Recent developments in the pressure wave propagation method - [Feature Article]

Performing measurements is an essential part of understanding physical phenomena. Hence, numerous measuring techniques have been developed to study space charge build-up in dielectrics. This macroscopic accumulation of charge modifies the electric field and can lead to aging and premature failure of electrical insulation. On the other hand, charge accumulation is essential to the operation of devices such as electret- or piezoelectric-based sensors. It is therefore essential to study charge build-up in order to design insulating materials in which excessive charge does not accumulate, or electrets in which a sufficiently large amount of charge accumulates to ensure adequate device operation, and the stability of that charge in time is sufficient to ensure an acceptable device life-time. This paper focuses on the measurement method using a pressure wave to perturb the charges. That method is commonly called the pressure-wave-propagation (PWP) method. A pressure wave enters the sample through one of its interfaces. As it travels through the material at the velocity of sound, the charges encountered are mechanically displaced.     

Feature article - Polymeric HVDC cable design and space charge accumulation. Part 2: insulation interfaces

The electric field distribution of cable insulation systems under HVDC can be affected significantly at interfaces due to space charge build-up. In this article, the second part of a three-article series, face and space charge accumulation are analyzed first in terms of macroscopic physics, then through approximate mathematical models that will be used to fit experimental data obtained for model cables having two insulation layers and constituting cylindrical interfaces.     

Nanodielectrics - How Much Do We Really Understand? [Feature Article]

This article aims to provide an introductory overview of the topic of nanocomposites, with a particular emphasis on their use in electrical applications in which their dielectric response is likely to be of importance. Composites are widely used engineering materials because of the enhancement in properties that can result from combining a number of distinct components. Reducing the size of the filler particles into the submicron range can overcome such problems and influence combinations of properties in a complex and often non-intuitive manner. It is this that has resulted in the rapid expansion of nanocomposites research in recent years.     

Electric field calculations for needle-plane geometry and space charge in polyethylene

Electric fields in polymeric insulation are calculated with the boundary element method (BEM) in the presence and absence of space charge. A 3-D version of the BEM software is employed to determine the profiles of divergent electric fields produced by metallic and semicon needles embedded in the polymeric insulation. It is shown that the value of the electric field in the insulation decreases rapidly and within 5 /spl mu/m from the tip it is less than 50% of its initial value. The BEM can also provide 3D profiles of the electric field distributions in the presence of space charge.     

Insulation Failure Mechanisms of Power Generators [Feature Article]

This article discusses about the abnormal or faulty operating condition of premature power generator insulation failures that can be classified as: design failures, material failures, manufacturing failures and maintenance failures. The final failure of the mica insulation is usually caused by electric breakdown through a tree channel. Formation and propagation of trees in the insulation therefore reflect the aging process of the insulation.     

Physicochemical aspects of gassing of insulating liquids under electrical stress - [Feature Article]

This article describes the physico- chemical aspects of the gassing phenomenon and proposes a new method to determine the resistance to gas formation of insulating liquids.     

Review of space charge measurements in dielectrics

In the last two decades, a significant effort has been directed to understand the internal charge distributions in dielectrics. This has resulted in the development of a number of methods which give detailed information about space charge distributions. This review presents the evolution of several experimental approaches for the determination of spatial charge and field distributions in dielectrics, including the nondestructive methods of direct probing such as the thermal-pulse methods and the methods using pressure waves that propagate through the charged sample. Detailed information is also provided on the principles used in each technique, typical resolution, limitations and advantages associated with each method. The applications of the methods to several insulating materials charged with various mechanisms are described. The paper concludes with a discussion on a number of quite interesting techniques which seem most promising     

Asset-Management of Transformers Based on Condition Monitoring and Standard Diagnosis [Feature Article]

In this paper, a methodology is developed to use data acquisition derived from condition monitoring and standard diagnosis for rehabilitation purposes of transformers. The interpretation and understanding of the test data are obtained from international test standards to determine the current condition of transformers. In an attempt to ascertain monitoring priorities, the effective test methods are selected for transformer diagnosis. In particular, the standardization of diagnostic and analytical techniques are being improved that will enable field personnel to more easily use the test results and will reduce the need for interpretation by experts. In addition, the advanced method has the potential to reduce the time greatly and increase the accuracy of diagnostics. The important aim of the standardization is to develop the multiple diagnostic models that combine results from the different tests and give an overall assessment of reliability and maintenance for transformers.     

Correlation between space charge distribution and water-treelocation in aged XLPE cable

The correlation between charge distribution in an aged crosslinked polyethylene (XLPE) cable and the location of water trees has been studied. The space charge radial distributions at different angle in the insulating wall of a service-aged XLPE coaxial cable, induced by application of a DC stress, were measured using the pulsed electroacoustic method. Furthermore, a cross section of sliced insulating wall was observed with an optical microscope to compare with the radial space charge distribution. The cable under investigation was electrically aged in service at 6.6 kV AC for 25 years in wet conditions. Water trees were found at the position in which space charge was observed. On the other hand, no water tree was found at the position in which the space charge was not observed. Judging from these results, the space charge measurement seems to be useful for the diagnosis of water trees in aged cables     

Life mangement of SF6 circuit breakers based on monitoring and diagnosis - [Feature Article]

In order to provide an accurate estimate of a circuit breaker's remaining lifetime, reliable measurements of ageing and failure characteristics and condition monitoring, are necessary. In order to obtain this documentation, it is necessary to monitor the performance parameters which are most sensitive to aging. Following comprehensive data acquisition and data analysis, it should be possible to develop a comprehensive electro-thermo- mechanical life model which accurately describes the aging behaviour of circuit breakers.     

Understanding treeing phenomena and space charge effect in gamma-irradiated XLPE cable insulation

In the present work, electrical trees were experimentally generated in virgin and gamma-irradiated XLPE cable insulation, under AC voltages. Fibrillar type of electrical tree structure forms from the defect site in gamma-irradiated specimens. Failure times due to electrical trees were analysed by adopting Weibull distribution studies. The results of the study indicate that gamma-irradiated specimens, where the rate of tree propagation is high, causing early failure of insulation. Pulsed electro acoustic emission (PEAE) technique was adopted to understand the space charge formation in XLPE cable insulation. The results of the study indicate that homo and hetero charges form near to the electrodes especially in gamma-irradiated XLPE cable insulation. Variations in space charge density at different voltage magnitudes with time were analysed. It is observed that the electric field in the insulating material becomes highly non-uniform on occurrence of hetero charges. It is also realized that on removal of applied voltage, the charge decay is less with gamma-irradiated specimens. Based on the present study, it could be concluded that Space charge density in insulating material and life of insulating material due to electrical tree formation shows inverse relationship.     

直流XLPE电缆绝缘中空间电荷的抑制方法综述

交联聚乙烯(XLPE)因其优异的介电、理化性能而被广泛应用于电缆绝缘领域。在电缆的服役过程中,电缆绝缘内部会积聚空间电荷,严重时可引发电场畸变,导致电缆击穿事故发生。对于直流XLPE电缆,空间电荷的积聚及影响更加不容忽视。针对直流XLPE电缆绝缘中产生的空间电荷积聚效应,目前学界主要采用共混改性、聚合物链段接枝极性基团、纳米掺杂改性及制备高纯净绝缘料等方法来进行控制,改性后的直流XLPE电缆绝缘对空间电荷产生的抑制效果均有所提升。文中首先对上述直流XLPE电缆绝缘中空间电荷的抑制方法进行综述,介绍其抑制原理以及相应的抑制效果,然后对比总结不同抑制空间电荷方法的优缺点,最后对未来直流XLPE电缆绝缘中空间电荷抑制方法的研究发展作出展望。     

Electric field and space charge of spherical electrode at highvoltage concentric with a grounded conductive sphere

The solution of the space-charge Poisson's equation is presented for a spherical electrode at high voltage concentric with a grounded conductive sphere. The obtained electric field is given in terms of the ion current emitted by the electrode and an integration constant. Both parameters are functions of boundary conditions. Using measured values of the ion current for a range of boundary conditions and applying these boundary conditions to determine the integration constant suggest a minimum value for the ion mobility of 1.9×10^-4 m² /V.s. Approximate formulas for the ion current and the electric field in terms of the independent parameters were also developed. Also, it was shown that this spherical system with its solution can be used to make accurate evaluation of the ion mobility. The obtained solutions can be applied with good approximation to many practical electrostatic systems involving point electrode at high voltage generating ions. Although the subject matter is very general and could have a fundamental research aspect, in this paper, it is meant to be applied to the area of electrostatic powder paint coating. Therefore, all the examples of boundary conditions and illustrations are typical of the electrostatic powder paint coating     

Holidays in space [space tourism]

IN FEBRUARY 1994, the American Society of Civil Engineers held a conference in Albuquerque, New Mexico, to talk about holidays in space.     

Mind the gaps - [comms white space]

The radio spectrum is a valuable resource. Even if we?re unlikely to see another government benefit to the tune of ￡22bn, as happened in the UK?s 3G auction of 2000, access to key spectrum bands around the world routinely sells for billions of pounds, dollars or euros.     

Luminescence and space charge in polymeric dielectrics - [whitehead memorial lecture (2008)]

Polymers are extensively used as insulating material in high voltage devices, such as underground power cables, power capacitors and transformers. During normal operation the polymeric insulation of a power device is not only subjected to electrical stresses, but could also be subjected to other stresses that can cause the degradation and ultimately lead to insulation failure. It has been well established that electroluminescence and charge injection, which gives rise to space charge in the polymeric insulation, occur at ac, dc and impulse field above a certain threshold value. Space charge can cause dissipative energetic processes such as photon and phonon emission, increase the local electric field and reduce the withstand voltage of the insulation. This paper describes the characteristics of electroluminescence in polymeric insulation subjected to ac voltage and shows its relevance to space charge injection in the material. It is shown that insulation subjected to high voltage could emit various types of light but electroluminescence emission which is related to space charge injection can be clearly distinguished from these other types of light emission. The electroluminescence technique is a valuable tool to evaluate the dielectric properties of novel insulating materials, such as nanodielectrics.