First trial of the electric power transmission of 3.8 kV–460 kVA through the prospective power transmission model system integrated under superconducting environment (PROMISE)

Superconducting technology is regarded as a breakthrough to future electric power transmission because of its highly densified and large transmission capability. This paper proposes a concept of the future power system composed of various superconducting apparatuses. A prototype model system called “PROMISE (PROspective power transmission Model system Integrated under Superconducting Environment)” is constructed to prove the realization of the above concept. PROMISE is composed of a superconducting transformer (60 Hz, 6/3 kV, 1000 kVA class), superconducting fault current limiter (6 kV, 200 A class), and superconducting power cable (5 m, 6 kV, 650 A class). This paper also shows that PROMISE realized the transmission of the electric power of 3.8 kV–460 kVA (50 Hz). This is the first achievement in the world. The voltage-current synthetic test verified that PROMISE can withstand ac voltage of 6 kV while carrying ac current of 170 A (60 Hz). The ac loss of superconducting cables, the heat leak of cryostat and the core loss of the superconducting transformer are measured to estimate the transmission loss of PROMISE. These fundamental performances of PROMISE may indicate the feasibility of the future introduction of superconducting technology for electric power systems.     

AC losses in superconducting coils

AC superconducting wire is being developed for such electrical equipment as superconducting transformers and superconducting generators. AC loss reduction is of primary concern in the development of such high-efficiency equipment. In reducing ac losses, it is necessary to develop assessment methods for ac loss in test samples which have shapes similar to the end-product equipment. This paper describes a least-square calculation of ac losses of superconducting wire as a function of frequency and magnetic field strength measured in test coils. Two sample solenoid coils were made to test the influence of different capacities and winding methods on ac losses in ac superconducting coils with rated capacities of 500 kVA and 20 kVA, and impregnated (epoxy resin) and nonimpregnated windings. The ac losses in the superconducting coils were measured by a calorimetric method using the evaporating rates of liquid helium. Estimated ac losses in the superconducting wire of the two coils were compared with Joule losses of copper conductors at ambient temperature. As a result of this comparison, a low-loss ac superconducting wire winding can be made for electrical equipment rather than employing conventional copper winding when used under low magnetic fields under 0.5 T.     

Ac losses in superconducting coils

AC superconducting wire is being developed for such electrical equipment as superconducting transformers and superconducting generators. AC loss reduction is of primary concern in the development of such high-efficiency equipment. In reducing ac losses, it is necessary to develop assessment methods for ac loss in test samples which have shapes similar to the end-product equipment. This paper described a least squares calculation of ac losses of superconducting wire as a function of frequency and magnetic field strength measured in test coils. Two sample solenoid coils were made to test the influence of different capacities and winding methods on ac losses in ac superconducting coils with rated capacities of 500 kVA and 20 kVA, and impregnated (epoxy resin) and nonimpregnated windings. The ac losses in the superconducting coils were measured by a calorimetric method using the evaporating rates of liquid helium. Estimated ac losses in the superconducting wire of the two coils were compared with Joule losses of copper conductors at ambient temperature. As a result of this comparison, a low-loss ac superconducting wire winding can be made for electrical equipment rather than employing conventional copper winding when used under low magnetic fields less than 0.5 T.     

630 kVA三相高温超导变压器的研制和并网试验

研发了630 kVA、10.5 kV/0.4 kV三相交流高温超导变压器，并成功进行了并网试验运行。该变压器导线采用多芯Bi2223/Ag不锈钢加强高温超导带材；高压绕组采用多层圆筒式结构，层间置有绝缘和冷却通道；低压绕组采用并联饼式结构。铁心采用非晶合金材料，为三相五柱式结构。绕组置于环形带有室温孔径的玻璃钢低温杜瓦中，以保证铁心处于室温环境。超导线的绝缘利用自主开发的包扎机以聚酰亚胺薄膜采用双半叠包工艺进行绝缘包扎。经过对该变压器进行基本性能测试，满足并网试验运行要求。     

Development and test in grid of 630 kVA three-phase high temperature superconducting transformer

Abstract A 630-kVA 10.5 kV/0.4 kV three-phase high temperature superconducting (HTS) power transformer was successfully developed and tested in a live grid. The windings were wound by hermetic stainless steel-reinforced multi-filamentary Bi2223/Ag tapes. The structures of primary windings are solenoid with insulation and cooling path among layers, and those of secondary windings consist of double-pancakes connected in parallel. Toroidal cryostat is made from electrical insulating glass fiber reinforced plastics (GFRP) materials with room temperature bore for commercial amorphous alloy core with five limbs. Windings are laid in the toroidal cryostat so that the amorphous core operates at room temperature. An insulation technology of double-half wrapping up the Bi2223/Ag tape with Kapton film is used by a winding machine developed by the authors. Fundamental characteristics of the transformer are obtained by standard short-circuit and no-load tests, and it is shown that the transformer meets operating requirements in a live grid. __________ Translated from Proceedings of the CSEE, 2007, 27(27): 24–31 [译自:中国电机工程学报]     

A shell type superconducting transformer for experiments using Nb3Sn superconducting cables

A shell-type superconducting transformer was developed for experiments using Nb₃Sn superconducting cables. The designed capacity is 667 kVA (single phase), the voltage is 440/220 V, the current is 1515/3030 A and the percent impedance is 16 percent. Main features of the transformer are as follows: (1) Magnetic field in superconducting coils is decreased by increasing the number of high and low coil groups. (2) Large-scale superconducting cables are not needed when the number of high and low coil groups is increased. (3) Epoxy impregnated coils are used to withstand an electromagnetic force at 120 Hz. The Nb₃Sn basic strand was manufactured by the internal tin diffusion process. The cable consists of seven insulated subcables, and the subcable consists of seven strands. The primary (HV) coil of the transformer was excited, in which the secondary (LV) coil was shortened. The primary current reached 1618 A_rms without quenching, and the reached capacity corresponds to 712 KA. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 118 (3): 13–21, 1997     

1250kV·A三相高温超导变压器的系统集成与试验

对1250kV·A/10.5kV/0.4kV三相高温超导变压器的系统设计、集成、试验与并网示范运行进行了研究。该超导变压器的一次侧绕组为螺线管型,二次侧绕组为饼式线圈结构,均采用Bi2223/Ag铜合金加强高温超导带材制备;铁心为三相三柱式,采用取向硅钢片叠成;低温恒温器带有室温孔,采用耐低温的玻璃钢制作。测试表明,该超导变压器的空载损耗为2319.2W,空载电流为0.254%,短路阻抗为5.6%,负载损耗为249W。此外,对一、二次侧之间的主绝缘进行了35kV/1min/50Hz工频耐压测试,测试最大泄漏电流12.6mA;对一次侧绕组纵绝缘进行了负向75kV/1.2μs/50μs全波雷电冲击电压测试。完成相关测试后,该超导变压器于2014年9月9日开始并网示范运行,长时间运行可靠。     

Fabrication and fundamental characteristics of a superconducting air-core autotransformer

With the recent advent of high-performance ac superconductors with very small ac loss and large current-carrying capacity, the possibility of an air-core superconducting transformer is being studied. Although the exciting current is very large, the air-core transformer has the following merits: (1) no iron loss; (2) no insulation care of the core; (3) no harmonics, and no in rush current induced by iron saturation, etc. Thus, the authors fabricated and tested a small experimental transformer (2.5 kVA, 330/150 V). As easily predicted, the exciting current becomes very large. It occupies about 45 percent of the rated current. Meanwhile, the leakage reactance increase considerably. The %IX becomes about 28 percent. These experimental results show good agreement with the theoretical analysis based on an equivalent circuit.     

单相3000kVA高温超导变压器设计

本文介绍了现阶段国内变压器研究领域的现状，说明了高温超导变压器的研究意义，描述了其优势和特点．正文是3000kVA高温超导变压器（同心式）的设计，通过Ansys软件计算给出它们的径向和轴向分量云图及短路阻抗。得出结论：同心式的绕组排列方式磁场区域的径向分量比较小，对于大容量变压器，绕组需要必要的导线换位来减小环流。     

Practical design of superconducting generator—electrical characteristics equations

A superconducting generator (SCG) consists of a nonmagnetic cryogenic rotor, which contains a super-conducting field winding cooled by liquid helium, and an airgap armature winding, which makes it possible to obtain a high magnetic field in the airgap. Since the SCG construction differs from that of the conventional generator, practical application requires that its electrical and mechanical design methods be established. This paper describes electrical characteristics equations of the SCG, electromagnetic shield characteristics of double dampers and proof test results of a 50-MVA experimental SCG. Equations for the generator parameters (voltages, output capacity, reactances, time constants, flux densities), which are useful for the practical design of the SCG, are derived from two-dimensional electromagnetic analysis considering winding thickness and the edge effect. From comparison with measured and calculated values of no-load, sudden 1–3 phase short-circuit and slip extended tests, these equations were confirmed as valid.     

A high-temperature superconducting transformer with localized magnetic field

This paper describes a high-temperature superconducting transformer with a bar-type magnetic core and concentric windings with alternating layers, with single-channel and multi-channel arrangements. There is given the design concept of high-temperature superconducting windings of the transformer, made in the form of newly developed first-generation high-temperature superconducting ribbon wires, with localized magnetic field intended for producing maximum transport currents in the windings, as well as for reducing the consumption of a high-temperature superconducting material, cooling agent, and energy losses in these windings.     

Development of a 400 kJ Nb3Sn superconducting magnet for an SMES system

An Nb₃Sn superconducting magnet to store 400 kJ was developed as a unit magnet for a 2.4-MJ SMES system used for stabilization studies of electrical power systems. The superconducting magnet consists of a cryostat and an Nb₃Sn coil. The dimensions of the coil are: 340 mm inner diameter, 700 mm outer diameter and 177 mm axial length. The pool-cooled coil is a stack of 20 Nb₃Sn double pancakes, and the cooling channels are aligned between pancake coils. To reduce Joule loss in electrical power converters, the maximum operating current of the coil is designed to be 350 A, which is one order of magnitude less than the operating currents of similar scale coils for pulse use. The conductor is an Nb₃Sn monolithic conductor with cross section 1.50 × 2.38 mm. For good superconducting stability and high dielectric strength of the coil, the Nb₃Sn double pancakes were wound by the react-and-wind technique. Operation of dc current to 105% (367.5 A) of the design operating current was achieved without quench. After the whole of the coil was exposed out of liquid helium, the coil did not quench under 120 A current operation for more than 2 hours. It was verified that the coil was stable for the SMES system. © 1998 Scripta Technica, Inc. Electr Eng Jpn, 121(3): 44–52, 1997     

Characteristics analysis of superconducting power transformer without iron core

With the progress of superconducting wires for ac power use, research on superconducting power transformers is increasing. These transformers can be divided into two types: the iron-core type [1, 2]; and the air-core type [3–9]. The latter type has such advantages as absence of iron losses and magnetic saturation, and greater possibility of reduction of size and weight. However, the air-core transformer has a large magnetizing current due to the absence of iron core. Hence, research has been carried out on the possibility of using the air-core transformer also as a shunt reactor in a power transmission system. However, the operating characteristics of the air-core transformer, such as voltage regulation and reactance voltage, are not clear at present. In this paper, the equivalent circuit without losses is proposed first. Since this equivalent circuit is expressed by means of the magnetic coupling factor and self-inductances of windings, the effect of these parameters on the transformer characteristics can easily be investigated. Then, based on this equivalent circuit, the per-unit expressions for the air-core transformer characteristics are derived and the characteristics are analyzed in detail. The validity of the theoretical results are confirmed by experimental results obtained by the use of an experimental superconducting transformer.     

Development of high-Jc NbTi multifilamentary superconducting wire with Nb artificial pins for AC applications

High critical current densities of 1.61 × 10¹⁰ A/m² at 1 T, 6.1 × 10⁹ A/m² at 3 T and 2.9 × 10⁹ A/m² at 5 T were achieved by controlling the distribution of Nb artificial pins in NbTi multifilamentary superconducting wires for ac applications. The critical current densities attained are over two times higher than those of conventional ac superconducting wires. This increase can be attributed to the shift of the peak of pinning force density to higher magnetic field by optimizing the pinning parameters. The benefits of increasing critical current densities for ac applications are demonstrated. A 2.5 T/100 kVA ac superconducting magnet was designed and made by using high J_c wire with controlled distribution of Nb artificial pins. Compared with conventional ac superconducting magnets, the new magnet exhibits a drastic decrease in size as well as in ac losses.     

高温超导变压器高压绕组的绝缘设计和试验

为了研究630kVA/10.5kV三相高温超导变压器雷电冲击情况下的绝缘问题,设计研制了一台与630kVA超导变压器高压绕组1∶1的模型。导线采用与超导变压器所用高温超导Ag合金包套不锈钢加强Bi2223多芯带材同样尺寸的CuNi合金带材,导线以聚酰亚胺薄膜采用双半叠包工艺进行绝缘。高压绕组模型为多层圆筒式结构,层间置有层间绝缘和冷却通道。液氮温度77K下远高于国家规定的该电压等级标准(75kV)要求的155kV,雷电全波冲击试验、波过程的测量试验以及对波过程仿真计算的结果表明,测量结果和理论计算结果比较吻合,冲击电位分布接近线性,没有形成震荡,为630kVA/10.5kV高温超导变压器的成功研制提供了依据。     

Design guidelines of a flux‐lock superconducting fault current limiter with ac magnetic field coil for a 6.6‐kV distribution system

We have proposed a new type of fault current limiter, which consists of a flux‐lock reactor with high‐Tc superconducting (HTS) elements and an ac magnetic field coil (Flux‐Lock‐Type Fault Current Limiter: FLT‐FCL). The FLT‐FCL can increase both the current capacity and the limiting impedance by means of a transformer action and an ac magnetic field application mechanism. This paper reports the conceptual design of an FLT‐FCL for application to a 6.6‐kV/200‐A distribution system. Theoretical expressions for the current limiting behavior are derived and the new concept of “quench power” is proposed in order to estimate the required number of HTS elements for two types of FLT‐FCL and for a basic FCL type consisting only of HTS elements. Design guidelines for the FLT‐FCL are derived from the calculation results. © 2001 Scripta Technica, Electr Eng Jpn, 135(4): 17–25, 2001     

Line fault test of model power system with three-phase superconducting fault current-limiting reactors

A novel three-phase conducting fault current limiting reactor (SCFCLR) is fabricated. The SCFCLR has three superconducting windings with the same number of turns wound on an iron core. The rating of SCFCLR is 200 V, 10 A. Two SCFCLRs are inserted in the sending and the receiving ends of the model power-transmission line. The line fault test of a model power system with two SCFCLRs is undertaken. The fundamental behavior of this reactor is confirmed. In the case of single-line-to-ground fault, the fault current is limited to a very small value by the large zero-phase sequence reactance of the SCFCLR without quench. In the case of a three-phase short circuit, the SCFCLR quenches, and the short-circuit current is limited by the normal conducting resitance of the winding. It is confirmed that the transient stability of the system during line faults is greatly improved by the insertion of SCFCLRs.     

Stabilization for thermomagnetic instability of CuNi/NbTi superconducting wire in spatially distributed magnetic field

Thermomagnetic instability in superconducting wires composing multistrand cables is a problem in the development of cables with large current capacity. This paper elucidates the quenching properties of ac superconducting wires in a distributed magnetic field applied to the strands in the cable, and the stabilization of the ac superconducting wires considering the effect of the longitudinal magnetic field or the fraction of copper embedded in each strand. First, the degradation of the quench current of CuNi/NbTi superconducting wires in a distributed magnetic field is exhibited with simple test samples. Second, the quench properties of the strand in a (6 + 1)³ cable and the optimal twist pitch of the cable for high stabilization are discussed. Last, the effect of copper on the quench properties of the strand and the appropriate fraction of copper for suppression of quench current degradation in a distributed magnetic field are discussed. © 2001 Scripta Technica, Electr Eng Jpn, 135(4): 26–34, 2001     

Development of kA-class alternating current superconducting conductors and fabrication of a prototype coreless superconducting autotransformer

Recently, multifilamentary superconducting wires with very low ac losses have been produced and practical applications will now be considered. To realize actualsize power machines and apparatuses, it is necessary to develop 1 - 10 kA ac conductors. However, the critical currents of multifilamentary wires at 1 T are several tens of A, and therefore it is necessary to use multistrand conductors consisting of several tens or several hundreds of strands. Such conductors sometimes show ac current degradation because of such factors as (1) nonuniform current distribution, (2) wire motion, (3) temperature increase, (4) longitudinal magnetic field effect, etc. Formerly, a coreless transformer was considered unpractical because of its large exciting current. However, Yamamoto and others proposed that a coreless superconducting transformer could be used as a stepdown autotransformer at the receiving side, utilizing its large exciting current as the reactive power source to cancel the charging current of an underground transmission line or UHV line, and therefore the shunt reactors could be eliminated. In this paper, development of ac-superconducting conductors aimed at prevention of current degradation are discussed, as well as quench test results of two small coils made with these conductors. In these conductors, low ac low strands with ultrafine NbTi filaments are twisted around a central bundle of stainless steel wires. One of the coils has been designed as a model coreless autotransformer, and its test result is also described.     

Specifying and loading cast-resin transformers

The thermal considerations in specifying and loading cast-resin transformers are presented. These considerations are based on conclusions reached from performing thermal tests on a 200 kVA cast-resin transformer with imbedded thermocouples. Specifications should require that the correct insulation temperature class be applied to individual windings and that hottest spot temperatures do not exceed this insulation temperature class at rated self-cooled and forced-air kilovoltamps. Specifications should also require calculation of hottest-spot temperature rises using mathematical models verified by thermal tests on a prototype transformer representative of the design family. The loading back-test method gives higher hottest-spot temperature rises than expected from the short-circuit test method. A method to correct the data to determine the hottest-spot temperature rise using the short-circuit method is presented. A specification form is suggested, and loading equations are presented to allow predictions of loading capability and comparisons of competitive designs