Sudden flashover of nonceramic insulators in artificialcontamination tests

This paper presents experimental and theoretical results to explain the phenomenon of sudden flashover on artificially contaminated nonceramic insulators. A sudden flashover in this paper is defined as a flashover occurring on a partially contaminated insulator, and which is preceded by little or no surface discharge activity. Sudden flashovers could occur in the field on those insulators where the wet surface resistance varies over a wide range along the insulator length. The mechanism of sudden flashover is significantly different from conventional contamination flashover that occurs in standard contamination tests. The insulators evaluated had weathersheds made from silicone and ethylene propylene diene monomer (EPDM) rubbers. The impact of the present study from the point of view of weathershed material, insulator design, development of a comprehensive laboratory test and field monitoring for evaluating the performance of nonceramic insulators in service are discussed     

Flashover mechanism of non-ceramic insulators

The contamination performance of non-ceramic (NC) insulators is better than porcelain insulators. The paper describes the pollution collection mechanism and concludes that silicone rubber insulators collect more pollution than porcelain insulators. Long term exposure of silicone rubber insulators produces a thin layer of pollution, which is a mixture of dust, salt and silicone oil. Fog or morning dew produces droplets on the flat surfaces and forms conductive regions. Spot discharge starts between the regions, which reduces hydrophobicity. Simultaneously, dry-band arcing starts on the shank of the insulator. The two arcs join together, which leads to flashover. The flashover voltage of polluted NC insulators is significantly higher than porcelain ones. Insulator performance is measured with laboratory tests. However, salt-fog and clean-fog tests can give different flashover values     

FTIR analysis of non-ceramic composite insulators

The chemical environment responsible for the brittle fracture failure of composite (non-ceramic) insulators is determined. Also previously reported observations by the authors are verified. Five non-ceramic composite suspension insulators and one composite guide were subjected to FTIR analysis. Out of the six field failed units, five insulators showed significant levels of nitrate on their brittle fracture surfaces with small traces of nitrate also found on the fracture surfaces of the composite guide. The results strongly indicate that the most probable cause of brittle fracture failure of composite RV insulators in-service is the formation of nitrate and thus nitric acid. This is consistent with the observations and conclusions previously reported by the authors.     

Survey on the use of non-ceramic composite insulators

This paper summarizes the result of the questionnaire on current status of non-ceramic (composite) insulators in the world conducted by the Non-ceramic (composite) Insulators Technical Committee, Japan. Current use status, reasons for use, maintenance, failure occurrence and research activity of 16 responding utilities are described. About 80% of the responding utilities answered that they would actively increase the use of non-ceramic (composite) insulators in the future. It is anticipated that the market for these insulators will keep expanding     

Prediction of leakage current of non-ceramic insulators in early aging period

The paper presents a neural network based prediction technique for the leakage current (LC) of non-ceramic insulators during salt-fog test. Nearly 50 distribution class silicone rubber (SIR) insulators with three different voltage classes have been tested in a salt-fog chamber, where the LC has been continuously recorded for at least 100 h. A boundary for early aging period is defined by the rate of change of the LC instead of a fixed threshold value. Consequently, the Gaussian radial basis network has been adopted to predict the level of LC at the early stage of aging of the SIR insulators and is compared with a classical network. The initial values of LC and its rate of change at 10 min intervals for the first 5 h are selected as the input to the network, and the final value of LC of the early aging period is considered as the output of the network. It is found that Gaussian radial basis function network with a random optimizing training method is an appropriate network to predict the LC with a 3.5–5.3% accuracy, if the training data and the testing data are selected from the same type of SIR insulators.     

Electrical performance of ice-covered insulators at high altitudes

This paper presents results on the flashover of ice-covered insulators at low air pressure. The flashover performance of two different types of insulators covered with polluted ice was experimentally determined and the effects of several determining factors, such as the air pressure, ice severity, and pollution rate, were discussed. Based on the laboratory investigations, a mathematical model for predicting the critical flashover voltage of ice-covered insulators at low air pressure was established. There is optimal agreement between the calculated and experimental results.     

Leakage current characterization for estimating the conditions of non-ceramic insulators’ surfaces

In this work both detection of the beginning of dry-band arcing and correlating the average value of leakage current (LC) with non-ceramic insulator surface damage have been investigated. Silicone rubber insulators were tested in salt-fog under different voltage and conductivity levels. The autocorrelation function (ACF) was calculated for both the fundamental and third harmonic components of LC during the early aging period (EAP). It has been observed that distinct differences exist in the behavior of both the fundamental and that of the third harmonic components of the LC during EAP. Although the fundamental component of the LC begins to grow immediately after starting the test, the third harmonic requires a much longer period of time to begin. Dry-band arcing is highly correlated with distortion in the LC and hence to its third harmonic component. But it has been observed that the level of the fundamental component of LC at which the third harmonic component started to increase is different from one case to another. As such, it is more appropriate to use the ACF of the third harmonic component of LC as an indication of dry-band arcing rather than a simple threshold value. Moreover, the average value of LC during late aging period (LAP) was correlated with the damage of non-ceramic insulators. It has been found that the average level of both the fundamental and third harmonic component of LC is well correlated with the different degrees of damage of non-ceramic insulators’ surface.     

Can water cause brittle fracture failures of composite non-ceramic insulators in the absence of electric fields?

It was postulated by J. Montesinos et al. (see ibid., vol.9, p.236-43, 2002), based on experimental evidence, that brittle fracture failures of composite (non-ceramic) HV insulators could be caused by water and mechanical stresses. It was also claimed therein that the brittle fracture process was more likely to happen with water than acids. This postulation could be of major importance as its ramifications might affect the entire composite insulator technology and, in particular, the usage of glass fiber polymer matrix composites in HV applications. Such an important statement should not be left without an independent verification. Therefore, attempts have been made in this research to initiate this process in unidirectional E-glass/modified polyester and E-glass/vinyl ester composites, used in non-ceramic insulators, by subjecting them to water under four-point bending conditions. This was done to independently verify the main conclusion of J. Montesinos et al. that water may be more damaging to unidirectional E-glass/polymer composites than acids. It has been clearly shown in this work that water, in the absence of electrical field, cannot cause stress corrosion cracking of unidirectional E-glass/polymer composites and thus brittle fracture of composite non-ceramic insulators. Thus the main results of J. Montesinos et al. could not be independently reproduced.     

Electrical tree tests. Probabilistic inference and insulatingmaterial evaluation

In this paper the application of neural network (NN) to the probabilistic inference of partial discharge (PD) phenomena generated from electrical tree growth is presented. On the basis of experimental results of measurements of trees occurring in a needle-plane arrangement, stochastic quantities are derived, which are relevant to PD pulse amplitude and phase. The NN trained by these quantities shows the feasibility of evaluations that connect tree-growth stage, i.e. the amount of damage produced by the tree, with a reduced set of these quantities. This set is, in turn, obtained applying a NN operating for data compression. In this framework, the NN can also recognize a material, among those used for training, associating to it the specific tree-growth feature     

Optimizing the performance of flat-surface, high-gradient vacuum insulators

High-gradient insulators (HGI) are periodic assemblies of conducting and insulating layers that have been shown to withstand higher pulsed voltages in vacuum than homogeneous insulators of the same length. We carried out calculations and experimental studies to understand the effect of geometry on the performance of well-conditioned, flat-surface HGI assemblies. We tested stacks with several different values of I/M (where I is the axial length of an insulating layer and M is the length of a metal layer). The experiments showed that HGI performance was substantially better than conventional insulators for I/M<3 and somewhat worse for I/M>3. Numerical calculations of electron orbits showed: 1) that the electric fields in HGI assemblies may have the favorable property of sweeping charged particles away from the surface and 2) that electron multiplication on the surface is suppressed when I/M<3.     

AC flashover performance of insulators covered with artificial ice

A field observation of ice accretion on Hydro-Quebec HV insulators was carried out, as well as a laboratory investigation of the AC flashover performance of various types of insulators covered with artificial ice. The field observations made it possible to identify the type and physical aspect of naturally occurring ice accretions produced during freezing rain precipitation. The laboratory investigation was conducted in a 4.8×2.8 m×3.8 m climate room using a HV transformer of 120 kV, 240 kVA with a short-circuit impedance of 5%. A method based on the standard IEC 507 method was developed for measuring the maximum withstand voltage (V_WS) of ice-covered insulators. Various factors were investigated, including the effects on the insulator V_WS of such particulars as type, thickness, and uniformity of the ice, as well as the arcing distance of the insulators and the conductivity of freezing water. The effects of uniform ice, 2 cm thick, on 5 IEEE insulator units was thus considered to be equivalent to the effect of an ESDD of about 0.13 mg/cm² on the same insulators     

Role of non-soluble contaminants on the flashover voltage ofporcelain insulators

The influence of non-soluble or inert contaminants on the flashover voltage of HV outdoor porcelain insulators is examined. A dynamic arc model, presented earlier for predicting the flashover voltage of porcelain insulators covered with water soluble contaminants, has been modified suitably. The extended model has been used to predict the flashover voltage of suspension and station post insulators of various shapes for both ac and dc voltages. Good correlation with experimental data has been shown. The model can thus be used as a computer aided tool for design and maintenance of HV porcelain insulators     

Impact of corona on the long-term performance of nonceramic insulators

This work discusses with the impact of corona on the long-term performance of nonceramic insulators. The aging mechanism of insulating materials and the partial discharges measurement (PD) on insulating materials is also discussed. In this measurement the rise of surface temperature of some solid insulating materials during activity of corona in point-dielectric-plain electrode arrangement by means of noncontact infrared (IR) measuring system. As the method measures the mean temperature of the sample surface, it is important to keep the diameter of the measuring target, in a given electrode arrangement, as small as possible. The smallest spot diameter of the sensor was 2.5 mm. The measurements made in the same electrode arrangements but with another IR camera proved that the spot diameter is even smaller than 2.5 mm. It means that the actual surface temperature is higher than indicated one.     

Impact of corona on the long-term performance of nonceramic insulators

A majority of nonceramic insulator (NCI) application is under relatively clean conditions where leakage current may be nonexistent. Occurrence of corona for long periods of time on NCI operating under relatively clean conditions is possible due to localized high electric field (E-field) brought about by design and manufacturing deficiencies. Therefore, resistance to corona-induced degradation of NCI housing materials is an important factor that impacts NCI long-term reliability, and is the focal point of this paper. Two sources of corona have been investigated: corona from a metal electrode, and corona from water droplets. Field inspections were performed on 230 and 500 kV insulators to demonstrate the existence of corona even under relatively clean and dry conditions. Three commonly used NCI housing materials, namely high temperature vulcanized (HTV) silicone rubber (SIR), ethylene propylene diene monomer (EPDM) and a blend of EPDM and silicone polymer were evaluated in the laboratory. In addition, housing materials from several insulators removed from the field were analyzed. Data gathered from electrical and physicochemical analyses show that corona induced degradation is dominantly due to a combination of electro-chemical reactions rather than thermal effects.     

复合绝缘子表面憎水性和污秽对其湿闪电压的影响

提出一种染污复合绝缘子的湿闪络试验方法，给出不同憎水性和污秽条件下复合绝缘子的干、湿闪电压，并根据试验结果对其干、湿闪机理进行了分析。试验结果表明：复合绝缘子的湿闪电压随表面憎水性的减弱而降低，干闪电压与表面憎水性无关；表面存在污秽时，绝缘子的湿闪电压较清洁表面有明显下降；其表面憎水性的不同仍然影响着复合绝缘子的湿闪电压；在不同盐密条件下，涂污的复合绝缘子湿闪电压也随表面憎水性的减弱而降低；重污秽条件下其表面憎水性较差时的湿闪电压要比憎水性良好时下降11％。     

Influence of sintering temperature on the flashover performance of alumina insulators in vacuum

This paper reports on the influence of sintering conditions on the flashover/pre-flashover characteristics of alumina insulators in vacuum under an impulse voltage. Four kinds of alumina insulator were studied, which were prepared using the same formula with different sintering conditions. The surface pre-flashover and flashover phenomena of different samples in vacuum have been observed under impulse voltage. Based on the experimental results, the characteristics of pre-flashover and flashover vary with different kinds of test samples. It was found that the insulators with a higher sintering temperature had a lower flashover voltage, and more active preflashover. From a scanning electron microscope (SEM) analysis of alumina insulators, we conclude that the methods of preparing alumina samples can affect their microstructure resulting in a change in the pre-flashover and flashover performance as alumina insulators.     

Effect of insulator profile on aging performance of silicone rubber insulators in salt-fog

The paper presents the results of the study on the influence of insulator profile on the aging performance of silicone rubber (SIR) insulators in salt-fog. Experiments have been conducted on various two-shed arrangements at 35 V/mm average stress and 0.25 S/m salt-fog conductivity level. The work is also extended to include commercial 15 kV class insulators with different profiles. Shed spacing, shed diameter, alternate shed design and shed shape are the parameters investigated in this study. The low frequency harmonics of the leakage current, early aging period (EAP), and equivalent salt deposit density (ESDD), are used to evaluate the aging performance of different designs. Insulator profile is shown to greatly influence the aging performance of SIR insulators. Shed shape proves to be the most important parameter to be considered in designing non-ceramic insulators profiles. Also, as the shed spacing decreases, the performance of SIR insulators improves. Simulation results using FEMLAB show that the electric field on insulators is below the corona onset at both dry and wet conditions. Dry band arcing is therefore the main electrical cause for aging in distribution class insulators and it is possible to improve the pollution performance of SIR insulators using appropriate profiles as suggested in this work     

酸雾对污秽绝缘子的交流电气特性的影响

根据酸性湿沉降水pH值与电导率之间关系的试验结果。得出结论：绝缘子交流闪络电压随酸雾pH值的减小而降低。为此，分析了污秽绝缘子在酸雾中发生污闪的原因，以及试验溶液pH值对合成绝缘子憎水性的影响，可供有关工程人员参考和借鉴。     

Role of Q-factor and of time jitter in the performance evaluationof optically amplified transmission systems

The role of Q-factor and of time jitter in the evaluation of the system performance has been investigated for return-to-zero (RZ) optically amplified links. We show that, also if the decision variable is not Gaussian, it is possible to predict, by means of a limited number of simulations, the conditions in which the bit-error rate (BER) is lower than a fixed value (10^-9), if both the Q-factor and the time jitter are evaluated including patterning effects. This analysis has been confirmed by the comparison among simulations and some experimental results, obtained in the framework of ESTHER and UPGRADE ACTS European projects, in very different propagation conditions