Electrical performance of non-ceramic insulators in artificialcontamination tests. Role of resting time

The paper presents the results of an investigation on the electrical performance of artificially contaminated non-ceramic insulators as a function of resting time. Resting time is defined as the time interval between the application of contamination and start of testing. New (unaged), full scale, 69 kV non-ceramic insulators using different types of silicone rubber and ethylene propylene diene monomer (EPDM) rubber as weathersheds were studied. Their electrical performance was assessed using the clean fog technique. The applied voltage was kept constant throughout the test, while contamination severity and resting time were varied as parameters. The transfer of hydrophobicity through the contamination layer was demonstrated by flashover tests. This was also visually depicted by analyzing samples in a scanning electron microscope. The results show that resting time exerts little influence on the electrical performance of EPDM insulators. However, for silicone rubber insulators, resting time is shown to drastically improve its electrical performance. Experimental evidence indicates that the transfer or recovery process of hydrophobicity in silicone rubber insulators occurs as a progressive superposition of silicone oil layers with time. Thus, the net effect of resting such insulators before tests can be thought of as a gradual reduction of the effective contamination layer thickness. Such a reduction alters the way in which the contaminant layer interacts with external stressing agents, which could impact the insulator's electrical performance in service. In this work, an attempt has been made to identify and quantify the electrically significant changes introduced by a reduction in the effective contamination thickness. It is in the light of this reduced effective contamination layer that we explain how a seemingly wettable silicone rubber insulator is still able to hold the applied voltage without flashover     

Mechanism of brittle fracture in nonceramic insulators

The results of an investigation concerning a mechanism of brittle fracture in glass reinforced plastic (GRP) rods used in non-ceramic insulators (NCI) are presented. Commercial grade GRP rods and GRP rods from actual insulators were exposed to ultra-pure water (UPW) and acids while being subjected to mechanical stresses. The experimental results revealed that water has the potential of inducing stress corrosion cracking on the fibers and hence brittle fracture in the rods. It is observed that the fracture proceeded faster when the rods were exposed to UPW than when exposed to acids. Furthermore, a brittle fracture in an epoxy cross-arm, which was installed in a region where the formation of acids in the atmosphere can be neglected, is analyzed. Based on these evidences, it is postulated that the failure of in-service NCI in the brittle mode can occur under the influence of water and mechanical stresses, and that the failure is more likely to happen with water than with acids.     

Diagnostics for nonceramic insulators in harsh environments

One of the main reasons for using nonceramic insulators in polluted regions is the reduction of maintenance needed to keep a line in service. Because the useful lifetime of these insulators depends largely on the pollution conditions, it is advisable to inspect the insulators on a regular cycle. The article discusses several diagnostic techniques that can be applied to detect risky conditions before failure.     

Quantification of corona discharges on nonceramic insulators

This paper attempts to establish a correlation of the visual images of corona obtained from a camera with discharge magnitude measured with conventional partial discharge equipment. A linear relationship is shown between the transformed image parameters and the discharge magnitude, thereby providing a means for quantifying corona observations made during routine maintenance inspections, of insulators from ground. Different insulator designs using silicone rubber (SIR) and ethylene propylene diene monomer (EPDM) housings were examined. The effect of fog has been examined by performing experiments inside a fog chamber. It is shown that this information can be used along with the corona degradation characteristics of housing materials to identify discharge patterns that can pose problems to the integrity of the insulator.     

Flashover of contaminated nonceramic outdoor insulators in a wetatmosphere

This paper deals with the flashover mechanism and performance of contaminated nonceramic (also known as composite and polymeric) outdoor insulators under wet conditions. Two types of wetting that occur in service, namely fog and rain, were considered. Different profiles of line insulators having weathersheds made from material families such as, high temperature vulcanizing (HTV) silicone rubber (SR) and ethylene propylene (EP) diene monomer rubber, were evaluated. The results demonstrate that the performance, and hence the relative ranking of nonceramic line insulators is influenced by the wetting mode and insulator geometry; that flashover of contaminated insulators can occur with rain at a much lower contamination severity than with fog; and that the mechanism of contamination flashover under rain is significantly different from that under fog condensation. New insulator profiles have been identified and tested to show that they can provide improved flashover performance, when compared to existing designs. This work proves that a comprehensive evaluation of outdoor insulator performance can be obtained by performing flashover tests that include more than one form of wetting, and hence illustrates the limitations of current standard contamination tests that employ only one form of insulator wetting     

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.     

Vacuum flashover on diamond-like carbon-coated insulators

Thin coatings of diamond-like carbon (DLC) were applied to cylindrical alumina and high temperature polyimide insulator samples. The coated insulators and uncoated controls were tested for resistance to surface flashover in vacuum under a dc voltage gradient. The DLC-coated insulators had significantly higher flashover potential than the controls and showed no visible tracking on their surface, even after multiple discharges. DLC coatings, applied at temperatures <350°C, show promise for improving the performance of HV vacuum feedthroughs, switches, and in other applications     

污秽绝缘子闪络特性试验研究

通过EMTP仿真分别获取0.5/5μs和2.6/50μs绝缘子串反击和绕击条件下的电压波形,并选取电阻-电容隔离回路作为工频回路与冲击电压发生器的隔离电路,同时确定污秽绝缘子制备方法和湿润方式。通过试验,得到在0.5/5μs和2.6/50μs两种冲击电压波形条件下,污秽绝缘子在工频叠加冲击时闪络电压变化的基本规律。     

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.     

Surface charging and flashover on insulators in vacuum

Investigations have been carried out on right cylindrical Teflon spacers in vacuum under dc stress to study the effect of cathode geometry on surface charging and flashover. Three cathode support arrangements were used. Type 1 involved a recess in the cathode into which the spacer was located, type 2 was a simple flat plate cathode, while type 3 had a raised insert on the cathode surface over which the spacer was located. In all cases the surface charge density and distribution is primarily dependent upon the magnitude of the electric stress. Using the type 1 arrangement inception for charge deposition was always ~10 kV even when the spacer had been subjected to repeated flashovers. At applied voltages higher than this the density of the charge, which was fairly uniformly distributed around the surface, was more or less proportional to the applied voltage. For the type 2 and type 3 arrangements, a distinct, uniformly distributed negative charge of ⩽20 μC/m² was always detected at low values of applied stress and at a fairly well-defined transition voltage this gave way to a distribution which was substantially uniform in the case of type 2 but quite filamentary in type 3, and both involved the deposition of positive charge     

The pollution flashover on high voltage insulators

The pollution flashover, observed on insulators used in high voltage transmission, is one of the most important problems for power transmission. It is a very complex problem due to several factors such as the modeling difficulties of complex shapes of the insulators, different pollution density at different regions, non-homogeneous pollution distribution on the insulator surface and unknown effect of humidity on the pollution. In the literature, some static and dynamic models were developed by making some assumptions and omissions to predict the flashover voltages of polluted insulators. In this study, historical development of insulator modeling was investigated and a dynamic arc model was proposed. For this purpose, scaled shape of a concerned insulator was firstly partitioned into triangular elements, then finite element method (FEM) was implemented and finally potential distribution on the insulator surface, variation of pollution resistance and flashover voltage were determined. The computed flashover voltage values of the selected string insulator have been compared to results from other research.     

Leakage current and flashover of field-aged polymeric insulators

In this paper results from leakage current (LC) measurements under steam fog and voltage withstand tests under rain conditions obtained on various field aged polymeric insulators are reported. Included in this work are several silicone rubber (SIR) and ethylene propylene diene monomer (EPDM) rubber insulators of ac and dc design that have been energized at suitably high ac and dc voltages under coastal conditions for a period of more than nine years. Their performances are compared to two sets of identical insulators, namely one that was outdoors exposed without being energized and one that was stored indoors. The LC of each insulator was studied three times, namely as received from the field, after it was gently washed to remove soluble pollution, and finally after a voltage withstand test. In between the measurements, the insulators were left to recover for a period of a week. The results show that the LC of the SIR insulators were generally lower than those of the EPDM. In both cases, the LC of the insulators that were stored indoors were significantly lower than those of the outdoor exposed insulators. The voltage withstand tests revealed an almost linear relation between the flashover voltage (FOV) and the arcing distance of the insulators. In general, for similar SIR and EPDM insulators the FOV of the SIR ones were higher than those of EPDM insulators     

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     

Causes and potential remedies of brittle fracture failure of composite (nonceramic) insulators

Existing brittle fracture models have been reviewed and their applicability to explain the in-service brittle fracture failure of composite (nonceramic) insulators is evaluated. It is shown that the only brittle fracture model that can explain all aspects of the brittle fracture process is a model based on the formation of nitric acid solutions in-service. The chemical cause of brittle fracture is identified in this work and recommendations are made on how to avoid brittle fracture in-service by proper selection of composite insulator rods resistant to brittle fracture. An attempt is made to clarify misconceptions that exist in the literature regarding the causes of brittle fracture and the most suitable prevention methods.     

Failure analyses of nonceramic insulators. Part 1: Brittle fracture characteristics

Nonceramic insulators, also referred to as composite, polymer or polymeric insulators, are used in overhead transmission lines with line voltages in the range of 69 to 735 kV. Despite the many benefits that nonceramic insulators offer in comparison with their porcelain counterparts, they can fail mechanically in service by rod fracture. One of the mechanical failure modes of the insulators is a failure process called brittle fracture, which is caused by the stress corrosion cracking (SCC) of the GRP rods. The process is catastrophic and unpredictable, leading to the drop of energized transmission lines. The most important characteristics of the brittle fracture process, which can occasionally affect high voltage nonceramic transmission line insulators, leading to their catastrophic in-service failures, have been presented in this article. In addition, several experimental techniques were suggested for the simulation of brittle fracture under laboratory conditions. Only the most important aspects of brittle fracture process have been discussed here.     

直流线路绝缘子冰闪特性研究

叙述了在人工气候室内对不同形式绝缘子进行的覆冰直流闪络特性研究,根据试验结果分析了不同直流绝缘子的闪络特性,并研究分析了人工污秽与自然污秽对冰闪电压的影响.结果表明,不同材质和型式的绝缘子对覆冰直流闪络电压影响不大,随着冰厚增加以及污秽增加,覆冰直流闪络电压都会下将,提出了绝缘子串直流U50%闪络电压与覆冰厚度及污秽的关系表达式,指出自然污秽下绝缘子的冰闪电压高于相同污秽状况下的人工污秽绝缘子.     

The pollution flashover of ac energized post type insulators

This paper investigates the application of a theoretical model to predict the flashover of practical post type insulators. In particular, the model is applied to two 132 kV designs with different profiles and leakage path lengths. It is found that arc shortening due to inter-shed and inter-rib flashover and arc movement results in a significant deterioration of the pollution performance of the insulator. The theoretical results are confirmed by salt fog tests and field experience     

A new flashover dynamic model of polluted HV insulators

In this work, a model based on field criterion has been developed to represent the flashover phenomenon, which occurs due to surface pollution on high voltage insulators, under ac voltage. The values of potential and electric field on an insulator surface have been determined using the finite element method (FEM). The open model of the insulator has been used for calculating the resistance in series with the arc, in addition to the values of the leakage current and the arc gradient. As a new approach, this dynamic model uses Lagrange multipliers for the solution of the pollution flashover problem. Both the impedance and the electric field criterion have been used for the propagation of arc on the surface. A computer program called NFDM (new flashover dynamic model) has been developed to achieve this. The results obtained from the program have been compared with theoretical and experimental results of other researchers.     

输电线路绝缘子冰闪防治措施的研究

结合湖南省电网的基本情况,进行覆冰绝缘子(长)串交流闪络特性的仿真试验研究,找到冰闪形成的机理和规律,根据影响冰闪的因素,探索合理的防冰闪技术措施;建立有限元仿真模型,进行受力分析,提出了防治冰闪的综合措施.