilizing this optimized flux diverter, the axial short circuit forces have been reduced significantly. Electromagnetic modeling and simulations, by the application of finite element method, have been employed for the verification of the analytical method results. A high degree of consistency has been observed between the analytical results and the simulation results.

Asynchronous Discharge of an Eight-Module Superconducting Pulsed Power Supply for Driving an Electromagnetic Railgun

Researches on the synchronous parallel discharge of multimodule pulsed power supply have confirmed that it can output large pulse currents with high amplitude. For electromagnetic railguns, excess current may cause orbital erosion and other problems, and the ideal driving current is rectangular pulse currents. Therefore, the asynchronous discharge of multimodule superconducting pulsed power supply (SPPS) is studied in this paper to reduce the amplitude and increase the pulsewidth for driving an electromagnetic railgun. A toroidal SPPS based on eight coupled high-temperature superconducting pulsed power transformers and a dynamic simulation model of electromagnetic railgun is established. The simulation results show that armature currents with different amplitude and pulsewidth can be obtained by adjusting the trigger delay time of each module. With the increase of the trigger delay time, the amplitude decreases but the pulsewidth increases. With a set of trigger delay time, the asynchronous discharge of the eight-module SPPS can output an approximate rectangular pulse current with amplitude of 126 kA. The armature and projectile with a total mass of 50 g can be accelerated to 346 m/s, giving the efficiency of 16.2%.

Design and Simulation of a Multimodule Superconducting Inductive Pulsed-Power Supply Model for a Railgun System

The superconducting inductive pulsed-power supply (IPPS) is considered to have broad application prospects in the field of electromagnetic launch because of its low electrical loss. In order to obtain a higher efficiency in continuous electromagnetic launch, a repetitive IPPS circuit topology based on a high-temperature superconducting pulse power transformer (HTSPPT) has been proposed in our previous studies. It can recover most of the remaining energy for generating continuous current pulses by using a bridge-type capacitive switching circuit. As a follow-up work, this paper presents a multimodule repetitive superconducting IPPS model for a railgun system. This multimodule model consists of 24 HTSPPT modules connected to an XRAM-like circuit topology. In order to verify the feasibility of the repetitive superconducting IPPS and study its driving characteristics, the dynamic launching process of a simple 4-m-long railgun system was simulated. The influence of the capacitance in the bridge-type capacitive switching circuit on the main performance of the system, such as the maximum output current, the maximum primary voltage, the launch efficiency, the maximum capacitive energy ratio, and the remaining energy recovery ratio, was analyzed. The simulation results show that the repetitive superconducting IPPS is suitable for driving the simple railgun. In addition, it also concluded that the system performances conflict with each other and the optimal capacitance value for each HTSPPT module should be determined by the tradeoff of system performances.

Keeping Cool Under Pressure: Dielectric Strength of Sheet Insulation in Boiling Liquid Nitrogen

According to our previous research on the dielectric strength of Nomex T410 paper under a horizontal turn-to-turn electrode system, a significant decrease by approximately 38% of the dielectric strength was found after the introduction of film boiling. In this paper, a wide range of sheet insulation samples has been tested under a turn-to-turn electrode system in vertical arrangement, more practical to the real design of high-temperature superconducting power transformers. Samples from three different types of paper (NMN, Nomex, and Kapton) have been investigated under a static situation with no boiling introduced. In addition, their dynamic dielectric characteristics under different levels of electric stress have also been tested by introducing a transient thermal stress. The experiment has been conducted in open bath liquid nitrogen (77 K) as well as in subcooled liquid nitrogen (67 K). Both of the tests are conducted under one atmospheric pressure. The result shows that all samples have enhanced dielectric performance when immersed in the subcooled liquid nitrogen. Moreover, the Kapton film is found to always have the best dielectric performance. However, its poor impregnation of liquid nitrogen largely affects its dielectric strength, especially at 77 K. When compared to Nomex paper with a similar thickness, the NMN paper is better due to its nonporous structure.

A Repetitive Inductive Pulsed Power Supply Circuit Topology Based on HTSPPT

Inductive energy storage for pulsed power supplies is considered to have great potential because its energy density is 1 order of magnitude higher than that of capacitive one. Associating with the superconducting technology and the STRETCH meat grinder circuit, which proposed by the Institute of Advanced Technology, a superconducting inductive pulsed power supply (SPPS) circuit has been proposed by using an air-core high-temperature superconducting pulsed power transformer (HTSPPT) in our previous study. It can not only recapture the energy in the leakage flux and slow down the current change in inductors by using a capacitor, but also reduce the coil loss and the power requirement of the primary source by using superconducting inductors. However, it can be found that the SPPS may generate a large residual current during the discharging phase, which results in an adverse influence on the working frequency and the energy transfer efficiency of the whole system. This paper proposes a modified circuit based on the bridge current switching circuit and HTSPPT. The main principle of the circuit is that the recaptured energy in the capacitor and the residual energy in the secondary are used to precharge the primary inductor for the next cycle, which shortens the charging time, generates continuous current pulses, and improves the energy transfer efficiency. The procedure of the repetitive SPPS is analyzed in detail, and simulations are carried out to explain how the circuit works clearly.

The Dielectric Strength of Nomex 410 Paper in Liquid Nitrogen Under Boiling Situations

It is crucial for the successful application of high-temperature superconducting (HTS) power transformers that they are able to survive under abnormal operations, such as a short-circuit fault. Modeling, based on the design of a four-winding three-phase HTS power transformer, suggests severe boiling will occur on the surface of the winding during a short-circuit fault and will jeopardize the turn-to-turn insulation of the HTS power transformer. In this paper, a turn-to-turn insulation breakdown experiment platform involving the introduction of boiling on the surface of electrode has been built. Nomex 410 paper, one of the candidates for turn-to-turn insulation in the design of the HTS power transformer, was tested. The characteristics of dielectric strength of Nomex 410 in three different thicknesses (0.13, 0.18, and 0.25 mm) have been investigated, without boiling, as well as under different boiling situations, in an open bath of liquid nitrogen. The voltage application methods applied during both experiments test the electrical breakdown of Nomex 410 under a dynamic situation. The experiment results show that when under film boiling, compared to the test results for nucleate boiling, the dielectric strength of Nomex 410 paper decreases by approximately 38% of the value when no boiling is involved. Moreover, it is verified that the presence of an electric field accelerates the heat transfer between the liquid nitrogen and the heated electrode.

Analysis and Exploiting of Mutual Coupling Effect on Inductive Pulsed Power Supply Consisting of Multiple HTS Pulse Power Transformers

Inductive pulsed power supply (IPPS) based on multiple high-temperature superconducting pulsed power transformer (HTSPPT) modules is a novel technology for electromagnetic launching. In a compact multimodule IPPS, the mutual couplings between HTSPPT modules are unavoidable. In this paper, an IPPS with the method of improved pulse-forming process which proved to be feasible by preliminary experiment is proposed to analyze the effect of the mutual couplings. The characteristics of the mutual coupling effect on the IPPS are presented in detail by theoretical analysis and parametric simulations. The evaluation indices reflecting performances of the IPPS under the mutual coupling effect are derived from simulations, including the energy storage capacity, flux linkage, axial tensile force, output current amplitude, and so on. Parametric simulations are executed with respect to the module arrangement, the number of modules, and the arrangement radius. Finally, the analysis of mutual coupling effect is summarized, and a strategy for exploiting mutual couplings between modules is put forward to improve IPPS performance.

Development of a single-phase 330kVA HTS transformer using GdBCO tapes

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 / 10kV / 0.4kV 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 77K. The efficiency and AC loss at rated load is 99.90% and 243.7W, respectively. In addition, an overload test was also performed.

Design and Electromagnetic Analysis of a 330 kVA Single-Phase HTS Transformer

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.

Experimental Study of Inductive Pulsed Power Supply Based on Multiple HTSPPT Modules

For electromagnetic emission applications that need a very high amplitude and an appropriate pulsewidth, a pulsed power supply based on multiple high-temperature superconducting pulsed power transformer (HTSPPT) modules is verified in this paper. First, the working processes of multiple HTSPPT modules are analyzed and two small experimental HTSPPTs are described in detail. Then, simulation is carried out to show the major pulse characteristics of two different discharge methods. To verify the feasibility of two discharge methods, high-current testing is carried out with the two HTSPPTs. The results show that a higher amplitude of current pulse can be achieved using synchronous parallel discharge and a larger pulsewidth of current pulse can be achieved using asynchronous parallel discharge.

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.

Electric and magnetic properties measurement and analysis of a conventional and a superconducting power transformer

Power transformers based on High Temperature Superconductors (HTS) technology have revealed potential for several practical applications, offering economic, environmental and operational benefits. In this work, two 650 VA single-phase transformers prototypes were developed, tested and characterized: a conventional one, using copper windings, and another with the same primary copper winding, but with a secondary winding made of HTS BSCCO tape. The two prototypes were compared regarding magnetic properties, losses, electric parameters and efficiency, and the results are presented and interpreted. Also, several measures to determine AC critical current of the HTS tape were made. The results are compared with DC critical current for the same tape.

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.

Design and Testing of a Novel Inductive Pulsed Power Supply Consisting of HTS Pulse Power Transformer and ZnO-Based Nonlinear Resistor

Higher energy density makes inductive energy storage more promising than capacitive storage for pulsed power supplies in industrial and military fields. To realize high amplitude of pulsed current and relieve stress of opening switch, this paper proposes a novel inductive pulsed power supply consisting of high-temperature superconducting pulse power transformer and ZnO-based nonlinear resistor. First, working processes and laboratory setup are described in detail. Then, simulation using the software SIMPLORER is built to show major pulse characteristics and comparisons of two different nonlinear resistors. For verifying the feasibility of this mode, high-current testing is carried out and the results show that large amplitude of pulsed current 3 kA with energy transfer efficiency 60% is achieved, and the ZnO-based nonlinear resistor can help to limit the voltage of the opening switch to a small constant below its clamping value as current is interrupted.

The short-circuit test results of 6.9 kV/2.3 kV 400 kVA-class YBCO model transformer with fault current limiting function

We are developing an elemental technology for 66/6.9kV 20MVA-class superconducting power transformer with fault current limiting function. In order to obtain the characteristics of YBCO conductor when the AC over current supplied to the conductor, the model coils were manufactured with YBCO tapes and tested. Based on these results, we manufactured the 6.9kV/2.3kV 400kVA-class YBCO model transformer with fault current limiting function and performed short-circuit test. At the 0.25s after short-circuit, the short-circuit current of primary winding was limited to about 174A for a prospective current of 559A. It was consistent with the design. The I–V characteristics of the winding did not change before and after the test. We consider the model transformer to be able to withstand AC over-current with the function of current limiting. The results suggest the possibility to design YBCO superconducting transformers with fault current limiting function for practical power grid.

Over-current characteristics of model coil using Y-based multi-filament wire for superconducting power transformers

In the Japanese national project to develop superconducting power applications using Y-based coated conductors, the present focus is on various aspects of technological development of superconducting power transformers. Since Y-based multi-filament wire contributes to reduction in AC loss of superconducting transformer coils, a 5mm-wide long-length multi-filament, between the filaments of 3 and 5, has been developing by a laser scribing method. In the case of the project target, a 66kV/20 MVA power transformer coils require a Y-based multi-filament wire between the lengths of 200m and 300m. It also must withstand over-current of about seven times the rated current, which is generated when a power system fault occurs. In this paper, over-current characteristics of model coil using Y-based 3-filament wire were evaluated.

Development of REBCO Power Transformers

REBCO superconducting power transformers, which are incombustible, lightweight and compact, are expected to have practical applications in equipment for substations and office buildings in urban areas. In recent years, there have been remarkable improvements in the performance of REBCO tape wires, which have the characteristic of a large critical current in highly magnetic fields. Their use is expected to reduce AC loss through the application of slit processing on the tape wire. Based on these advantages, the authors have developed element and system technologies of REBCO power transformers. Wire-scribing technology, coil-winding technology (for example, short-circuit characteristics), cryogenic technology, fault current-limiting (FCL) technology and a design study of a 66 kV/6 kV, 20 MVA transformer are examined.