Nuclear magnetic resonance imaging of electrical trees in PE

Test samples of LDPE (low-density polyethylene) were electrically aged until PD (partial discharge) and electrical treeing occurred. Investigations on the spatial distribution of microscopic domain sizes in the lamellar morphology of the samples were carried out applying a novel method of proton NMR imaging which probes magnetization transfer by spin diffusion. Spatial changes of LDPE morphology are detected due to the appearance of PD     

Understanding electrical trees in solids: from experiment to theory

A review of recent developments made in the understanding of the electrical tree mechanism is presented. The life of the tree is covered from initiation, through propagation, to long-term changes in shape. The initiation process is examined in terms of the injection of space charge and its ability to transfer energy to the polymer to create damage. Theoretical models for the processes involved are assessed in terms of the experimental data and an outline for the sequence of events in tree initiation developed. The inter-relationship between tree discharges, tree propagation, and tree shape is discussed. Theoretical models for these processes are evaluated in terms of their ability to reproduce experimental data, especially tree shapes and discharge sequences in time and space. The chaotic nature of the tree propagation mechanism is discussed both through experimental data and the results of a completely deterministic theoretical model. Some special features of electrical trees such as the existence of conducting trees, acceleration at long times and slow growth in thick insulation are briefly touched upon. Finally a summary of the state of the art is presented.     

Mechanism for impulse conversion of water trees to electrical treesin XLPE

A lightning impulse causes substantial capacitive current in a water tree channel which, as a result of its small cross section, has relatively low conductance. Transient, nonlinear finite element computations with coupled thermal and electric fields for the geometry of a 15 kV XLPE dielectric cable indicate that an 80 kV lightning impulse can cause the water within a water tree channel to boil over a range of four orders of magnitude in water conductivity, from 10^-2 S/m to 10² S/m. Boiling of the water in a water tree channel reduces substantially the yield stress of the XLPE, raises the pressure within the water tree channel, and is likely to leave a cavity which can support partial discharge resulting in electrical tree initiation     

Initiation and growth of electrical trees in LDPE generated byimpulse voltage

The initiation and growth of electrical trees generated by impulse voltages were studied in a molded low-density polyethylene (LDPE) specimen with a metal foil electrode. The Weibull plots of the number of impulse voltage shots to the tree initiation and the puncture of the specimen were analyzed. The difference between the plots of positive and negative impulses demonstrates the difference between the initiation and growth of positive and negative trees. Taking account of the estimation of the electric field strength in the vicinity of the electrode, the tree initiation process was examined. Through the examination of the Weibull plots of the number of impulse voltage shots up to the puncture of the specimen the deterioration of LDPE caused by the impulse tree was analyzed     

XLPE 电缆绝缘中的电树枝种类及其影响因素

通过大量样品实验研究,本文发现,在同等实验条件下,由于材料聚集状态的差异,在同一XLPE电缆绝缘中引发和生长的电树枝可分为枝状、丛林状、藤枝状、松枝状和混合状等五类;不同结构的电树枝对高压XLPE电缆运行可靠性的影响不同;对产生这几种电树枝结构的内在材料聚集状态特征进行了研究。     

Characteristics of PD pulses in electrical trees and interfaces inextruded cables

The characteristics of partial discharge (PD) occurring within electrical trees in extruded cable insulation, and also at model interfaces between a cable and its accessories, have been investigated. A wideband PD detection system was used that enabled the true shape of the PD pulses to be measured. The rise- and fall-times, pulse width, amplitude and frequency of the pulses were recorded as electrical trees grew through the insulation, and also as the tracking progressed along a model cable/accessory interface. The PD pulse characteristics in electrical trees varied with time of voltage application and also with the electrical stress in the insulation. Differences in the PD characteristics of electrical trees in the bulk of a polymer and at interfaces between polymers were observed     

Morphology and the growth of electrical trees in apropylene/ethylene copolymer

Growth of electrical trees under 50 Hz high electric stress has been studied in a clarified propylene/ethylene copolymer, to explore the effects of the applied field and the material microstructure. Crystallization of the copolymer at low temperatures (<128°C) produces a continuous lamellar texture and the material consequently is optically transparent. At higher crystallization temperatures (134°C), more sporadic nucleation occurs and, as a result of the larger scale structural features that develop, the material becomes optically scattering. Nevertheless, CCD images of evolving tree structures could be obtained in both systems. Electrical treeing was found to occur reproducibly, but in a markedly different manner in the two morphologically different but chemically identical materials. In the low temperature crystallized copolymer, electrically conducting tree structures were found to develop with a growth rate that increased monotonically with increasing applied voltage. Conversely non-conducting tree structures formed in the 134°C crystallized copolymer that mimic the well documented decreasing tree growth rate with increasing applied voltage behavior of both low density polyethylene and a flexible epoxy resin     

Ultrasonic measurement of three-dimensional structure of electrical trees using characteristics of tree structure

In this paper we propose a method to distinguish noise-covered ultrasonic signals by making use of the distinguishing features of signals reflected from electrical trees extending toward the direction of depth within the specimen. This method utilizes the characteristics of signals due to the spatial continuity of a tree branch and also to the change of the delay time of the signal reflected from the electrical tree when the ultrasonic transducer is moved slightly near the target point. As a result, it is shown that the three-dimensional structure of the bush-type tree in polyethylene can be visualized in a nondestructive way. The resultant shapes of the trees agreed well with those obtained by actual sectioning of the specimen using a microtome. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 119(3): 7–13, 1997     

A fuzzy logic algorithm to detect electrical trees in polymeric insulation systems

Inter-time (i.e., the time that separates a PD event from the next one) distribution analysis has been recently proposed as a tool to infer the presence of trees in solid insulation systems. In this paper, inter-time distribution analysis is improved to infer more accurately trees at an early growth stage. Measurements from both artificial specimens as well as practical objects are presented to support the effectiveness of this tool. Moreover, considerations on inter-time analysis robustness with respect to both noise interference and other partial discharge sources are reported.     

Fractal dimension in the analysis of medical images

The analysis of cardiac magnetic resonance (MR) images and X-rays of bone is considered. Each image type is approached using a different form of fractal parameterization. For the MR images, the goal of the study is segmentation, and to this end small regions of the image are assigned a local value of fractal dimension. For the bone X-rays, rather than segmentation, the large-scale structure is parameterized by its fractal dimension. In both cases, the use of fractals leads to the classification of the parameters of interest. When applied to segmentation, this analysis yields boundary discrimination unavailable through previous methods. For the X-rays, texture changes are quantified and correlated with physical changes in the subject. In both cases, the parameterizations are robust with regard to noise present in the images, as well as to variable contrast and brightness     

The effect of space charge on the initial growth of electrical trees in polyethylene

This paper describes the influence of space charge on the growth of an electrical tree which is an important factor of the dielectric breakdown in polyethylene-insulated power cables. Polyethylene and ethylene vinyl-acetate copolymer with a semi-conducting needle were used as the treeing specimens. The statistical analysis showed that the growth rate and the unbalance of positive and negative partial discharges increased as the vinyl-acetate content increased, and that the growth per cycle decreased with the increase in frequency. A model of the tree growth accompanied by the space charge behavior was proposed considering the above experimental results. The tree growth is considered to be caused by the dielectric breakdown due to the field enhancement at the tree tip during the positive half cycle of the applied voltage, the field enhancement being due to residual negative space charge injected during the previous negative half cycle. The negative space charge in polyethylene would be mainly supplied through the partial discharge inside the tree.     

Decision trees and wavelet analysis for power transformerprotection

This paper proposes decision trees (DTs) and wavelet analysis for the protection of large transformers. The DTs assimilate a large number of measured and derived quantities such as wavelet decomposition and harmonic coefficients. Wavelet decomposition is performed on 32 samples from a one-cycle window. The DT input vector contains eight wavelet coefficients from one level of resolution and other variables such as RMS differential current, RMS restraining current, percent differential current, and second and fifth harmonics. The eight detail coefficients are sorted so that the DT input vector remains relatively constant for a quasiperiodic signal as the observation window moves along the time axis. DTs are trained on data extracted from a large number of simulations. Accuracies around 95% are obtained in the presence of CT saturation when using either wavelets or harmonics in addition to the differential and restraining currents     

Reliability analysis of electrical engineering power semiconductor devices

A new approach to predicting reliability indices based on the numerical analysis of nonuniform temperature fields of power semiconductor devices (PSDs) is presented. Thermal analysis of the power diode module is carried out in a two-dimensional formulation with junction temperature T_junc = 125°C. The finite difference method is used to solve the differential equation of heat conduction. During the numerical experiments, the ambient temperature (from 25 to 45°C) and dimensional orientation of the diode module vary. It was found that the temperature difference is more than 100°C. To analyze the reliability indices of the diode module, two mathematical models, Arrhenius and multiplicative (statistical), are selected. 1t is found that raising the ambient temperature from 25 to 45°C approximately halves the reliability indices of the power diode module. The vertical orientation of the module reduces the heat transfer and causes an increase in the failure rate indices to 10% under natural convection for T_amb = 25°C. When the diode module is lowered, the reliability indices drop by 18%, all other things being equal. The largest differences in the estimates of the reliability of PSDs are observed at a lower location of the diode assembly. For example, the failure rate for the Arrhenius model was 325 times higher than that of the multiplicative model for an ambient environment of 45°C. The necessity of taking into account the real unsteady temperature fields to increase the prediction reliability resource of PSDs is shown.     

电气控制系统自动化设计分析

电气控制系统自动化设计过程中需要根据网络技术发展状况,按照电气系统设计的基本原则开展有效的工作,提高系统的综合管理水平,达到节约设计成本、减少系统维护量的根本目标。系统设计过程中需要保持简单可靠、兼容性好、可操作性强的基本原则,按照系统设计创新,提高电气系统总体设计水平,为电气系统完成相应功能奠定重要基础,能够更好地提高电气系统的自动化水平,促使电气系统功能更加完善。     

江苏省电动汽车发展可行性分析

从电动汽车产业及充电站建设的政策支持、国外经验总结、电网接入技术、电动车电池储能及管理系统技术、电机及控制技术等关键技术保障方面探讨了电动汽车产业的发展。结合江苏省社会、经济、科技及汽车产业的发展趋势,对江苏省电动汽车的发展作了初步展望。     

当前电工行业经济运行形势分析

2003年电工行业“开门红”，各项主要经济指标继续保持快速、稳定增长的势头。主要产品产量、经济效益及进出口总额增势不减，行业经济运行的轨迹在2002年第四季度基础上继续呈现良好的经济运行态势。     

低压电器机械开关电接触可靠性分析及应用

电接触可靠性是低压电器机械开关可靠性的一个重要方面,特别对小容量低压电器机械开关(如继电器、断路器),二者的关系更加密切。电触头是低压电器中的关键元件,其性能直接影响到低压电器机械开关电接触可靠性。本文介绍了低压电器产品中常用电触头的特性及其应用领域。给出了低压电器产品选用电触头的建议和应用实例。     

故障树分析法在炉膛安全监控系统可靠性分析中的应用

阐述了控制系统的可靠性分析的意义,使用故障树分析法对炉膛安全监控系统进行了可靠性分析,建立了炉膛安全监控系统各个部件之间的联系,从而用整体可靠性来评价系统。     

Fractal pattern of second-order non-linear digital filters: A new symbolic analysis

This paper studies the three-valued symbolic dynamics associated with the second-order non-linear digital filter which has been shown to be a chaotic system recently. A new necessary and sufficient condition is given for a periodic sequence to be an admissible one. the condition is further used to prove that four types of periodic sequences cannot be admissible.