Verification tests and insulation design method of cold dielectric superconducting cable

A High‐Temperature Superconducting (HTS) cable has a bulk power transmission capacity as a candidate for the replacement of aged cables and/or for the increase of the power transmission capacity, and its diameter is preferred to be smaller than the inner diameter of the duct for the existing cables. To reduce the diameter of HTS cable, the cold dielectric (CD)‐type electrical insulation in which a cable core is immersed into liquid nitrogen (LN₂) should be adopted, and the thickness of its electrical insulation layer has to be optimized. Since a partial discharge (PD) in the electrical insulation layer of the CD‐type HTS cable is considered as a major cause for the aging of the insulation layer, PD‐free design must be adopted for the CD‐type HTS cable. This paper describes a design method for the electrical insulation layer of the CD‐type HTS cable adopting the PD‐free design under AC stress, based on the experimental results such as a PD inception stress (PDIE), an impulse breakdown stress, and PD extinction characteristics under AC stress superimposed with an impulse stress. Moreover, the proposed design method was applied to a 500‐m HTS cable and was verified by a field test. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 164(2): 25–36, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20512     

高温超导电缆发展及其应用概述

随着高温超导技术的发展,高温超导电缆已经在输电系统中有了实际应用。与传统电缆相比,高温超导电缆具有传输容量大、损耗低、体积小、重量轻、可靠性高、节约资源、环境友好等优势,有望在未来电网发展中发挥重要作用。本文介绍了高温超导电缆的结构及特点、基本设计原理、传输电流与导体层电流分布及交流损耗等技术问题,并对高温超导电缆在交流和直流输电系统中的应用以及目前世界各国对高温超导电缆的研究及成果做了介绍。     

高温超导电缆在城市地下输电系统应用的可行性研究

大城市有可能最先采用商业化运行高温超导电缆 ,用于城市地下交流输电系统。其主要应用目标是用于地下电缆工程改造 ,利用现有排管以高温超导电缆取代现有的常导电缆 ,增加地下电缆传输容量以及采用高温超导电缆将巨大电能 (1GVA以上 )输入到城市负荷中心。采用常导电力电缆传输 1GVA以上的电能进入中心城区 ,输电电压一般要求为 5 0 0 k V。在城市中心区不可能建设 5 0 0 k V变电站。 5 0 0 k V电缆线路所需的 5 0 0 k V大长度电缆和相应附件 ,目前尚未研制开发。采用高温超导电缆将有可能降低输电电压等级 ,可以采用 2 2 0 k V高温超导电缆将 1GVA以上的电能输入到城市负荷中心 ,满足特大型城市负荷中心供电需求。采用 110 k V高温超导电缆 ,亦有可能传输 1GVA左右电能。本文通过对交流高温超导电缆系列设计计算对额定电压 35 k V、110 k V、2 2 0 k V的高温超导电缆 ,按不同传输电流 (或传输容量 ) ,以高温超导电缆的传输效率 (损耗与传输容量比 )、高温超导电缆外径限值和超导导体绕制结构限制条件 ,确定高温超导电缆适用性界定条件 ,提出城市地下输电、配电系统用高温超导电缆可行方案。     

Design study of a high‐temperature superconducting transformer

Many research and development projects on high‐temperature superconducting apparatus such as a transformer, fault current limiter, and cable are being actively pursued in the power field as a result of performance improvement of Bi silver sheath high‐temperature superconducting (HTS) wire. HTS transformers are considered to be among the most promising applications in view of efficiency improvement, the interface with cryogenic cable and normal temperature devices, incombustibility, overload capability, the function as a reactor, and the possibility of use as a fault current limiter. The necessary technical development items of an HTS transformer in order to achieve performance superior to conventional devices are examined. For this purpose, three‐phase 66‐kV 100‐MVA transformers were designed for the comparison of an HTS transformer and a conventional one with oil cooling. The desirable development items are an HTS wire current density of about 40 A/mm², an allowable winding strain of about 0.5%, a percent impedance of 7.5%, an air gap flux density of about 0.3 T, and AC losses of about 0.3 W/km‐A. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 142(1): 25–31, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10087     

高温超导电缆的研发现状和发展趋势

与常规电缆相比,高温超导电缆具有体积小、重量轻、容量大、电流密度高、损耗低、环境友好等优势,为未来电网提供了一种新的电力传输方式。随着高温超导线材取得的重要研究进展,国际上相继开展了高温超导电缆的研发,已有多条超导电缆工程成功地进行了挂网示范运行。本文介绍高温超导电缆的研发进展情况,并简单对高温超导电缆的发展趋势和关键技术做出展望。     

Analysis model of leakage current in a long shield power cable in inverter‐fed AC drives

Each time switching occurs, a PWM inverter produces high‐frequency leakage currents flowing through stray capacitors. Since the inverter for a trash conveyance crane is installed far away from the motor, the shield power cables between them become long, and leakage currents pass through the stray capacitances of the shielded cables and flow to the ground. This paper proposes a model circuit for analyzing leakage currents which flow into the shields of the cables. The effectiveness of the proposed model circuit is verified by comparison of simulated leakage currents using the proposed method and measured leakage currents. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 140(2): 65–72, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.2002     

Evaluation by numerical analysis of magnetic shielding characteristics of bulk high‐Tc superconductor containing weak links

High‐T_c superconductors (HTS), which have the characteristics of critical current density over 3 × 10⁴ A/cm² at liquid nitrogen temperature (77 K) and 1 T, can be produced. Thus, they are promising for many practical applications such as a magnetic bearing, magnetic levitation, flywheel, and magnetic shielding. Since the HTS characteristics are not homogeneous in some specimens due to grain boundaries and cracks, the distribution of magnetic characteristics should be assessed. Thus, we have measured the distribution of the magnetic flux density on the surface of the HTS using a Hall element, and have evaluated its magnetic characteristics. The measurement of magnetic characteristics using a Hall element is difficult regarding the distribution of the magnetic flux density on the actual surface and inside of the HTS sample. In this research, we conducted a quantitative evaluation of the magnetic shielding characteristics of the HTS including weak links under a static magnetic field with the three‐dimensional finite element method analysis. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 146(1): 9–17, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10209     

A transfer‐function approach to the analysis and design of zero‐power controllers for magnetic suspension systems

A transfer function approach is applied to the analysis and design of zero‐power controllers for magnetic suspension systems. The general structures of controllers achieving zero‐power control are derived for both current‐ and voltage‐controlled magnetic suspension systems. For the former type of system, there are two basic approaches: feeding back the velocity signal and introducing a minor feedback of the integral of the current. Both approaches are applicable to the latter type of system. In addition to them, the self‐sensing suspension also achieves zero‐power characteristics automatically. A direct synthesis method for zero‐power control is developed based on the analysis. Several experiments are carried out with a single‐degree‐of‐freedom model. The experimental results show the effectiveness of the proposed synthesis method. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 141(2): 67–75, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10049     

应用第2代高温超导体的冷绝缘超导电缆输电导体层间均流技术

冷绝缘高温超导电缆的导电层一般设计为多层结构以满足大电流载流特性,但伴随层数的增加,超导体上的集肤效应会引起电缆输电导体各层电流分布不均匀的问题,从而造成电缆损耗增加和传输性能下降。采用基于动态惯性权重因子的粒子群优化算法,提出了电缆导体层电流层间均流优化的设计方法。应用第2代高温超导材料钇钡铜氧涂层导体,通过建立超导电缆的等效电路模型,考虑电场、磁场等约束因素,对一根1km长,110kV/3kA等级的冷绝缘高温超导电缆进行优化设计,获得了电缆本体结构参数及输电导体层和屏蔽层的电流分布。比较优化前后层电流的结果可知,优化后超导电缆各导体层电流与平均电流相比最大不平衡率小于3.5%,各屏蔽层电流达到均布,较好地实现了电缆各导体层电流均匀分布的优化目标。最后,超导模型样缆载流特性实验也验证了优化设计方法的有效性。     

基于第二代高温超导带材的高载流超导导体研究进展

由于其高临界电流密度以及优越的机械性能和电磁特性,第二代高温超导带材(也叫涂层导体)在高温低场的电力传输和低温高场下的磁体应用具有广阔的应用前景。在电力传输的低场应用中,高温超导导体在低电压大容量场合需要几千安培甚至上万安培的传输电流。在大型高场磁体应用方面,为了避免由于过高电感在磁体失超和快速关断过程中的感应高压问题,大载流容量、高电流密度高温超导导体在运行于4.2K及以下温度的大型高场超导磁体方面具有很好的应用前景。近年来,基于第二代高温超导带材,国际上相继提出了几种高载流容量的高温超导导体,本文介绍几种高温超导导体的结构及研发现状和进展,并对其结构、性能和工艺进行简单的比较和评述。     

Fundamental performance of superconducting fault current limiter with two air core coils

In this paper, we theoretically compared the fundamental characteristics of fault current limiter (FCL) with high‐Tc superconductor (HTS) and two coaxial air‐core coils based on steady‐state analysis. Two types of FCL are possible. One is parallel type and the other is transformer type. The parallel type can be divided into two types according to the combination of the winding direction of coils. That is, there are two cases that the coils are wound so that the magnetic fluxes induced by coils reduce and increase each other. In this paper, we called them parallel type 1 and 2, respectively. There is no significant difference in the HTS volume required to satisfy both the specified limiting impedance Z_FCL and initial current I_ini in limiting operation among those three FCLs although the HTS in each type of FCL has different length and cross‐sectional area. In the cases of those FCLs, we can improve the current limiting performance by arranging the HTS in the coils and applying the magnetic flux to the HTS in the limiting operation. The magnitudes of the magnetic flux density are almost the same. From the viewpoint of the FCL impedance in normal operation, parallel type 1 has the most desirable structure. On the other hand, transformer type is the best to eliminate the magnetic flux applied to the HTS in the normal operation. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 165(3): 29–36, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20477     

Investigation of Water Tree Degradation in Dry‐Cured and Extruded Three‐Layer (E‐E Type) 6.6‐kV Removed XLPE Cables

Dry‐cured and extruded three‐layer (E‐E type) 6.6‐kV cross‐linked polyethylene (XLPE) cables were introduced into electric power systems more than 30 years ago, but they do not experience failures because of water tree degradation. Also, the degradation index of water treeing for these cables has not been established. Therefore, investigating results of residual breakdown voltage and water tree degradation of these cables will help us plan for cable replacement and determine water tree degradation diagnosis scheduling, and will be fundamental data for cable lifetime evaluation. In this study, the authors measured the ac breakdown voltages of dry‐cured and E‐E type 6.6‐kV XLPE cables removed after 18 to 25 years of operation and observed the water trees in their XLPE insulation. As a result, it was observed that breakdown voltages were larger than the maximum operating voltage (6.9 kV) and the ac voltage for the dielectric withstanding test (10.3 kV). Water trees were mainly bow‐tie water trees and their maximum length was approximately 1 mm. Although the number of measured cables was limited, the lifetime of this type of cable was estimated to be approximately 40 years, even experiencing water immersion.     

Development of new conductors of the cable‐in‐conduit type for HTS coils with high performance

A new type of cable‐in‐conduit conductor composed of HTS tapes was proposed as a winding for high‐performance HTS coils with high current capacities and low AC losses. In the fabrication of the conductor, the twist of stacked tapes around their axes was made before inserting them inside conduits. The twist angle should be changed continuously along the axis to reduce face‐on oriented magnetic fields applied to the tape in the winding conductor during coil operation. In order to confirm the high current capacity of this type conductor, two single‐layered solenoidal coils wound with sample conductors composed of five stacked Bi‐2223 tapes with bias angles of 20 and 0° were fabricated and tested in liquid nitrogen. A copper magnet system was used in this experiment to generate the spread magnetic field with a spread angle of 20° from the coil axis to the radial direction, which simulates the profile of magnetic fields near the edge windings of practical coils. A large improvement on critical‐current degradation affected by spread magnetic fields was successfully observed for the test coil wound with the 20° bias conductor. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 153(4): 12–19, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20218     

Study of shunt type SFCL equipped with electromagnetic repulsion switch for large capacity

Using a high‐temperature superconductor, we constructed and tested a model Superconducting Fault Current Limiter (SFCL). The SFCL that we proposed has a vacuum interrupter with electromagnetic repulsion mechanism. We set out to construct a high‐voltage‐class SFCL. We produced an electromagnetic repulsion switch equipped with a 24‐kV vacuum interrupter (VI). However, the opening speed becomes slower, because the larger vacuum interrupter needs a heavier‐weight contact. For this reason, the current which flows in a superconductor may not be interruptible within a half cycle of current. In order to solve this problem, it is necessary to change the design of the coil connected in parallel and to strengthen the electromagnetic repulsion force at the time of opening the vacuum interrupter. Thus, the design of the coil was changed, and in order to examine whether the problem is solvable, a current limiting test was conducted. We carried out a current limiting test using second‐generation (2G) HTS wire. The element used in this experiment has a stainless steel stabilizer on both sides of the wire. In the experiment we succeeded in interrupting the current of a superconductor within a half cycle. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 173(4): 20–27, 2010; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21039     

A new approach to an individual‐phase reactive power compensator for nonsinusoidal and unbalanced three‐phase systems

Many researchers have attempted to clarify the definitions of active power, reactive power, active current, reactive current, etc. for unbalanced and nonsinusoidal three‐phase situations. The so‐called pq theory has given a new definition of instantaneous reactive power, and it has been discussed and developed by many authors. In this paper, the merits and demerits of the instantaneous reactive power compensator are discussed. It is shown theoretically that applying instantaneous reactive power compensation to unbalanced three‐phase systems has a serious disadvantage in that it causes third‐order harmonic currents on the source side, which problem cannot be avoided. To overcome this problem the authors propose a new approach, and name it the “quasi‐instantaneous” reactive power compensator. It compensates individual‐phase reactive currents. The basic principles of the quasi‐instantaneous reactive current compensator are discussed in detail, and its validity is confirmed using digital simulation. In particular the authors show that the power factor of each phase becomes unity on the source side, but the source currents remain unbalanced when the proposed method is applied. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 139(3): 73–81, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.1162     

浅析高温超导电缆在城市电网的应用前景

高温超导电缆具有不同于常规电缆的优越特性,已在国内外研制成功并在工程中实验和应用。介绍了高温超导体特性和高温超导电缆,简要分析了国内外高温超导电缆的研究进展、应用现状和项目实例,对高温超导电缆在城市电网的应用前景作了初步探讨。     

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     

Banishing blackouts

This paper presents the use of superconductive cables as a cost-effective way to upgrade urban power networks while protecting the environment. High-temperature superconductor (HTS) cable, which is capable of serving very large power requirements, is one of the technologies with the greatest promise to address these concerns. Because they are actively cooled and thermally independent of the surrounding environment, they can be fit into more compact installations than conventional copper. This advantage reduces environmental impacts. In addition, HTS cables exhibit much lower resistive losses     

Approach for wide use of diagnostic method for XLPE cables using harmonics in AC loss current

A water tree is one aspect of the degradation of XLPE cables used for underground distribution or transmission lines. We have developed the loss current method using the third harmonic in AC loss current for cable diagnosis. The harmonic components in loss current arise as a result of the nonlinear voltage– current characteristics of water trees. We confirmed that the third harmonic in the AC loss current has good correlation with respect to water tree growth and breakdown strength. After that, we applied this method to the actual 66‐kV XLPE cable lines. Up to now, results on over 130 lines have been obtained. In the case of cable lines terminated at gas‐insulated switchgear (GIS), we have to remove the lightning arrestor (LA) and the potential transformer (PT) from the test circuit. The reason is that we are afraid that each LA and PT disturbs the degradation signal from cable lines. It requires extra time (1 or 2 days) and costs more to remove the LA and PT in GIS from a circuit. In order to achieve easy and reasonable diagnosis, we have developed a new method for cable lines terminated at GIS by utilizing a technique that enables one to cancel the signal of the LA and PT from disturbed signal of the cable lines. We confirmed the effect of the new method through experiments with actual cables. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 165(4): 52–59, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20510     

高温超导输电技术的应用

介绍高温超导（HTS）输电技术的基本原理、主要优势及其冷介质同轴设计和热介质设计的特征与比较。重点介绍Pirelli公司50m长、400MVA HTS原理电缆的研制过程和美国南方导线公司为大型制造厂安装的30m长,12.5kV的HTS电缆供电系统。