Dry Band Research Articles

Modeling of Dry Band Formation and Arcing Processes on the Polluted Composite Insulator Surface

This paper modeled the dry band formation and arcing processes on the composite insulator surface to investigate the mechanism of dry band arcing and optimize the insulator geometry. The model calculates the instantaneous electric and thermal fields before and after arc initialization by a generalized finite difference time domain (GFDTD) method. This method improves the field calculation accuracy at a high precision requirement area and reduces the computational complexity at a low precision requirement area. Heat transfer on the insulator surface is evaluated by a thermal energy balance equation to simulate a dry band formation process. Flashover experiments were conducted under contaminated conditions to verify the theoretical model. Both simulation and experiments results show that dry bands were initially formed close to high voltage (HV) and ground electrodes because the electric field and leakage current density around electrode are higher when compared to other locations along the insulator creepage distance. Three geometry factors (creepage factor, shed angle, and alternative shed ratio) were optimized when the insulator creepage distances remained the same. Fifty percent flashover voltage and average duration time from dry band generation moment to flashover were calculated to evaluate the insulator performance under contaminated conditions. This model analyzes the dry band arcing process on the insulator surface and provides detailed information for engineers in composite insulator design.

Is the Dry-Band Characteristic a Function of Pollution and Insulator Design?

This paper assesses the dry-band formation and location during artificial pollution tests performed on a 4-shed 11kV insulator with conventional and textured surface designs in a clean-fog chamber and with the application of a voltage ramp-shape source. The different designs present the same overall geometrical dimensions, but the textured ones are characterized by the application of a patented insulator surface design. Three pollution levels, extremely high, high and moderate, were considered. A newly developed MATLAB procedure is able to automatically recognize the perimeter of the insulator, the trunk and shed areas on infra-red recordings. In addition, using the vertical axis identification, all trunks are subdivided into zones and into left and right areas, significantly increasing the capability of abnormalities detection. Any temperature increase within these areas enables to detect the appearance and the extension of dry bands. The results of the analysis of the statistical location and extension development over time of the dry bands during these set of comparative tests show a clear distinction between designs and pollution levels. These results may offer interesting design guidelines for dry-band control.

Effect of micro-nano additives on breakdown, surface tracking and mechanical performance of ethylene propylene diene monomer for high voltage insulation

Ethylene propylene diene monomer (EPDM) is a polymer widely used for insulation in high voltage outdoor insulators and cables. It is well accepted that appropriate addition of micron particles to form a composite can enhance its insulation performance. This work reports improvement on the dielectric breakdown strength, tracking failure time, mechanical properties and volume resistivity of EPDM composites co-filled with boron nitride (BN) micron and nano–particles. Test specimens were fabricated by melt-blending and hot press techniques. AC breakdown tests were performed as per IEC60243-1 Standard. The tracking test was performed following IEC 60587 Standard and volume resistivity measurement as per ASTM D257. Experimental results show improvement in electrical properties with increasing particle loading up to a certain dosage but enhancement in the mechanical properties is observed up to 30 wt% particles addition. The co-filled composite exhibits considerably higher dielectric breakdown strength (89.24 kV/mm) and volume resistivity (~ 5.0 × 1015 Ω cm) relative to Micro-20 wt%. The tracking failure time of the co-filled is much improved due to excellent resistance against dry band arcing and thermal accumulation in the discharge region. Moreover, co-filled composites show improvement in mechanical properties as compared to the micron–filled counterparts. The improved thermal conductivity, better thermal stability and overall higher surface area of the particles are possible factors which impart better performance to the co-filled composites.

Estimating leaf nitrogen concentration from similarities in fresh and dry leaf spectral bands using a model population analysis framework

ABSTRACT Fresh leaf spectral reflectance is primarily influenced by leaf water content and structural aspects such as the inter-cellular spaces within the spongy mesophyll, which also interfere with the estimation of the leaf nitrogen content. It is therefore essential to identify spectral bands that are least affected by the above perturbing factors for improving leaf nitrogen estimation for fresh leaves across any landscape. Wavelengths selection plays a vital role in identifying the best spectral features for assessing leaf nitrogen concentration from hyperspectral data of dry and fresh leaves. The primary objective of this study was to determine typical optimal bands for leaf nitrogen estimation from spectra (400–2500 nm) of whole fresh and dry leaves for the same specimens of Eucalyptus grandis. This was achieved via the use of competitive adaptive re-weighted sampling (CARS), and Monte Carlo cross-validation-competitive adaptive re-weighted sampling (MCCV-CARS) band selection approaches. Bands selected (931 nm, 1003 nm, 1027 nm, 1036 nm, 1177 nm, and 1180 nm) via the MCCV-CARS approach yielded the highest estimation accuracy for both fresh predicted coefficient of determination (R 2 cal) = 0.82 and predicted root mean square error (RMSEP) = 0.14) and dry leaves (R 2 P = 0.88 and RMSEP = 0.13) when compared to CARS (2044 nm, 2107 nm, and 2188 nm) only. The identified spectral features could be relevant for assessing leaf nitrogen concentration for different seasons, for example, wet to dry season.

Prediction of DC flashover voltage of cap‐and‐pin polluted insulator

The aim of this work is to estimate the flashover voltage of cap-and-pin insulator using a novel improvement of the pollution resistance evaluation. In cap-and-pin suspension insulators, local arcing starts around the pin where the dry bands first appear due to the evaporation of the conductive layer. In appropriate conditions, a unique dominant arc starting from the pin propagates toward the cap resulting in a complete flashover. Since the highest current density is concentrated around the arc root during propagation, the pollution layer residual resistance variation, with respect to the arc position, is non-linear. Thus an original three-dimensional model, which depicts a real insulator using COMSOL Multiphysics TM to evaluate the residual resistance, is developed. This model takes into consideration the actual geometry of the insulator and the current restriction at the arc root for a specific arc radius. A novel mathematical model for calculating the residual resistance of this polluted insulator was elaborated for all possible arc radii. The mathematical model and the 3D simulation results are in satisfactory agreement. To evaluate the flashover voltage, a new approach based on Saddle Point Finding Method is proposed. The new approach and model have been validated using experimental results of previous studies.

Influence of Moist Environment on Aging Performance of Energized Silicone Rubber Used for Outdoor Insulation

The influence of moist environments on the aging performance of silicone rubber (SR) used for outdoor insulation was investigated by a salt-fog test. High-temperature vulcanized (HTV) SRs with different alumina trihydrate (ATH) filler contents were adopted as the test specimens. For the test of a single water droplet, the vibration and elongation of the water droplet caused a flashover, and the flashover voltage decreased with increasing ATH content. In the first cycle of the salt-fog test, the leakage current increased with increasing ATH content. The relatively higher hydrophobicity of the specimen with a low ATH content than the specimen with a high ATH content had an inhibitory effect on the initial leakage current development. In the last cycle of the salt-fog test, the heat from dry band arc discharge became the dominant factor affecting the surface aging after the loss of hydrophobicity, and the specimen with a higher ATH content inversely had a better inhibitory effect on leakage current development. According to the X-ray photoelectron spectroscopy (XPS) analysis results, when the energized SR was exposed to a moist environment, the heat from dry band arc discharge and the hydrolysis reaction caused a change in the binding state between Si and O, which is considered the mechanism responsible for material degradation. During this process, the higher content of ATH had a remarkable effect on alleviating surface aging.

The Discharge Mechanism Leading to the Tracking Progress in Polyvinyl-Chloride-Sheathed Flat Cord

This paper suggests the mechanism of the discharge phenomenon that leads to tracking progress. For this purpose, a polyvinyl-chloride-sheathed flat cord (PVCSFC), which is widely used as a distribution cord, was employed and an artificial crack was intentionally made on the surface. Tracking was progressed using electrolyte droplets. The mechanism of the discharge phenomenon was suggested from the measurement and analysis of the current waveform. As carbonization progressed during the tracking progress, the frequency of scintillation discharge also significantly increased. The mechanism for scintillation discharge during the tracking progress was based on the change in the electric field of the dry band, initial electron, electron avalanche, photoionization, secondary electron generation (electron detachment, photoemission, and secondary electron emission avalanche), space charge due to positive ions, and formation of conductive channels. This discharge mechanism could explain the physical phenomenon and change in the current waveform that occurs during the tracking progress.

Critical current for phase shift of dry-band discharge on wet polluted insulators

This paper presents experimental results on the influence of the electrolyte on the phase shift of dry-band discharges. Two types of electrolytes (NaCl and MgCl2) were introduced to compare the case of potassium chloride (KCl), and a model electrode system including glass-filter electrodes wetted with an aqueous solution of the electrolyte was employed to simulate a dry band. In both cases, the continuous and intermittent dry-band discharges were observed. The gas temperature of the continuous and intermittent discharges was 6000–6200 K and 1700–1800 K, respectively. The gas temperature was independent of the electrolyte. The spectral emission intensity originated from nitrogen molecules (N2) was also independent of the electrolyte. On the other hand, the spectral emission intensity originated from the electrolyte depended on the electrolyte and was strongly affected by the peak value of the current during the continuous dry-band discharge. The peak value of the continuous current was independent of the electrolyte. Hence, the relation between the current peak and the spectral emission intensity originating from the electrolyte during the continuous discharge was examined. As a result, the critical current was found to be approximately 3 mA irrespective of the electrolytes examined (KCl, NaCl and MgCl2).

Effects of thermal properties on tracking and erosion resistance of micro-ATH/AlN/BN filled silicone rubber composites

Silicone rubber is a hydrophobic polymer which is widely employed for high voltage outdoor insulation. The material offers excellent electrical performance under contaminated environments. However, pristine silicone rubber has low thermal properties and this may cause tracking and erosion failure due to severe dry band arcing and ohmic heating on the insulating surface. This work investigates the effect of material thermal characteristics on the tracking and erosion resistance of silicone rubber filled with micron sized alumina tri-hydrate (ATH), aluminum nitride (AlN) and boron nitride (BN) particles. Composites with different loading were synthesized by dispersing particles in pristine room temperature vulcanizing (RTV) silicone rubber and IEC 60587 inclined plane test (IPT) was conducted to evaluate tracking and erosion resistance. Apart from physical parameters and leakage current, an infrared thermal imager was used to measure the surface temperature distribution during the course of IPT. Experimental results showed tracking and erosion resistance is significantly enhanced with addition of BN particles followed by ATH. AlN composites exhibit poor tracking and erosion resistance, similar to pristine silicone rubber. It is concluded that addition of BN-composites improves ability to impede the tracking and erosion process, the reasons being better thermal stability and enhanced thermal conduction in the discharge region. On the other hand, infrared analysis revealed thermal accumulation is remarkably higher in AlN-composites which promotes dry band arcing and results in tracking and erosion failure.

Flashover Performance and Process of Suspension Insulator Strings Artificially Covered with Snow

Snow accumulates on the surface of insulator string, causing a decrease in its electrical performance, seriously threatening the reliable operation of the power grid. Most previous studies have focused on iced insulators; however, there is a lack of research on snow-covered insulators. In this paper, to reveal the influencing mechanism that snow has on the electrical characteristics of insulator string, based on an artificial snowing test in a chamber, the effects of equivalent salt deposit density, applied voltage type, and snow thickness on the flashover performance of snow-covered insulators are analyzed, and the flashover process is investigated. The results show that the relationship between the arc flashover gradient and the equivalent salt deposit density is a power function with a negative exponent, which is similar to that of polluted and ice-covered insulator strings. For the insulator strings with the same snow accretion, the direct current (DC) arc flashover gradient is lower than the alternating current (AC) arc flashover gradient. The relationship between arc flashover gradient and snow thickness is also a power function. The formation of a dry band during the flashover of snow-covered insulator string is similar to the flashover of the polluted insulator, and the arc propagation along the surface of the snow-covered insulator is similar to the flashover of the iced insulator.

Examination of Wind Effect on Adss Cables Aging Test

DOI: 10.5152/iujeee.2018.1818 ADSS (All dielectric self supporting) cables are generally installed on high voltage transmission lines and mostly used under different environmental effects, such as temperature, wind, UV radiation, etc. In this study, wind affect on ADSS cables was investigated by using dry band arcing (IEEE 1222 Electrical surface degradation) test method. A wind source was implemented to the test setup to evaluate the performance of PE (Poly-ethylene) cable jacket on ADSS cable under different wind speeds. Test results indicate that life time of the ADSS cable decreases while the wind speed is increasing.

Thermal distribution analysis and suppression mechanism of carbonized tracking and erosion in silicone rubber/SiO2 nanocomposites

For high voltage outdoor silicone rubber insulators, carbonized tracking growth and erosion of the material are the major failure modes and it can adversely influence the reliability of power delivery networks. Thermal accumulation in the dry band arcing region is a key factor for temperature build-up and it causes thermal depolymerization and promotes tracking on the insulator surface. This work investigates the effect of nano–SiO2 doping on the thermal accumulation induced tracking growth and erosion of silicone rubber. A tracking-erosion model is presented to explain how doping directly affects such mechanisms. Experimental results show excellent enhancement in the performance of silicone rubber with higher doping contents in terms of physical tracking growth, erosion, leakage current and thermal accumulation in the arcing region. These favorable findings could be attributed to the better thermal conductivity which allows the high energy thermal flux to dissipate away more efficiently. Moreover, thermogravimetric analysis results indicate that nano doping enhances the physical and chemical cross-linking points which restrict the segmental motion at the particle matrix interface. It can promote thermal stability of the nanocomposites which is clearly witnessed in the physical results. The proposed tracking-erosion model suggests that colliding scattering of nano–SiO2 particles reduces the secondary electron collapse which impedes the release of high energy and restricts thermal degradation.

Influence of Single and Multiple Dry Bands on Critical Flashover Voltage of Silicone Rubber Outdoor Insulators: Simulation and Experimental Study

Dry band formation on the surface of outdoor insulators is one of the main reasons leading to flashover and power outages. In this paper, a dynamic arc model is proposed for single and multiple dry bands configuration to predict the critical flashover voltage for silicone rubber outdoor insulators. An arc is modelled as a time dependent impedance consisting of a Resistor Inductor Capacitor (RLC) circuit. The effect of dry band location and existence of multiple dry bands on critical flashover voltage is investigated. To validate the proposed model, experiments were conducted in a climate chamber under controlled environmental conditions on rectangular silicone rubber sheets polluted using improved solid layer method based on IEC 60,507. Tests were conducted at different dry band configurations and pollution severity levels. A good correlation was found between experimental results and simulation results. This model can provide a good foundation for the development of mathematical models for station post insulators having multiple dry and clean bands and can be used in the design and selection of outdoor insulators for polluted conditions.

Discharge behavior and dielectric performance of artificially polluted hydrophobic silicone rubber

Flashover voltage and discharges behavior were investigated with silicone rubber (SiR) polluted with fog, red clay or carbon particles. For a SiR polluted with carbon, partial discharges occur between water droplets and form interspersed dry bands. For a SiR polluted with red clay, the leakage current induces uniform dry bands and dry-band arc discharges. Because flashover occurs through the dry bands, the noticeable difference in flashover voltage between the polluting materials is not seen. The pollution deposit changes the surface state and causes partial discharges which lead large leakage current, however, the partial discharges don't significantly lower insulation performance.

Enhancement of tracking and erosion resistance of silicone rubber with platinum/amino-silane by modulation of crosslinking density

Tracking and erosion resistance of silicone rubber with N-[3-(trimethoxysilyl)propyl]aniline (NTSPA) and Karstedt catalyst (Pt) was improved significantly by increase of crosslinking density through incorporation of polymethylvinylsiloxane with high vinyl content of 2.93 mol% (HPMVS). Silicone rubber with 15 phr of HPMVS passed the incline plane test (IPT) at 4.5 kV with eroded mass of merely 0.65 wt%, whereas silicone rubber without HPMVS failed the test. TGA and TG-FTIR results indicate that increase of crosslinking density facilitated radical crosslinking reactions induced by Pt/NTSPA at elevated temperature. Residues from silicone rubbers after IPT were characterized via Raman spectroscopy, 29Si NMR and SEM. The results reveal that highly crosslinked network promoted formation of a compact silicon oxycarbide ceramic barrier to protect inner material from further degradation during dry band arcing. Moreover, conductive graphitic carbon in the residue decreased remarkably, thereby inhibiting the formation of conductive path.

Thermal behavior and resistance against electrical discharges of modified silicones for outdoor application

Silicone composite insulators are state of the art for outdoor applications. The development of these insulators has reached a high level so in a next step the weight of these insulators can be reduced. One way to do so is by adding a low-density filling material. In this paper, the thermal stability of silicones filled with hollow microspheres (HMS) with diameter in the micrometer range is examined. High thermal stability is a key property of outdoor insulation materials as it determines the resistance against arc discharges as well as erosion due to dry band discharges. Therefore, arc resistance tests, inclined plane tests and thermogravimetric analysis are used to investigate the influence of HMS on thermal stability of silicone. The influences of thermal material properties like heat conductivity, heat capacity and temperature conductivity are examined and a method to calculate heat conductivity of porous composite materials is discussed. Results show no influence of thermal material properties on thermal stability, thus observed decrease of thermal stability of silicone filled with glass HMS cannot be explained by thermal characteristics. Instead, increase of surface area of the silicone is identified as key parameter, which causes a stronger thermal degradation. In addition, application of glass HMS in liquid silicone rubber (VMQ1) hinders the curing and reduces thermal stability further. Adding of ceramic HMS and silanization of glass HMS decreases erosion and increases arc resistance. This can be explained by an increased interphase quality between the silicone and these fillers, which prevent accelerated thermal degradation due to an increased thermally active surface area of the silicone.

Evidence of organized intraseasonal convection linked to ocean dynamics in the Seychelles–Chagos thermocline ridge

The Madden–Julian oscillation (MJO) is the dominant driver of intraseasonal variability across the equatorial domain of the global ocean with alternating wet and dry bands that propagate eastward primarily between 5°N and 5°S. Past research has shown that MJOs impact the surface and subsurface variability of the Seychelles–Chagos thermocline ridge (SCTR) (55°E–65°E, 5°S–12°S) located in the southwest tropical Indian Ocean (SWTIO), but investigations of how SWTIO internal dynamics may play an important role in producing MJO events remain limited. This study uses Argo, in conjunction with several remote sensing and reanalysis products, to demonstrate that SWTIO oceanic dynamics, particularly barrier layer formation and near surface heat buildup, may be associated with MJO genesis between August and December of most years between 2005 and 2013. A total of eight SWTIO specific MJO events are observed, all occurring between August and December. Four of the eight events are correlated with positive SWTIO total heat content (THC) and barrier layer thickness (BLT) interannual anomalies. Two others formed over the SWTIO during times when only one of the variables was at or above their seasonal average, while two additional events occurred when both variables experienced negative interannual anomalies. Lacking complete 1:1 correlation between the hypothesized oceanic state and the identified SWTIO MJO events, we conclude that additional work is required to better understand when variability in key oceanic variables plays a primary role in regional MJO genesis or when other factors, such as atmospheric variability, are the dominate drivers.

Effects of RTV coating on the electrical performance of polymer insulator under lightning impulse voltage condition.

Located near the equator, Malaysia is a country with one of the highest lightning densities in the world. Lightning contributes to 70% of the power outages in Malaysia and affects power equipment, automated network systems, causes data losses and monetary losses in the nation. Therefore, consideration of insulator evaluation under lightning impulses can be crucial to evaluate and attempt to overcome this issue. This paper presents a new approach to increase the electrical performance of polymer insulators using a Room Temperature Vulcanisation (RTV) coating. The evaluation involves three different settings of polymer insulator, namely uncoated, RTV type 1, and RTV type 2 upper surface coatings. All the insulators were tested under three different conditions as dry, clean wet and salty under different impulse polarities using the even-rising test method. The voltage breakdown for each test was recorded. From the experiment, it was found that the effectiveness of the RTV coating application became apparent when tested under salty or polluted conditions. It increased the voltage withstand capabilities of the polymer insulator up to 50% from the basic uncoated insulator. Under dry and clean conditions, the RTV coating provided just a slight increase of the breakdown voltage. The increase in voltage breakdown capability decreased the probability of surface discharge and dry band arcing that could cause degradation of the polymeric material housing. The RTV type 1 coating was found to be more effective when performing under a lightning impulse. The findings might help the utility companies improve the performance of their insulators in order to increase power system reliability.

Measurement of surface resistance of silicone rubber sheets under polluted and dry band conditions

Ageing and degradation of silicone rubber insulators in contaminated environments is a major concern due to their organic nature. Understanding the flashover process of silicone rubber insulators under contamination is very important and needs standardized tests and procedures. Surface resistance measurement can be used as an indication of ageing and contamination severity of outdoor silicone rubber insulators. This paper presents experimental and mathematical results on surface resistance of high-temperature vulcanized silicone rubber sheets under various environmental and contaminated conditions. Rectangular silicone rubber sheets were polluted artificially in laboratory using a modified solid layer method based on IEC 60507 and tested in a climate chamber where fog rate, humidity and temperature can be controlled. Influence of non-soluble salt deposit density (NSDD), pollution severity, relative humidity, ambient temperature, dry band location and width, and multiple dry bands on surface resistance have been investigated. The obtained results show that surface resistance decreases with increase in NSDD, pollution severity and relative humidity. Increase in surface resistance was observed with the formation of dry bands. Although surface resistance was effected by all parameters, the most dominant parameter was noted down to be pollution severity level and dry band formation. Repeatability of results was confirmed by performing each test five times and concluded that surface resistance measurement can be standardized to evaluate ageing and pollution severity of silicone rubber insulators. These results along with the available literature on equivalent salt deposit density (ESDD) measurement could be used to formulate a relation between ESDD and surface resistance that could be useful for evaluating the performance of silicone rubber insulators in wet and contaminated environments.

Study on Acceleration Factor at 5000 h Multiple Stress Test and Aging Evaluation of EVA Insulator

SUMMARYThe authors carried out an outdoor exposure test and the 5000 h multiple stress test to ethylene vinyl acetate (EVA) polymer composite insulator used for 22 kV distribution line and an attempt was made to estimate the acceleration factor of the 5000‐h multiple stress test corresponding to the climate condition of the Kyushu area. By comparing dry band arc cumulative charge of leakage current at 5000‐h multiple stress test with that in the outdoor exposure test, the authors estimated the acceleration factor of the 5000‐h multiple stress test to be about 20 times. The acceleration factor was found to almost agree with the evaluation result of the absorption of hydroxyl group by FT‐IR analysis and the surface roughness of the insulator, respectively. Finally, for the electrical property, the expected life of EVA insulator that the authors developed was confirmed to be more than 40 years.