A study of treeing phenomena in the development of insulation for500 kV XLPE cables

This review summarizes research on treeing phenomena, i.e. the formation of electrical trees and water trees, that has been undertaken in Japan for the development of 500 kV XLPE cable. Section 1 presents the results of factors affecting XLPE cable insulation breakdown under commercial ac and lightning impulse voltages. Section 2 verifies the phenomena of electrical tree formation in XLPE cable insulation using block samples and model cables, and gives the results of studies to determine the level electrical field stress initiation for such trees. Section 3 summarizes the results of studies on long-term aging characteristics, which is a particular problem under commercial ac voltages, while Section 4 explains how this research influenced the design of 500 kV XLPE cable insulation. All authors were members of `The investigation committee of fundamental process of treeing degradation' under IEEJ     

热老化对交联聚乙烯电缆绝缘中水树的影响研究

热老化过程不但会影响交联聚乙烯电缆绝缘的电磁学和物理化学性能,还对绝缘内水树的产生与生长有着一定的影响。通过研究热老化过程对XLPE电缆绝缘中的水树现象的影响,以及在几个有可能的影响因素当中,哪个因素对水树现象的影响最大。实验结果表明,在与XLPE电缆绝缘的热老化有关的各种因素对水树现象的影响中,热氧化对XLPE电缆绝缘表层水树的产生和生长的影响最大。尽管热氧化所引起的缺陷有可能就是XLPE电缆绝缘中水树生长过程中的起始点,但是它在一定程度上抑制着水树的成长,甚至有着"水树延迟效果"的美称。     

The effect of pulsed voltages on dielectric material properties

The problem of premature failure of rotating machines fed by inverters appears to be of crucial importance. Understanding of the phenomena caused by this new type of supply on the stator insulation of motors provides enhanced reliability of the inverter/motor set. After identifying the stresses created by this type of device, a study of aging of the materials used as models, and subjected to similar stresses, is undertaken. For the voltage, rise time and frequency under study, the intrinsic part of the aging is studied. The results are presented and discussed according to the nature of the interaction existing between the electrical stress and the material properties     

The Performance of Strippable and Bonded Screened Medium-Voltage XLPE-Insulated Cables Under Long-Term Accelerated Aging

XLPE high-voltage underground cables are being extensively used in Arabian Gulf countries. Most of these cables are being manufactured locally. Although XLPE has excellent dielectric characteristics, extensive premature failures have been reported by the local power utilities. The main causes of such premature failures are related to manufacturing defects and insulation degradation mainly caused by water treeing phenomena. In this research work, five types of 15-kV, XLPE-insulated cables were subjected to accelerated aging under simultaneous electrical, thermal, and environmental stresses for periods of 60, 120, and 180 days. After each aging period, partial-discharge (PD), mechanical properties, ac dielectric strength and the extent of degradation due to water treeing of these cables were measured. The results show that PD and mechanical properties show just little variations whereas the ac-retained dielectric strength decreases rapidly in the first 60 days after which the rate of its decrease retards. Correlations are established between the retained dielectric strength and the lengths of the longest water trees. Pros and cons of the insulating materials that are used as well as the role of strippable and bonded semiconducting screens are also discussed     

Electrical treeing in mechanically prestressed insulation

For several years it has been apparent that the mechanical characteristics of insulating materials have a pronounced influence on their electrical breakdown by treeing. As a consequence of this discovery, it is possible to look again from a different perspective at the treeing behavior in materials containing fillers and barriers. The effect of temperature on treeing can be interpreted in terms of the effect of temperature on the various mechanical properties of the material, and of any particulate or solid barrier inclusions. The internal mechanical stresses produced in resin samples containing cast-in metallic electrodes can affect treeing behavior both positively and negatively, depending on whether this stress in the material is tensile or compressive. In the light of this experience it has been found possible to enhance the treeing resistance of electrical insulation by using these mechanical effects in the most advantageous way, viz. by mechanically prestressing the material by the inclusion of fibers in tension; a method which is akin to that employed in the prestressing of concrete     

A versatile method to study electrical treeing in polymeric materials

Electrical treeing in high voltage cable insulation has been studied widely as it is one of the major breakdown mechanisms for solid dielectrics subjected to high electrical stresses. The most frequently experimental set-up used to study electrical treeing is utilizing needles to achieve a high divergent field to promote the initiation of electrical trees. In this paper a new type of electrode configuration is described, which will be beneficial when examining electrical treeing in terms of both the capability to follow the course of events in real time and the small amount of insulation material is needed. This new electrode configuration makes use of a thin tungsten wire as electrode to obtain the high divergent electrical stress needed to initiate electrical trees. Multiple electrical trees are at times formed at different points along the wire during the testing, this indicates that the field are homogenous along the wire. This allows for determining the tree initiation field and the rate of tree growth. It is also shown that this electrode configuration is able to distinguish the increased electrical tree inception obtained by the addition of voltage stabilizers.     

AC and DC performance of polymeric housing materials for HV outdoorinsulators

An experimental study was conducted to investigate the performance of several polymer outdoor insulator formulations under AC and DC electrical stresses. The performance of the polymers was evaluated in terms of their charge accumulation and decay characteristics as a function of humidity, and their tracking and erosion resistance. The results obtained from this study indicate that significant differences exist in surface charging phenomena between AC and DC. A likely contribution of DC polarization processes leading to hydrophobicity loss is believed to influence additionally the performance of materials subjected to pollution and DC electrical stresses. A substantial reduction in the tracking and erosion resistance of the polymeric materials is observed with DC stresses, in comparison to AC. The poorer electrical performance of the materials stressed with DC voltage was found to be due to higher magnitudes and longer duration times of the discharge current     

Application of thermal analysis techniques to appraisal and qualitycontrol in Japan

This review deals with both inorganic- and organic-based electrical/electronic materials, including enameled wires, insulating varnishes, epoxy resins, thermoplastic resins, thermoplastic films, thermosetting resins, ferroelectric polymers, mineral-based transformer oils, zinc oxide varistors, rotating machine insulating systems, photoresists, glass-fiber-reinforced plastics, electronic ceramics, and high-T_c (critical temperature) oxide superconductors. The thermal analysis techniques considered are thermogravity, evolved gas analysis, differential thermal analysis, differential scanning calorimetry, thermomechanical analysis, thermally stimulated current measurement, and photoinduced calorimetry. An attempt is made to clarify how successfully a particular technique can be used for developing new materials or for evaluating a specific practical application of an electrical or electronic material     

Influence of temperature and water on the breakdown electric field strength of XLPE cables under divergent fields

The present paper introduces experimental results on treeing breakdown phenomena in actual power cables. Using a water capillary electrode, cable samples immersed in silicone oil have been tested by alternating (50 Hz) and direct voltage stresses. Examination of the needle geometrical breakdown electric field strength resulted in a detailed analysis of the influence of temperature and the presence of water on the breakdown of XLPE cables under divergent fields.     

交流叠加冲击电压下XLPE绝缘中电树起始特性

ＸＬＰＥ绝缘在交流叠加冲击电压作用下电树起始特性的试验研究表明,电树的冲击起始场强不仅与预加的交流电压的大小有关,还与冲击电压与交流电压的相对极性有关,并有明显的累积效应,起晕也有明显的时延现象。     

用有限元分析交联聚乙烯电缆中水树成长行为的研究

水树是电缆主绝缘的主要劣化现象之一,严重影响电缆的使用寿命。正确评价长有水树的材料的电场分布,对于理解水树的生长机理以及水树引起的介质击穿是非常重要的。采用具有不同物理化学性能的热老化XLPE电缆绝缘和与外加电场梯度方向成一定角度的非常规的水针电极结构进行水树枝加速老化实验,用有限元方法对水树加速老化实验中采用的结构模型进行数值电场分析,研究了水树尖端处电场增强对水树枝老化发展的影响。     

DC electrical treeing phenomena and space charge

Influence of space charge on dc tree propagation is studied using many insulating polymer materials under divergent fields. The inception voltage and the propagation of dc treeing are affected strongly by space charge. Space charge formation in polymers was observed directly by using thermally stimulated current (TSC) techniques under divergent field. These results of the space charge effect on dc treeing phenomena are consistent with those obtained by the TSC method. From these results, the origin of space charge in polymers and the suppression techniques for space charge formation were discussed     

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.     

Specifying and Designing Flexible Shaft Electrical Machines

Recent developments relating to the undestanding, analysis, and measurement of vibratory motiohs, forces, and instabilities of rotating systems and their bearing support structures are now becoming available to the designers of systems involving electrical machines and their component suppliers. The status of the more pertinent of these developments and their applicability to such systems is reviewed with special considerations to those involving flexible shaft machines.     

Partial discharges. Their mechanism, detection and measurement

Different partial discharge (PD) detection and measurement procedures suitable for use on cables, capacitors, transformers and rotating machines are examined and compared. Both narrow and wide bandwidth PD detectors are considered; particular attention is given in regard to their suitability to different types of electrical apparatus and cable specimens under test as well as their applicability to discharge site location and their capability to detect different forms of PD. A rather substantial portion of the discussion is devoted to the use of intelligent machines as applied to PD pattern recognition in terms of either PD pulse-height/discharge epoch (phase) distributions or discharge pulse shape attributes.     

交联聚乙烯电缆热老化与电树枝化相关性研究

热老化过程不但会影响交联聚乙烯（XLPE）电缆绝缘的物理化学性能,还对绝缘内电树的产生与生长有着一定的影响。研究了热老化后XLPE电缆绝缘中的电树行为．探讨XLPE电缆绝缘中电树枝过程与材料热老化的关系。采用带循环通风的热老化箱对XLPE电缆绝缘进行3个温度等级的热老化实验：采用针板结构电极进行电树枝实验,并利用数码显微镜观察电树枝的产生和发展情况;利用差示扫描量热法（DSC）、傅里叶红外光谱分析（FTIR）测试了不同温度热老化下XLPE电缆绝缘的物理化学性能;最后探讨了几种不同结构电树枝的生长机理．认为热老化并没有加速电树枝的生长．反而有一定的抑制作用．     

Measurement of space charge in XLPE insulation under 50 Hz acelectric stresses using the LIPP method

Space charge is believed to play an important role in ac electrical ageing of polymeric insulation, particularly where electrical treeing is the dominant cause of premature failure. The majority of work to date has been on the space charge characteristics under dc electric stresses, whereas work on the dynamics and the role of space charge on electrical breakdown under 50 Hz ac conditions have only received limited attention. Consequently, a full understanding of space charge trapping and dynamics under ac operating conditions is required if ever more reliable polymer insulated cable systems are to be developed. The present paper reports on aspects of an on-going investigation into the measurement of space charge in XLPE insulation under 50 Hz ac stress using the laser induced pressure propagation (LIPP) technique. Some important features of the method are described such as, point on wave control of the laser and use of correction factors to offset variations in the laser power and ablation of the target material. Emphasis has also been placed on establishing a simple method for analysing data, termed "X-plots". Some preliminary results of the space charge and electric stress distribution of un-degassed and degassed crosslinked polyethylene (XLPE) plaques aged electrically under ac stress (30 kV_peak/mm) for 24 h are also presented     

Performance characteristics of dielectrics in the presence of spacecharge

The short and long-term dielectric behavior of a number of representative electrical insulating systems is compared in the presence of space charge. Dielectric materials, used both in the communications and power application areas, are considered. In this overview, particular attention is given to thin inorganic films, organic solid-liquid and solid polymer systems as regards to the manner in which space charge affects their dielectric loss, voltage breakdown, treeing and electrical aging characteristics     

Testing of low-voltage motor turn insulation intended forpulse-width modulated applications

The problem of premature failure of low voltage rotating machines fed by inverters could have serious economic impact if it is not addressed. Indeed, even though much already has been written on the different topics related to the use of such power supplies, there is a great need for a better understanding of the physical mechanisms involved, particularly with regards to aging mechanisms of insulating materials. The aim of this paper is to highlight the effect of adjustable speed drives (ASD) on the stator finding insulating materials. Pulse-like electrical stresses have been applied to three types of samples (thin film, twisted pair, and layer to layer). Physical characterization and/or lifetime measurements have been performed. The results are presented and discussed according to the nature of the interaction existing between the electrical stress main characteristics (magnitude, frequency, dV/dt) and the material properties. According to these results, the development of a standardized insulation quality control test, based on a physical understanding of involved stresses and material behavior, appears now to be necessary