Flashover Performance Research Articles

Prediction of Critical Flashover Voltage of High Voltage Insulators Leveraging Bootstrap Neural Network

Understanding the flashover performance of the outdoor high voltage insulator has been in the interest of many researchers recently. Various studies have been performed to investigate the critical flashover voltage of outdoor high voltage insulators analytically and in the laboratory. However, laboratory experiments are expensive and time-consuming. On the other hand, mathematical models are based on certain assumptions which compromise on the accuracy of results. This paper presents an intelligent system based on Artificial Neural Networks (ANN) to predict the critical flashover voltage of High-Temperature Vulcanized (HTV) silicone rubber in polluted and humid conditions. Various types of learning algorithms are used, such as Gradient Descent (GD), Levenberg-Marquardt (LM), Conjugate Gradient (CG), Quasi-Newton (QN), Resilient Backpropagation (RBP), and Bayesian Regularization Backpropagation (BRBP) to train the ANN. The number of neurons in the hidden layers along with the learning rate was varied to understand the effect of these parameters on the performance of ANN. The proposed ANN was trained using experimental data obtained from extensive experimentation in the laboratory under controlled environmental conditions. The proposed model demonstrates promising results and can be used to monitor outdoor high voltage insulators. It was observed from obtained results that changing of the number of neurons, learning rates, and learning algorithms of ANN significantly change the performance of the proposed algorithm.

Feasibility of C 3 F 7 CN/CO 2 gas mixtures in high‐voltage DC GIL: a review on recent advances

With the rise of renewable energy resources, especially offshore wind power plant, the DC transmission concept has attracted more and more attention. The selection of eco-friendly insulating gas, as most critical insulating part of gas-insulated equipment is top priority for developing highly reliable system. Moreover, it is acknowledged that eco-friendly C3F7CN/CO2 gas mixtures have become a potential alternative to SF6 gas due to its excellent performance. This study reviews the basic physical properties of C3F7CN gas/gas mixtures and insulation properties including the gas gap breakdown and surface flashover performance at DC, as well as lightning impulse voltage. Investigation about the gas stability is presented, including the compatibility of gas with solid materials in gas-insulated transmission line (GIL), and the decomposition characteristics under long-term electrical and thermal stresses. The important progress on the charge accumulation characteristics of gas–solid interface in the novel gas environment under DC voltage stress is analysed. Finally, the key issues that need to be paid attention to and further followed through regarding the application of this novel gas in GIL are summarised and put forward. This study hopefully can provide a complete reference for the development of eco-friendly DC gas insulation equipment with C3F7CN/CO2 gas mixtures.

Flashover Performance of Electrodes With Different Roughness Coated by Dielectric Film

The design and operation of many pulsed-power devices are affected by surface flashover of insulator, resulting in reduction of voltage and current output; therefore, the physical research under the corresponding parameter is limited. Since the primary electron emission plays a critical role in the flashover process, methods that can prohibit electron emission from the cathode are expected to improve flashover performance effectively. In this article, stainless steel electrodes of different roughness are coated with alumina films by means of thermal vacuum evaporation, and the surface flashover performance is studied and compared with untreated electrodes. The results show that the first breakdown voltage, condition voltage, and hold-on voltage of coated electrodes increased by 39%, 26%, and 39% for Ra 0.8, and the increase of Ra 3.2 was quite limited. To substantially improve the flashover performance, it still needs to figure out other factors which will affect the electron emission threshold.

Performance of Composite Outdoor Insulator Under Superimposed Direct and Switching Impulse Voltages

High Voltage Direct Current (HVDC) systems are increasingly being adopted around the world to integrate renewable resources, increase power transfer, and allow flexible grid operation. HVDC overhead lines play a critical role in this, and focus is needed on the research of composite outdoor insulators failure. In the case of a pole to ground fault of a bipolar scheme, the healthy pole experiences a slow transient overvoltage similar to a Switching Impulse (SI) superimposed on the operational DC voltage. However, there is no standard test for such voltage superimposition. This paper investigates the performance of a composite insulator stressed with such superimposition under dry and rain conditions. To determine the direct voltage influence on the flashover voltage, the results have been compared with the SI only case. Four different rain conductivities have been used in the test program because these insulators can be exposed to different acid conductive rain types, typical of industrialized environments. Both vertical and horizontal orientations have been studied. It was possible to conclude that, in most cases, the direct voltage pre-energization has a negative impact on the flashover performance. The increase in rain conductivity leads to lower flashover values, and the horizontal orientation outperforms the vertical configuration. Under these test conditions, the flashover voltages were all larger than a typical DC operational switching surge. Thus, in the case of pole to ground fault, the healthy pole failure risk appears to be low. However, further research is needed on higher voltages and polluted conditions.

Enhancing flashover performance of alumina/epoxy spacers by adaptive surface charge regulation using graded conductivity coating

Surface flashover properties of alumina/epoxy spacers, involving a surface charge accumulation process, are critical for the safe and reliable operation of a high-voltage direct-current (HVDC) gas-insulated transmission line (GIL). This study reports surface charging behavior and flashover performance of alumina/epoxy spacers with different surface conductivity graded coating (SCGC) schemes in SF6/N2 mixtures under DC stress. Four kinds of SCGC schemes, i.e. localized coating near high voltage (HV-coating), near grounded electrode (GND-coating), at the middle of spacer surface (SPM-coating) and near both high voltage and grounded electrode (HV-GND-coating), are designed by partially spraying SiC/epoxy composites on the spacer surface. Surface charge distribution patterns exhibit varied features with different SCGC schemes. The HV-coating and GND-coating schemes lead to aggravated homo-charge and hetero-charge accumulation respectively, whereas in the SPM-coating scheme surface charge shows a multi-tier distribution pattern with alternating polarity. A transition of the dominant surface charge mechanism from bulk conductivity to surface conductivity with increasing conductivity on the coated area is found. Flashover performance differs a lot with different SCGC schemes: the HV-coating and HV-GND-coating schemes increase the flashover voltage while the SPM-coating and GND-coating schemes degrade it. The optimal surface insulation strength is achieved in the HV-coating scheme with a coating width of about 10 mm. The impact of different SCGC schemes on flashover performance is revealed based on the electric field analysis by considering the effect of surface charges.

Effect of accumulated surface charges on DC flashover of SiR insulators under pollution and aging conditions

In this paper, the measurements and modeling of the DC flashover behaviors of silicone rubber (SiR) insulators under pollution and aging conditions in the presence of surface charges have been presented. For this purpose, four different types of standard HTV-SiR insulators were selected, and then, DC flashover performance was carried out on the studied insulators in the three states including non-aged, aged, and contaminated conditions in the presence surface charges. The surfaces of the insulators were charged by an external corona source in the experimental tests, and then, the surface electric charges were measured. The experimental results of DC flashover voltage showed that the positive and negative surface charges affected flashover performance in all the states. Based on obtained empirical models of the DC flashover performance, it can be concluded that the increased aging time, positive electric charges, pollution, and average diameter reduced DC flashover performance, while the ratio of shed spacing to shed depth and negative charges enhanced the flashover voltage level.

Experimental investigation on composite insulators AC flashover performance with fan-shaped non-uniform pollution under electro-thermal stress

Pollution deposition along with ultraviolet (UV) radiation and electro-thermal stress cause the composite insulators to face with critical conditions. The type of pollution which settles on insulators is not only uniform but also it can be non-uniform and uneven. In this study, the results of the uniform and fan-shaped non-uniform (FSNU) pollution tests on four various types of virgin and aged polymeric insulators under AC voltage are presented. To find the effects of accelerated aging on polymer chains of composite insulators, scanning electron microscopy (SEM) is carried out. Based on IEC 60507, the solid layer method is applied to settle pollution on the insulator surface. The results demonstrate that the flashover voltage of virgin and also aged samples decreased when pollution non-uniformity degree (J) increases from 1 to 13. The flashover voltage of studied insulators is reduced by increasing the occupation ratio (K). However, the impact of K was lesser than J on flashover voltage of the polymeric insulators. Investigated polymeric insulators have been aged up to six weeks (1000 h). Although the influence of two weeks (333 h) accelerated aging on flashover voltage and morphology of the composite insulators was not tangible, therefore flashover strength of the samples is decreased while the aging period time increases. Statistical analysis has been done to estimate the flashover voltage of accelerated aged composite insulators under uniform and FSNU pollution based on virgin insulators. The research aimed to select insulators in windy and sunny areas.

DC pollution flashover performance of HVDC composite insulator under different non-uniform pollution conditions

The non-uniform air flow field and electrical field distributing around insulators cause uneven deposition of pollution particles on different positions and parts of the insulator surfaces. Pollution-induced flashover is a serious threat to the safe operation of the power system, and non-uniform pollution distribution has great influence on flashover voltage of insulators. Towards this, in this study, the solid layer method was used to simulate different non-uniform pollution conditions. Then, DC pollution flashover tests of composite insulators were carried out and the flashover performances were analyzed. The results indicated that the increase of KL/W (from 1 to 15) decreased U50 by 27–32%. While the increase of KT/B (from 1 to 10) increased U50 by 35%, and the increase of KH/M (from 1 to 4) increased U50 by 25%. Where KL/W, KT/B and KH/M are the SDD (salt deposit density) ratio of windward to leeward side, top to bottom surface and high voltage to middle group respectively. Besides, higher proportion of leeward side area will reduce U50. U50 and SDD are still in agreement with the negative power function relationship, non-uniform pollution conditions have great effect on a (constant of power function), but less impact on n (characteristic index of power function). Finally, equations for calculating different non-uniform pollution flashover voltages were established and validated, also the relative errors are acceptable. The results are very important for guiding insulation design and anti-pollution works of operating transmission lines.

Pollution flashover performance of RTV coatings with partial damage

Under conditions of high temperature, high humidity, and strong ultraviolet radiation, RTV coating on the insulators may get aged, powdered and even peeled off. The physical damage of the RTV coating on the insulator surface can result in a reduction in the pollution flashover performance. Therefore, this study investigated the influence of location, lateral length, and longitudinal length of the damage on the pollution flashover voltage of RTV-coated insulators. From the experimental results, it was found that the longitudinal length of the damage along the creepage distance direction was the most influential factor on the pollution flashover voltage. According to the flashover voltage function obtained by fitting, the concept of “effective path” distance was proposed. This concept can be used to analyse the influence of multiple damages on the pollution flashover voltage. To validate the analysis, verification experiments were performed using typical industrial insulator. The results showed that the pollution flashover voltage of the insulator was also in a linear relationship with the proposed “effective path” distance, which confirmed the mechanism analysis.

Improvement of surface flashover in vacuum

Surface flashover of insulation systems is a basic issue in the field of high voltage and electrical insulation. To improve the surface flashover performance is of great significance for the development of advanced power transmission equipment and insulation materials. This study reviews the research progress of surface flashover in vacuum regarding to effective methods to improve the surface flashover performance in vacuum, including insulation system optimisation and material modification. The former one is beneficial to reduce the electric field distortion, and the later one is able to adjust the surface trap parameters of the material through physical and chemical methods. In addition, the ‘U-shaped’ curve is proposed to reveal the relationship between surface flashover voltage and surface trap level, discovering the synthetic effects of surface traps on surface flashover. It is expected that the ‘U-shaped’ curve will become a guidance to improve surface flashover performance through adjusting trap parameters. Moreover, several suggestions are made to build unified surface flashover model which is suitable to the range from high vacuum to high pressure.

Atmospheric-pressure plasma jet deposition of bumpy coating improves polypropylene surface flashover performance in vacuum

Polypropylene (PP) with low leakage current and high mechanical strength is a promising candidate for recyclable vacuum insulation applications, but its surface flashover voltage is low. Advantageously to widely used technologies such as fluorination and sanding, here it is shown that plasma polymerization can provide a new way to achieve higher flashover strength by depositing a bumpy coating in a two-step process using atmospheric-pressure plasma jet. The experimental results show that the suitable volume content of methyl methacrylate monomer in argon to obtain plasma polymerized coating is about 5%, and the corresponding vacuum flashover voltage is improved by 35.5%. Moreover, the morphology analysis of surface appearance shows that the PP has a bumpy surface after plasma jet deposition. The surface charge dissipation of the PP treated for 5 min is about 26% higher than that of the untreated sample, and the secondary electron emission yield decreases from 2.35 to 1.64, which can result in the surface flashover performance enhancement. This plasma-based method for depositing bumpy micropatterns is simple and may be applicable in other areas.

Surface charge transport behavior and flashover mechanism on alumina/epoxy spacers coated by SiC/epoxy composites with varied SiC particle size

Charge accumulation phenomenon on gas–solid interface greatly restricts the development of HVDC energy transmission system. In this study, the surface charge transport behavior and flashover performance on alumina/epoxy spacer coated by SiC/epoxy composites are experimentally investigated under DC stress. SiC/epoxy composites with varied SiC particle size are fabricated and deposited on spacer surface. Nearly a charge free surface is achieved especially at smaller SiC particle size, even when metallic wires are adhered on spacer surface and connected to high voltage electrode. The DC flashover voltage increases with the decrease of SiC particle size. On the one hand, smaller SiC particle size exhibits more outstanding nonlinear conductivity characteristics, contributing to accelerating charge dissipation, whereas on the other hand, it would introduce plenty of shallow traps due to the increased interfacial regions between SiC particle and epoxy matrix, resulting in aggravating charge accumulation. A theoretical model is proposed to reveal the control mechanism of SiC/epoxy coating with different SiC particle size on flashover performance. The validity of this model can be confirmed based on the surface trap distribution, carrier mobility and our previous investigations on gas–solid interface flashover development process.

The Shed Hole Influence on the Electrical Performance of Composite Insulator

In recent years, shed holes appeared on the surface of in-service composite insulators have been observed in transmission lines in the southern and middle regions of China. Their presence can shorten the insulator creepage distance and therefore may impair the composite insulator’s electrical performance. In this paper, extensive experiments were carried out to study the influence of shed hole (shed hole diameter, shed hole arrangement, shed hole percentage) on the electrical performance including pollution flashover voltage, pollution-rain flashover voltage, dry flashover voltage, wet flashover voltage, and lightning impulse voltage. The experimental results showed that the presence of shed hole had insignificant impact on the dry flashover voltage, wet flashover voltage, and lightning impulse when the shed diameter was 1.0 mm. However, the shed hole influence on the pollution flashover and pollution-rain flashover performance was appreciable. When the shed hole diameter is 1.0 mm, both the pollution flashover voltage and the pollution-rain flashover voltage are higher with alternating arrangement than that with straight-line arrangement. With the same shed hole diameter and pollution degree, the electrical performance of pollution-rain flashover is better than the pollution flashover. It is also found that the diameter of shed hole is increased by 3% to 60% after the pollution flashover test.

Influence of Sheds Damage on the AC Pollution Flashover Performance of Different Voltage Class Composite Insulators

The damage of high-voltage terminal shed of composite insulators can influence the flashover performance of the insulators. The study was carried out experimentally to investigate the flashover characteristics of composite insulators with sheds damage at different voltage levels. The results show that sheds damage would decrease pollution flashover voltage, creepage distance flashover voltage gradient E L and the dry arc distance flashover voltage gradient E H , and such decrease range was proportional to damage degree. Besides, with the increase of composite insulator's dry arc distance, effects of sheds damage on pollution flashover voltage, creepage distance flashover voltage gradient E L and the dry arc distance flashover voltage gradient E H were gradually decreasing. With the ESDD ranging from 0.1 mg/cm to 0.3 mg/cm 2 , AC pollution flashover voltage of 35 kV, 110 kV and 220 kV composite insulators decreased by up to 35.92%, 15.03%, 9.11% respectively; the creepage distance flashover voltage gradient E L decreased by up to 19.15%, 6.05%, 1.34% respectively; the dry arc distance flashover voltage gradient E H decreased by up to 35.92%, 15.03%,9.11% respectively.

Comparison of AC Flashover Performance of Snow-Accreted Insulators Under Natural and Artificial Simulation Environments

The research on snow-accreted insulators have been performed in an artificial climate chamber. However, the differences in the distribution, density and compactness of snow accretion under natural and artificial simulation environments cause differences in the flashover performance of snow-covered insulators. In the present work, a series of flashover tests were conducted on snow-coated glass and silicone rubber insulators in the climate chamber and at Xuefeng Mountain Natural Icing Test Base. The discrepancies in the electrical characteristics and flashover process of snow-covered insulators under two test environments were compared and analyzed. Results show that the arc flashover gradient of snow accreted insulator at high altitude site is higher than that in the laboratory because of the differences of the environmental parameters. Affected by the adiabatic effect and capillary action of snow layer, within the range of the study the flashover gradient of insulators covered with snow increases with the increasing in the snow thickness. The calculation of various forces applied on arcs during its propagation demonstrates that when the arc current is less than 0.2 A, the local arcs develop along the snow surface of the insulators under the action of an electrostatic force. As the current increases beyond 0.2 A, the thermal buoyancy is predominant, thereby causing arcs to levitate from the insulator surface.

Study on Surface Rainwater and Arc Characteristics of High-Voltage Bushing With Booster Sheds Under Heavy Rainfall

The flashover performance of insulators can be improved by BS (booster sheds) in the rain, which is mainly attributed to the reasons that BS break up long cascades of water and block connections of arcs. However, surface rainwater characteristics and arc characteristics of bushing have not been quantitatively studied under heavy rainfall. In this article, the artificial rain tests were conducted on a 500 kV transformer high-voltage bushing equipped with and without BS under the rainfall intensity of 10 mm/min. X (the total length of water column) and L arc (the critical length of arc) on the bushing surface were taken as the feature parameters of surface rainwater characteristics and arc characteristics, respectively. The effects of BS on E h (the rain flashover voltage gradient along the insulation height), X and L arc were investigated, respectively. Furtherly, the relationships were studied among Eh, X and L arc . Results indicate that E h has a sharp rise as the number of BS (NBS) is from one to two, however the rise of E h gradually decreases when NBS exceeds two. X decreases while L arc increases with the rise of NBS, however both the change ranges of them continually fall. Furthermore, L arc presents remarkable negative correlation to X because of the effect of the electric field. E h rises nonlinearly with the decrease of X, which is due to the change of the wetting uniformity on the bushing surface and the potential redistribution along air gaps in the presence of the local arc.

Studying AC Flashover Performance of Suspension Insulators Under Natural Cold Fog and Wet Deposition Conditions

The electrical performance of insulators under comprehensive conditions including low air pressure, pollution and icing, is an important basis for the selection of external insulation. However, the problem of cold wet deposition has not attracted extensive and considerable interest in research. Based on flashover tests of suspension insulators in field test station, the AC flashover performance of insulators was studied. The results show that the duration of existence of the arc throughout the composite insulator surface in case of condensation was significantly lower than that in case of freezing fog flashover because of a change of the sheds surface roughness, which leads the arc melt the frozen fog droplets on the sheds surface and extends its flashover time. Compared with the flashover characteristics of the hydrophilic insulators in frost fog, the composite insulators exhibited clear arc extinguishing and arc starting phenomena. Moreover, the wind velocity has affected the flashover performance of suspension insulators in cold fog. Studying flashover characteristics of suspension insulators can contribute to the design of the external insulation of transmission lines and substations under natural cold fog and wet deposition regions.

A coaxial film capacitor with a novel structure to enhance its flashover performance.

As one of the key components of an electromagnetic pulse simulator, the peaking capacitor is coaxially constructed with a laminate structure utilizing alternative thin metal rings and polymer film dielectrics, which is designed as a part of the antenna. This paper presents a coaxial film capacitor with a novel structure to improve its flashover performance. First, the relationship between the capacitance and the capacitor's structural parameters are deduced, and a theoretical basis of lengthening the polymer film dielectrics to enhance the flashover performance is discussed. Based on the theoretical analysis, two 150 pF, 3-layer coaxial film capacitors with different extended lengths for the polymer film dielectrics (10 and 60 mm) are designed and developed. For the capacitor with a 60 mm extended film dielectric, the polymethylmethacrylate rings are adhered to the thin metal rings to support the polymer film dielectrics using a soft adhesive. Electric field analyses show that a longer extended length could decrease the surface field at the edge of the polymer film dielectric by as much as 93%. Comparative insulation experiments show that the flashover voltage for the improved capacitor is 89.9% higher than the original one. The influence of the capacitor's polymer extensions (including the polymer supporting rings and the film dielectrics) on the radiating field of a cone antenna is analyzed numerically, and the results show that only slight changes are introduced into the radiating field. Finally, a designed 350 pF, 20-layer coaxial film capacitor with lengthened film dielectrics is presented.

Assessment of Overvoltage and Insulation Coordination in Mixed HVDC Transmission Lines Exposed to Lightning Strikes

Many geographical constraints and aesthetic concerns necessitate the partial use of cable sections in the High Voltage DC (HVDC) transmission line, resulting in a mixed transmission line. The overhead sections of mixed lines are exposed to lightning strikes. The lightning strikes can not only result in flashover of overhead line (OHL) insulators but can enter the cable and permanently damage its insulation if adequate insulation coordination measures are not taken. In this work, we have analyzed the factors that affect the level of overvoltage inside the cable by simulating a fast front model in PSCAD. It has been determined that surge arresters must be provided at cable terminals when the length of cable sections is less than 16 km to limit the core-ground overvoltage within the lightning impulse protective level (LIPL). The level of sheath-ground overvoltage is independent of the length of cable; however, it can be limited within LIPL by lowering the sheath grounding impedance to 1.2 Ω. Insulation coordination measures do not impact the likelihood of OHL insulators’ flashover. The flashover performance of OHL can be improved by lowering the footing impedance of the second tower closest to the cable terminals, which is otherwise most likely to flashover.

Unraveling the role of surface molecular structure on vacuum flashover for fluorinated copolymers

To investigate the influence of surface molecular structure on the electrical performance of fluorinated copolymers, ultraviolet (UV) curable resin containing different types of fluorinated monomers (ethers and alkyls) were prepared by photopolymerization. The X-ray photoelectron spectra indicated the presence of fluorinated monomers on the surface after UV radiation. Furthermore, the surface morphology measurement revealed that these monomers cause a slight change to the surface roughness. Subsequently, the electrical properties were examined through a pulsed flashover test in vacuum. The results showed an improvement in the electrical strength of the specimen doped with dodecafluoroheptyl methacrylate (alkyls). In contrast, the sample doped with perfluoropolyether chains (ethers) exhibited no alleviation in flashover performance, despite the fact that its surface F 1s/C 1s atomic ratio is 10 times higher than that of the former. A coarse-grained model based on quantum chemical calculations reveals that electron traps are introduced at the surface because of the grafted fluorinated chains. Further explanation based on the free volume theory indicates that the higher gas absorption arising from the amorphous structure during the electron-induced degassing process accounts for the deterioration of the insulation system, which was verified by surface charge behavior before and after the flashover.