Development of extruded ethylene propylene rubber insulated superconducting cable

A superconducting power cable is one of the promising ways of underground transmission of huge electric power in the future. The authors have long proposed the idea of the extruded polymer insulation for superconducting cables. The prominent features of the design are to exploit the excellent electrical properties of polymer in the cryogenic temperatures and to separate the helium coolant from the electrical insulation. Although the extruded cross-linked polyethylene cable has proved ability at the liquid nitrogen temperature, the cable insulation cracked due to mechanical stress during cooling to the liquid helium temperature. To overcome this problem, ethylene propylene rubber (EPR) was selected as a new insulating material considering the good results of mechanical and electrical tests of EPR samples at cryogenic temperatures. An extruded EPR insulated superconducting cable 15 m in length was fabricated and a cooling test down to the liquid helium temperature and a voltage test at the liquid helium temperature were carried out with fair success. This is a breakthrough in terms of the electrical insulation design of cryogenic cables.     

Tan δ of ethylene-propylene rubber in cryogenic temperature region–effects of additives and sample thickness

The authors have been developing extruded polymer insulated superconducting power cables. Dielectric loss in electrical insulation cannot be ignored in superconducting cables since conductor loss in the cables is minimal. Studies so far show that ethylene-propylene rubber (EPR) is suitable as an electrical insulating material in the extruded polymer insulated superconducting cable design because it demonstrates excellent mechanical and relatively good electrical qualities at a cryogenic temperature. Widely used EPR includes some kinds of additives; however, their effect on tan δ of EPR at cryogenic temperature remains unknown. The effect of additives such as crosslinking agent and fillers on tan δ of EPR was examined at temperatures of between 4.2 K and 300 K. Thickness dependence of tan δ was also measured using EPR films of different thickness and an extruded EPR insulated cable sample. The results show that additives increase tan δ of relatively thin films of EPR even in the cryogenic temperature region; however, they do not have an intolerable dielectric loss in comparison with conductor loss and heat inflow of the superconducting cable. The remedy to tan δ increase due to the polymer contraction relative to shielding wires has been established.     

Electrical insulation technology for superconducting devices inJapan

The abnormal voltage induced in a superconducting magnet is examined, and the need to utilize insulating materials that do not degrade electrically or mechanically at a very low temperature is demonstrated. Breakdown of the liquid helium in which most superconducting magnets are immersed is discussed. Different types of superconducting magnets are briefly described. Research activities in Japan on electrical insulation technology at cryogenic temperatures and several practical examples of electrical insulation for superconducting devices are reviewed     

ac treeing of ethylene-propylene rubber in cryogenic temperature region

The authors have proved that ethylene-propylene rubber (EPR) is suitable for the solid electrical insulating material of superconducting and cryoresistive cables due to its excellent mechanical and good electrical properties. It is imperative to study the treeing resistivity of EPR in view of long-term reliability as a cable insulator. The objectives of this paper are to establish an ac treeing test method of opaque EPR samples with fillers and to examine the treeing resistivity of EPR at both liquid-nitrogen and room temperatures and the effect of fillers on treeing resistivity. It was found that EPR has enough resistance to ac treeing at liquid-nitrogen temperature and that fillers improve ac treeing resistivity slightly. High reliability of EPR insulation is expected during long-term operation at cryogenic temperature.     

High voltage studies of dielectric materials for HTS power equipment

The discovery of high temperature superconductors (HTS) has triggered renewed interest in the study of dielectric materials at cryogenic temperatures. While considerable work was done in the 1970s and 1980s on dielectrics immersed in liquid helium for low temperature superconducting applications, there remains a need for dielectric research at liquid nitrogen temperature for HTS applications, requiring experimental data oriented toward practical situations. We report on AC breakdown (puncture and/or flashover), and impulse breakdown of solid materials in either vacuum or in liquid nitrogen. Solid materials which we examined, include fiberglass reinforced plastics, epoxies with and without filler, and polymeric tape. Combinations of some of these materials have also been studied at low temperatures. Additionally we have measured permittivity and dissipation factor for materials for which these parameters are not available at 77 K. Finally, we also discuss specific applications for HTS cables including breakdown and aging studies on model cables, with lapped tape electrical insulation, immersed in liquid nitrogen.     

Insulating materials evaluation for Cold Dielectric superconducting cables

The realisation of the so-called "cold dielectric" cable, where the insulation is kept at a cryogenic temperature, entails the choice of an optimised insulation system, with properties that are the best compromise between physical-electrical characteristics and design requirements. To this purpose, a specific study was planned to qualify different materials. On the basis of literature results, mostly carried out at liquid helium temperature, polypropylene laminate (PPL), paper, high-density polyethylene (HDPE) fiber and ethylene-propylene rubber (EPR) were chosen as potential candidates for the final use in liquid nitrogen. Electrical properties of materials were evaluated and compared by means of different models. Flat and cylindrical models were used to characterise the intrinsic properties of materials and determine the impact of the technological processes, respectively. Short term and long term tests were carried out at liquid nitrogen temperature and at different absolute pressures for the evaluation of electrical endurance. As far as short term tests are concerned, AC and lightning impulse (LI) breakdown step tests were performed. Long term tests were carried out under AC field; furthermore, power factor and partial discharge measurements were made to evaluate the impregnation process. The results of the whole testing procedure indicate that PPL shows the best behaviour for the application at cryogenic temperature in a superconducting cable.     

挤包绝缘高压直流电缆应用现状与研究展望

挤包绝缘高压直流电缆已成为远距离、大容量输电线路建设的主要装备之一,现阶段广泛应用的挤包绝缘直流电缆以交联聚乙烯(XLPE)绝缘为主.针对XLPE绝缘高压直流电缆研发和应用中的绝缘空间电荷特性、副产物脱气等主要问题进行全面综述,指出在工业化生产方式下,超纯净体系XLPE绝缘材料是目前用来缓解空间电荷问题和实现电缆批量化...     

The high voltage design of superconducting power transmissionsystems

The special problems that arise when designing a superconducting system are examined. These are: (1) that the voltage and current levels chosen must permit economic integration into existing transmission and generation facilities; (2) the mechanical design of components that get cold must permit relatively large absolute contraction and possibly differential contraction without damage that may affect successful operation, and cool-down and warm-up cycles throughout the life of the system should not cause deterioration of the dielectric insulation; and (3) the dielectric losses of the insulation must be carefully controlled in AC systems. Basic research on cryogenic dielectrics is briefly reviewed, and 11 projects around the world that have engaged or are engaging in electrical insulation testing of prototype cables are described. The development of accessories is discussed     

Insulation coordination of MV power cables

In medium voltage power cables, the concept of cost/reliability has been current since the end of the 1980s when the Brazilian Standards introduced ethylene-propylene-rubber (EPR) insulated cables, with different thicknesses of insulation for conventional cables and for those with blocked conductors or with water tightness constructions. However, despite the benefits obtainable by use of such cables, they have had limited use by electrical power utilities; perhaps because the concept has not become widely recognized. This paper intends to clarify and submit a new approach for design of the dielectric system of medium voltage cables, for use in underground and insulated overhead distribution feeders. Theoretical aspects of sizing the dielectric system, by means of coordinating the insulation, and also using reliability parameters, is dealt with in the paper. Results of practical applications are also presented and discussed     

The selection of the frequency range for high-voltage on-sitetesting of extruded insulation cable systems

High-voltage on-site tests of extruded insulation cables are necessary to check the quality of the installation of the system. The test voltage should simulate the stress under service conditions, and generate the same failure mechanism. With respect to an optimum design of test systems, a frequency range of 20 to 300 Hz, proposed in the latest IEC Draft for on-site testing of extruded insulation cables, seems to be reasonable. The test results on model cables can be used for real cables concerning the influence of the waveform on the breakdown voltage. The withstand voltage decreases with increasing frequency. The withstand voltage and the electrical breakdown field strength are very close together for frequencies between 20 Hz to 300 Hz. Mechanical defects, as well as water trees, reduce the breakdown voltage at 0.1 Hz more than at 20 to 300 Hz, but the absolute test voltage values are higher, and the breakdown mechanism is different compared with voltages of power frequency or adjacent frequencies. For on-site tests, frequency-tuned resonant test systems (ACRF) can be recommended because they have a very good weight-to-test power ratio, and very low power demands     

Performance of reduced wall EPR insulated medium voltage powercables. I. Electrical characteristics

Paper-insulated lead covered power cables (PILC) have had a long and successful heritage. After almost 100 years, this design of cable is still in operation and continues to be manufactured. However, electric utilities are now looking for a reliable replacement for PILC cables. This is due to two primary reasons: (1) the difficulty in installing and maintaining this type of cable; and (2) increasing pressure to replace these cables due to environmental concerns. To date, diameter limitations of conventional extruded dielectric cables has impeded their replacement in existing PILC conduits. This paper describes a study for the evaluation for reliably reducing the insulation thickness to achieve a lower diameter cable to effectively replace PILC cable in existing conduits. Part I of the investigation reviews the theory of insulation wall determination and the test program carried out to evaluate electrical performance of reduced wall EPR cables. Additionally, cable design concepts and constructions are discussed     

Polymer insulated high voltage cables

This article reviews the different components, conductors, semiconductive shields, insulation, sheaths, and jackets, that make up extruded, high-voltage cables. The different types of conductors are described as are the different insulation systems that are available for medium and HV cables. There is a comparison of the two main insulators used in extruded HV cables, EPR, and XLPE. Each material has some properties that are better than those of the other material.     

超高分子量聚乙烯用作低温电缆绝缘的试验研究

为了解决固体合成绝缘材料低温下的脆裂问题,本文研究了超高分子量聚乙烯用作低温电缆绝缘层的可能性。通过试验比较了各种电缆绝缘材料在低温下的脆裂情况,并在液氮温区中测定了 UHMW-PE 的电气绝缘性能。试验结果是满意的。     

Electrical Insulation Performance of Paper–Ice Composite Insulation System at Cryogenic Temperature

In recent years, research on the development of high-temperature superconducting cables has advanced. The application of a composite insulating system made of insulating paper-liquid nitrogen has been studied as an electrical insulating system for high-temperature superconducting cables. In paper-liquid nitrogen composite insulating systems developed on the basis of conventional techniques, microscopic bubbles easily remain inside the paper, and the partial discharge are supposed to occur easily. In order to overcome this drawback, we formulated a paper-ice composite insulating system as a substitute for the paper-LN₂ of the cryogenic electrical insulating system in high-temperature superconducting cables. In this study, the breakdown strength, partial discharge, V-t characteristics, effect of pressure on breakdown characteristics, and the self-healing breakdown characteristics of a paper-ice composite insulating system were investigated in order to understand the basic properties of new insulating materials for applications to high-temperature superconducting cables. As a result, it was clarified that the AC breakdown strength of paper-ice composite insulating system was higher than that of paper-liquid nitrogen insulating system, and that of paper-ice composite insulating system exhibited a self-healing type. Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Aging of polymeric and composite insulating materials. Aspects of interfacial performance in aging

Aging of polymeric and composite materials is reviewed mainly from the standpoint of their interfacial performance. Insulating materials can be divided into simple polymers and composites. Polymers for power cables, transformers, insulators and rotating machines consist of thermoplastics such as PE, PET and PPS, elastomers such as silicone, EPR and EPDM, and thermosets such as epoxy. Composites for GIS, rotating machines and insulators comprise epoxy/glass, epoxy/silica/alumina, and epoxy/mica systems. Aging processes are complicated in general, and take place under simultaneous multiple stresses such as electrical, thermal, mechanical and environmental stresses. Some of the phenomena covered in this paper are associated with the degradation by tracking and erosion and the loss of hydrophobicity in case of surface properties, and with PD, electrical treeing, water treeing and combined phenomena. The quality of cable insulation such as XLPE has been improved extensively from the standpoint of design electric strength. Interfacial problems will emerge for modification of cable joints. It is expected that polymers for outdoor use and filled epoxy resin systems should be improved from their environmental stability and from their design stress enhancement, respectively.     

Extending the service life of 15 kV polyethylene URD cable usingsilicone liquid

The authors describe methods and materials used to extend the useful life of extruded polyethylene insulated cables rated at 15 kV that have developed electrochemical trees in the insulation of sufficient severity to cause service failures. Earlier rehabilitation methods treated extruded dielectric cables with both gases and insulating liquids. This new method fills the cable and impregnates the insulation with a silicone liquid. This liquid, which is reactive, significantly improves the voltage breakdown strength of the cables, which had been reduced by electrochemical trees, and extends the useful service life of the cable. A review of the criteria used to develop the silicone liquid for this application and the methodology used to inject it into the cable are presented. An assessment of the costs for this life extension indicates a significantly lower cost than conventional cable replacement with little disturbance to the service and property of the customer     

Electrical insulation technology for superconducting magnets inJapan

Electrical insulation of superconducting magnets needs to endure a variety of voltages with a large range of shapes, magnitudes and duration under normal operation, during coil quenching, and at withstand voltage tests. The author reviews the current state of electrical insulation technology for superconducting magnets in Japan. He maintains that Japan will continue to aggressively pursue the research, development and application of superconducting magnets. In accordance with this, the main technical subjects to be solved in the insulation of the superconducting magnets in the near future will be: (1) the development of insulation that has high electrical, mechanical and radiation-resistant properties at the cryogenic condition; and (2) investigation into the long-range effects of insulation on the stability of superconducting magnets     

Properties of electrical insulating materials at cryogenictemperatures: A literature review

Many insulating materials and systems for superconducting electrical equipment have been developed in response to the severe demands of safe usage, such as in cryogenic temperature, high mechanical stresses, high radiation, etc. They have been designed to fulfill very extreme service conditions, either at liquid helium or liquid nitrogen temperatures. Whatever the future industrial development, there is already abundant knowledge about insulating materials at cryogenic temperatures, and the feasibility has been proven. Many laboratories in the world continue research to increase this knowledge and to fill in the remaining gaps     

Electrical aging of extruded dielectric cables: review of existingtheories and data

Despite the huge amount of data on so-called electrical aging of extruded HV cables, the fundamental phenomena responsible for it or evolving with aging time, are still far from well understood. It is therefore not surprising why it is so difficult to predict reliable cable lifetimes in service from accelerated aging experiments in the laboratory. The objective of this paper is to review critically the existing theories of electrical aging of solid dielectric materials. A relatively large number of models and theories exist but none of the most often used is known to yield reliable life predictions. One conclusion is that there is a need for a more comprehensive model of electrical aging of extruded dielectric cables. In order to develop this model, an extensive review of existing literature data was undertaken. This paper summarizes the data collected from more than 200 papers on aging of PE, XLPE and EPR cables. It appears that cable breakdown strength should not be plotted on log field vs. log time graphs to yield long-time (i.e. low-field) values, since results obtained over a long time period do not obey an inverse power law. In fact, high-field results are better described by an exponential relation between time and field. The models of Simoni, Montanari and Crine seem to give the best fit to experimental results obtained under a wide variety of experimental conditions. It is also shown that the lower field limit for the exponential regime with XLPE cable is in the 8 to 15 kV/mm range, which corresponds to the onset of strong charge injection. The influence of environment, insulation nature and morphology, and testing temperature are discussed     

中高压电力电缆用乙丙橡胶绝缘材料的研究

用于电厂封闭母线的电缆、盾构电缆、煤矿用电缆等中高压电力电缆的绝缘越来越多地采用乙丙橡胶。本文研究了适合于中高压电缆使用的乙丙橡胶绝缘材料,从基体材料的选择、配合剂的选择、加工工艺等方面进行了探讨,所研制的乙丙绝缘材料及其制品具有优良的机械电气性能和加工性能。