Clean Fog Research Articles

Application of Analytical Modeling of Flashover Derived for Flat Samples to Polluted 11-kV SiR Insulators

This article introduces a complementary criterion for flashover (FOV) of polluted insulators based on the concept of equivalent impedance variation with discharge length and leakage current. An analytical electrothermal model enabling the calculation of critical FOV voltage and critical current of polluted insulators is developed using this complementary criterion and the equivalent impedance criterion. It is shown that the critical parameters depend on the thermal characteristic of the discharge and the components of the equivalent electrical circuit. The comparison of the computed FOV voltage and critical current is in good accordance with the experimental ones for a laboratory insulation flat sample model with different applied voltage waveforms. The application of the developed model to actual insulators destined for 11-kV systems, by including equivalent salt density deposit (ESDD) and the form factor, shows that FOV voltage values match very well with the experimental data in clean fog test conditions.

Monitoring of Dry Bands and Discharge Activities at the Surface of Textured Insulators with AC Clean Fog Test Conditions

The aim of this study is the presentation of the results of an in-lab comparative study of electrical and thermal monitoring of artificially polluted, HTV-textured silicone rubber insulators, with different pollution levels. This work is a preliminary study of an in-situ monitoring of 400 kV SiR textured in a polluted environment. The results showed that the rms leakage current magnitude and pulses, and the average dissipated power depended on the pollution levels and the dry-bands formation. The discharge activity and their nature are governed by the pollution level and the voltage. A differentiation and a quantification between dry-band discharge onset and dry-band arc inception is highlighted.

Reliability assessment of RTV and nano-RTV-coated insulators concerning contamination severity

Currently, insulator contamination is one of the most critical causes of flashover and security problems in high voltage power systems. To overcome this problem, Room Temperature Vulcanized (RTV) silicone rubber coating and recently nano-RTV composite coating are used in the contamination area as maintenance measures for porcelain insulator in power transmission lines. Nano-RTV coating is an approach to improve the properties of porcelain insulators and reliability of the power grid. This paper proposes a new model for evaluating the relationship between contamination level and critical flashover voltage in the presence of nano-RTV coating based on experimental results. Artificial contamination test was carried out in clean fog test on coated and non-coated insulators. Besides, a comparison was made between the surface hydrophobicity of these insulators through evaluating contact angles. Finally, in order to evaluate the effect of contamination intensity through insulator electrical stability on reliability, the life time expectation model was improved and modified. This modified model was employed to evaluate reliability and performance of RTV-coated, nano-RTV-coated, and non-coated porcelain insulators. Results show that using the nano-RTV- coated insulators can improve life time and failure probability density function of insulators compared to traditional coatings and non-coated ones particularly in contaminated area.

Temporal and spatial variations of haze and fog and the characteristics of PM2.5 during heavy pollution episodes in China from 2013 to 2018

In recent years, the frequent occurrence of haze and fog events in East and Central China has caused a serious reduction in atmospheric visibility and had a huge impact on human health. Based on the relative humidity, visibility and PM2.5 concentration, the occurrence frequencies of haze as well as the spatial and temporal variations of fog, haze and mist in China from 2013 to 2018 were analyzed. The occurrence frequency of the three weather phenomena increased from 2013 until 2016, and then decreased. Regarding seasonal distribution, haze days are concentrated in winter, while mist often appears in spring and autumn. Clean fog mainly occurs in high-mountain stations and clean areas with high relative humidity, while polluted fog is more common in densely populated cities. In Beijing, the proportion of clean weather increases year by year, PM2.5 in haze weather in Beijing mainly consists of organic carbon, nitrate, sulfate and other secondary aerosols. Most fog events are accompanied by air pollution. The dominant haze is humid haze, which means that its occurrence and aggravation are usually related to aerosol hygroscopic growth.

Performance Evaluation of 230 kV Polymer Insulators in the Coastal Area of Saudi Arabia

This article investigates the technical performance of 230 kV field-aged composite insulators in the coastal region of Saudi Arabia. Two insulator samples removed from the 230 kV line after 20 years of service are assessed at KFUPM High Voltage Lab (KHVL). Different assessment techniques, such as visual inspection, contact angle, surface pollution severity, electrical withstand, and material characterization are utilized to evaluate the insulator performance. Equivalent salt deposit density (ESDD) and non-soluble deposit density (NSDD) techniques are used for the surface pollution severity measurements. Clear views of surface changes and surface hydrophobicity conditions are displayed. Scanning electron microscope (SEM), energy dispersive x-ray spectrometry (EDX), and Fourier-transform infrared spectroscopy (FTIR-IR) techniques are utilized for the material characterization. The visual inspection reveals small and big cracks in the insulator sheds. Hard breakable portion areas, many shed cuts, changed color areas, and whitish parts are visually noticed in the insulator surface. Such changes confirm the aging condition effect of the insulator units. Furthermore, the samples are subjected to laboratory clean fog tests to check the electrical performance of the samples. Informative results are given for the current condition and performance for the 20-years, field-aged insulators. Additionally, the experimental results are presented for the rest of the insulator life period. Utilization of the existing line insulators has been evaluated. Finally, discussions and recommendations for the future handling of the insulators at the 230 kV line are highlighted.

Efficient control of atmospheric sulfate production based on three formation regimes

The formation of sulfate (SO42−) in the atmosphere is linked chemically to its direct precursor, sulfur dioxide (SO2), through several key oxidation paths for which nitrogen oxides or NOx (NO and NO2) play essential roles. Here we present a coherent description of the dependence of SO42– formation on SO2 and NOx under haze-fog conditions, in which fog events are accompanied by high aerosol loadings and fog-water pH in the range of 4.7–6.9. Three SO42– formation regimes emerge as defined by the role played by NOx. In the low-NOx regime, NOx act as catalyst for HOx, which is a major oxidant for SO2, whereas in the high-NOx regime, NO2 is a sink for HOx. Moreover, at highly elevated NOx levels, a so-called NO2-oxidant regime exists in which aqueous NO2 serves as the dominant oxidant of SO2. This regime also exists under clean fog conditions but is less prominent. Sensitivity calculations using an emission-driven box model show that the reduction of SO42– is comparably sensitive to the reduction of SO2 and NOx emissions in the NO2-oxidant regime, suggesting a co-reduction strategy. Formation of SO42− is relatively insensitive to NOx reduction in the low-NOx regime, whereas reduction of NOx actually leads to increased SO42– production in the intermediate high-NOx regime. Distinct dependence of atmospheric SO42– formation on NOx levels in haze-fog events is revealed by SO42– production isopleths that are obtained through simulations of atmospheric chemistry with a box model.

Influence of the Shape of Composite Insulators Housing on their Aging Resistance in presence of High DC Voltage and Clean Fog

Influence of the Shape of Composite Insulators Housing on their Aging Resistance in presence of High DC Voltage and Clean Fog

Alternating Current Discharge Characteristics and Simulation Analysis of Rod-Plane Short Air Gaps under Salt Fog Conditions

In this paper, smog meteorological conditions in the natural environment is simulated by the salt fog method. The study of the alternating current (AC) discharge characteristics of rod-plane short air gaps in salt fog environments has important guiding significance for how to strengthen the external insulation strength of ultra-high voltage (UHV) transmission lines and electrical equipment in smog environments. The rod-plane short air gap is selected as the model to simulate the extremely uneven electric field. The AC discharge test is carried out in the salt fog environment with different conductivity, and the finite element method (FEM) is used to simulate the distribution of electric field in air gap under salt fog environment conditions. The results show that under clean fog conditions the AC discharge voltage in the air gap increased by 15.1% to 35.5% compared to that under dry conditions. With the increased conductivity of salt fog, the AC discharge voltage in air gap decreased by 4.1% to 9.2% compared to that under clean fog conditions, and the reduction is within 10%. The distortion of the electric field and the adsorption of free electrons in the gap by droplets lead to the decrease of the electric field intensity in the air gap. With the increase of the conductivity, the electric field intensity in the air gap increases slightly. Meanwhile, the influence of salt fog and its conductivity on the AC discharge voltage of rod-plane short air gap is examined, becoming saturated with the increase of the gap distance and the conductivity of salt fog.

The Development of Low-Current Surface Arcs Under Clean and Salt-Fog Conditions in Electricity Distribution Networks

Electricity distribution networks are built to deliver increasing amount of electricity from distributed energy resources (DERs) such as offshore and onshore wind and solar farms. Aging of network assets through low-current surface discharges is becoming a great concern for the security and reliability of the distributed energy supply. In particular coastal conditions can present a challenge for overhead line insulators through the environment of salt fog. Also, onshore areas may expose power lines to clean fog and agricultural or industrial pollution. In this paper, experimental studies are conducted to investigate electrical arc formation and growth on insulators, following the testing standards in accordance with IEC 61109. Dry-band arcs in both clean-fog and salt-fog environments are compared. It is identified that arcs are much faster to develop and have greater energy in salt-fog environments, due to the lower surface resistance and resulting higher leakage current. The time from a first arc to strike, to the situation where the arc is stable on the insulator surface is identified as a potentially critical parameter in controlling aging of an insulation surface. The time from initial appearance of an arc to full stability is eight times shorter in a salt fog of 16,000 $\mu \text{S}$ /cm compared to a clean fog of $600~\mu \text{S}$ /cm. This means the very existence of a stable low-current arc is much more likely in polluted conditions. Accumulated arc energy and heat dissipation have been calculated for both fog conditions. It is concluded that aging by low-current electrical arcs tends to be more severe in salt-fog environments, so that great care needs to be taken when designing overhead lines and insulators in such areas.

Leakage current analysis of polymeric insulators under uniform and non‐uniform pollution conditions

This study presents two different methods under uniform and non-uniform pollution layer in order to measure and calculate the leakage current (LC) of silicone rubber insulators. Experimental test for evaluating the LC analysis of polluted insulator have been done in a laboratory clean fog chamber. The electric field and potential distributions were obtained from finite element method software for 3D models. The mathematical background and circuit theory are described in details by a section of insulator and using the extended form factor formula. The surface conductivity used in the calculations was extracted from the measured LC after wetting rate. LC characteristics under 1 : 1, 1 : 2, 1 : 5 and 1 : 10 ratios of top to bottom surface salt deposit density on polymeric insulators are studied. To verify the proposed models of this study, the results of experimental data and two other approaches are compared with together before dry-band formation. Moreover, a dynamic LC model under uniform pollution layer has been introduced and extended in order to calculate the LC when the formation of dry-bands along the insulator surface occurs. The dynamic model is drawn from experimental data and measured surface conductivity.

Remote leakage current detector for identification of insulators discharges

Performances of high voltage insulators can be evaluated by analyses of leakage current (LC) patterns. When insulators are polluted, the development of the LCs may lead to discharges causing insulator flashovers. This paper proposes a simple method of identifying the state of discharge activities through remotely sensed LCs. A remote detector, which is capable of detecting LC by capturing resultant electromagnetic field, was first developed. Preliminary tests were conducted at the laboratory considering typical near-by interference effects on polluted insulators such as effects due to corona and line current in order to test the robustness of the proposed system under the said interference effects. The tests were extended by obtaining different LC patterns by conducting salt-fog spray tests on five different types of clean insulators (made of porcelain, glass and silicone rubber), at two salinity levels (10 g/L and 40 g/L) and six different voltage levels (50%–300% of rated voltage). LCs were also measured on five different field polluted insulators through clean fog tests. In order to verify the proposed method, LCs were measured directly for all the test cases. The directly measured LCs were classified in to three classes: sinusoidal, non-sinusoidal and discharge. The remotely sensed LCs were also classified to these three classes using a kurtosis analysis. The statistical measure of kurtosis was observed as a viable and robust parameter to distinguish these three classes based on the remote measurement. The sinusoidal class had kurtosis values less than 5; discharge group had kurtosis greater than 12; the non-sinusoidal class had kurtosis values between these two levels. Hence, this paper suggests the possibility of using kurtosis values of remotely sensed LCs to identify the state of discharge activity prior to the insulator flashover remotely while proving to be robust under typical interference effects.

Additional salt deposit density of polluted insulators in salt fog

The surface pollution layer of polluted insulator absorbs fog water and gets damp in salt fog. Meanwhile, the salts in the salt fog deposit on insulator surface, which increases the surface conductivity and degrades the flashover performance of insulators. At present, the effects of fog water conductivity (γ 20) and equivalent salt deposit density (ESDD) are analysed separately, which does not reveal the essence of the flashover in salt fog. In this study, a plenty of experiments on porcelain, glass and composite insulators are carried out in salt fog conditions, and the 50% flashover voltage stress, leakage current and variation of ESDD during the wetting process are studied. The concept of additional salt deposit density (ASDD) is proposed to quantitatively analyse the additional ESDD caused by salt fog. The test results indicate that the ac flashover voltage stress decreases with the increase of both ESDD and fog-water conductivity. Both the ESDD and leakage current become larger in salt fog compared with those in clean fog. The ASDD can be applied to analyse the combined effect of ESDD and fog-water conductivity and be expressed as ASDD = k × γ 20 × ESDD. Then the flashover voltage stress is E = A × (ESDD + ASDD)−a . The values of k for the porcelain, glass and composite insulators are 0.179, 0.191 and 0.230, respectively. The flashover in salt fog environment is a special kind of pollution flashover in essence.

Degradation of various polymeric materials in clean and salt fog conditions: measurements of AC flashover voltage and assessment of surface damages

This paper is aimed at the assessment of the resistance to tracking of insulators made of several polymers, widely used in electrical industry (medium voltage cable accessories such as terminations and insulators and anchoring clamps and mechanical screw fuses), under polluted environments. This new method performed on one insulator model is based on the electro-geometrical characteristics (AC flashover voltage versus leakage distance). The tested materials cover thermoplastics, (namely polyamide 6 (PA6/50), fireproofed polyamide 66 (PA66/50), polycarbonate (PC/40) and polyarylamide (PARA/50)) thermosetting (namely two cycloaliphatic epoxy resins (EP1 and EP2)) and elastomers (namely two EPDM materials noted EPDM and EPDM V0 (fireproofed)). Polluted environments such as clean (light) and salt (heavy) fogs have been created to assess the resistance of polymers to partial discharges (erosion and tracking). Results regarding the flashover voltage (FOV) show a significant reduction in the presence of clean fog equal to 50% of the value measured in dry conditions for all polymers. This decrease is more pronounced in salt fog (-80% reduction of FOV). Surface degradations suggest that epoxy resins and EPDM elastomers are subjected to fewer damages in salt fog when being compared to thermoplastics. It is established that epoxy resins behave better than EPDM elastomers against discharges. In addition, polycarbonate (PC/40) exhibits the worst properties because of large tracking paths that form on its surface.

배전용 피뢰기의 인공오손에 의한 누설전류의 고조파 분석

This paper reports the contamination performance of two distribution arresters (new and used ones) tested under three different contaminant conditions ? clean fog, ESDD (equivalent salt deposit density) level (A, B, C, and D), and kaolin contamination conditions, and their leakage current, total leakage current, and component of the resistive leakage current were measured in order to diagnose arrester deterioration. The 3rd harmonics was larger than 5th and 7th ones for the arrester under the clean fog, and as the ESDD contamination level was applied, 5th one became relatively larger than 3rd one. Therefore, these results indicated that the resistive leakage current could be used for the diagnosis of the arresters.

Ageing of silicone rubber insulators in coastal and inland tropical environment

This paper presents investigations on the performance of 33 kV silicone rubber insulators characterized by different creepage lengths, which aimed to find out whether the insulators could permanently work when electrically stressed beyond the recommended limits in polluted and clean tropical environments. The study was performed under natural field and laboratory conditions. The insulators tested included eight types of silicone rubber composite insulators, one type of hybrid silicone-ceramic insulator and one semi-conducting glazed porcelain insulator, while ordinary porcelain and glass insulators were used as reference. During the field investigation, two sets of the insulators were separately installed and energized in coastal and inland parts of Sri Lanka, being by that exposed to marine and clean tropical environments. Their performances were periodically evaluated by visual inspections and measurements of hydrophobicity class. After five years of field exposure, the insulator performances were evaluated in laboratory by measurements of leakage currents under clean fog conditions and of wet flashover voltage. A third set of the insulators was aged in laboratory for 1000 hours inside a salt fog chamber where the insulators were continuously energized and daily sprayed with salt solution for eight hours and left to rest for remaining 16 hours. This treatment represented conditions similar as those in the field i.e. insulators exposed to salt sprays during monsoons. The insulator performances were investigated by measurements of leakage currents and classifying their patterns into different categories, i.e. capacitive, resistive, non-linear, discharge and strong discharge types, by means of fast Fourier transform and short time Fourier transform analyses. It was found that the long-term field exposure yielded weaker insulator deterioration than the salt fog chamber ageing, which indicated for a possibility to increase the electric stress on silicone rubber insulators to levels higher than the ones used today on glass and porcelain counterparts.

Determining the probability of flashover occurrence in composite insulators by using leakage current harmonic components

Flashover of contaminated insulators in polluted areas is one of the most important factors influencing the operation of transmission and distribution lines and substations. Hence predicting an approaching flashover is of a great importance. In this paper, a new method and criteria have been proposed to predict the flashover occurrence of SIR composite insulators and its probability by using leakage current harmonic component. The tests were performed on various profiles of artificially polluted SIR insulators under clean fog condition. It is proposed that an important index is the ratio of 5th harmonic component to the 3rd for which the operational and critical limits have been determined. The probability of flashover occurrence has also been calculated by geometric distribution. Only the measurements of a minimal set of parameters are required. The presented criteria can be used to optimally schedule overhead lines maintenance teams and to develop proper Condition-Based Monitoring Systems (CBMS). Experimental results illustrate high performance of this approach.

Degradation assessment of nanostructured superhydrophobic insulating surfaces using multi-stress methods

This paper describes the electrical insulation property of modified superhydrophobic surfaces, which are prepared on epoxy based composites by using special nano-particles. After surface modification the samples exhibit an extreme unwettability with a static contact angle thetasges130deg. In order to investigate the long term stability under multiple stress, the specimens are subjected to electric fields and moisture in accelerated aging tests like modified-rotating-wheel-dipping-test (MRWDT), clean fog test and condensation test, according to the critical outdoor and indoor service conditions, respectively. It is shown that leakage current and effective power dissipation of contamination layers are significantly suppressed in the presence of superhydrophobic insulating surfaces. Thus, the pollution performance of the insulation systems can be enhanced to a great extent in clean fog test and MRWDT, whereas the insulating surfaces covered with nanoparticles seem to have no beneficial effect during the condensation experiments because of the different wetting mode. In addition, the impact of release agents and fillers on the surface superhydrophobicity is also examined in this study. Moreover, the surface analyses are employed to characterize the topographical change of the nanostructure in connection with the material degradation.

Mechanism for change in leakage current waveform on a wet silicone rubber surface-a study using a dynamic 3-D model

This paper describes the results of experiments and simulations made to examine the waveform of leakage current flowing through the surface of HTV (high temperature vulcanized) silicone rubber specimen exposed to clean fog. Water droplets were placed on the surface of specimen energized with AC voltage and investigations of the surface state were done by a high-speed video camera in a wet condition produced by sprayed clean fog. Simultaneously, according to the experimental condition, a dynamic 3-D model was built to calculate the electric field and current density distribution on the specimen surface by a finite element software. A conducting water layer is formed due to the deformation of droplets and the development of dry band arcing. This caused large distortion and nonlinearly which increased the odd harmonic components in the leakage current waveform. This is in good agreement with the simulating calculation. The information from the leakage current frequency characteristic was extracted and correlated with the insulation surface condition. It can be considered as a diagnostic index for electrical characteristics and insulation state of polymer insulators in the wet condition.

Nonstandard method for accelerated ageing tests of nonceramic insulators

A methodology for testing accelerated ageing of nonceramic insulators (NCI) is presented based mainly on polluting the insulators with several substances, such as kaolin, lime, fertilizer and natural field pollutants. The polluted insulators are evaluated inside a salt fog chamber by applying cycles of salt fog, clean fog, drying and constant voltage. During the tests, clear evidence of tracking, erosion, chalking and puncture was observed on the insulators. Flashover takes place on the early damaged insulators in a relatively short time with respect to the test time of 5000 h recommended by IEC 61109. Based on the results obtained it is necessary to modify the standard continuous 5000 h salt fog test to evaluate the long-term performance of NCI. The new test must consider low salinity, clean fog and drying periods and an initial pollution coating. Under these conditions, the leakage current magnitude is smaller and consequently much more discharge activity occurs over the polluted insulator surface leading to surface deterioration, since hydrophobicity recovery of the insulator surface is allowed during the drying period. Measurements show that the leakage current peak values were in a range from 50 to 120 mA.

Frequency characteristics of leakage current waveforms of an artificially polluted suspension insulator

In order to establish a method for monitoring contamination in insulators based on leakage current waveforms and their frequency characteristics, the leakage current waveforms and frequency characteristics of an artificially polluted 180 mm diameter cap and pin type insulator were investigated by the wet contaminant and the clean fog methods at fixed applied voltages. As a result it was found that leakage current waveforms become similar to the symmetrical wave when strong local arcs occur; hence, the intensity of the odd order of harmonic components, e.g. 50, 150, and 250 Hz, is high. Furthermore, it was clarified that the transition of the leakage current waveforms, until flashover occurs, is classified into six stages and that a threshold exists by which the occurrence of flashover can be predicted.