Search

Due to wind and uneven distribution of the field along the insulator, a kind of non‐uniform contamination is deposited on the insulator surface as fan‐shaped and longitudinal pollution. This paper investigates such non‐uniform pollution from theoretical and experimental points of view. For this aim, the residual resistance of the pollution layer is formulated based on the geometry of fan‐shaped and longitudinal non‐uniform pollution. Also, the arc current is formulated using the arc Obenaus's equivalent circuit for a constant arc voltage in terms of arc length propagation. Finally, the AC dynamic theoretical model of pollution flashover is proposed that can estimate flashover parameters for different values of the fan‐shaped density degree (J), extension degree (K) and longitudinal non‐uniform pollution density degree (Z) considering instantaneous variables in arc progression. It results that with increasing Z, the flashover voltage increases, while by increasing the J and K, the flashover voltage decreases. To validate this model, the experimental tests are performed on the sample silicone rubber (SiR) insulator that show acceptable accuracy of theoretical model. Furthermore, based on experimental results, a new correction coefficient is proposed under fan‐shaped and longitudinal pollution that varies in the range of 0.85–1.62.

It is usually seen that DC flashover voltage of the water droplet on the surface of silicone rubber (SIR) composite insulator is lower than AC flashover voltage. It is necessary to study the mechanisms and influencing factors of the surface properties. In this article, experiments are performed to study the moving processes of the water droplet and the influences of its dynamic behaviours on flashover voltage under DC/AC electric field. Besides, an electrohydrodynamic coupling method is employed to establish the simulating model. Moreover, the theoretical physical model is established to reveal the mechanisms of dynamic behaviours of a droplet. It is indicated that the simulation results are in good consistency with the experimental data. The elongation behaviours of a droplet on the surface of a composite insulator can influence the homogeneity of the electric field distribution along the sample's surface. It is shown that the different dynamic behaviours and flashover performances for a single water droplet on the SIR surface are both affected by the volume of the droplet and the type of voltage applied.

Ethylene–propylene rubber (EPR) cable is the most commonly used type in high-speed railway. Several flashover accidents take place in cable termination in an extremely cold environment. In this study, EPR cable terminations were tested in a simulated low-temperature system with varied applied voltages. The results showed that the differences in elastic modulus and free space volume between EPR insulation and stress control tube (SCT) in a frigid environment would lead to structural defects between them. Damages on the EPR insulation occurred in the form of flashover point when the ambient temperature declined to −30°C and below, with the growth rate of discharge and resulting extent of damage across the EPR/SCT interface being non-linearly related to ambient temperature. Besides, the partial discharge (PD) initial voltage and PD extinction voltage under low-temperature conditions reduced by >40% compared with those at 20°C. The flashover probability of EPR terminations increased significantly, and the shape characteristics of PD patterns have also changed obviously at low temperatures. It can be verified that under the high-voltage and low-temperature conditions, the EPR/SCT interface is prone to organisational mismatch, which will lead to the PD phenomenon and seriously affect the stable operation of EPR cable terminations.

Water droplets on the surface of insulators can distort the electric field and cause the generation of surface partial discharge or flashover. In this study, the inception and the development of surface partial discharges on a 110 kV composite insulator were evaluated by using a solar-blind ultraviolet imager. Several experiments were conducted by using water droplets differing in volume, conductivity, and position on an insulator housing. The results showed that the corona inception voltage decreases with the increasing volume of the water droplets. The relative permittivity of the water droplet decreases with the increase of the conductivity and then reduces the electric field intensity and increases the corona inception voltage. Installing the grading ring can increase the value of the corona inception voltage by 38% at most. To investigate the influence of the water droplets on the corona inception characteristics of the insulator, the sheath and shed models were proposed according to the feature of the electric field distribution. By changing the parameters of the water droplets, the corona inception voltage, discharge position, and electric field were analysed. Furthermore, the flashover voltage of the water droplets is influenced by its conductivity, while the corona inception voltage by its relative permittivity.

This study aims to predict the flashover voltage (FOV) of silicone rubber (SiR) insulators. Accordingly, it benefits from two methods, including an artificial neural network (ANN) model and a FOV gradient fitting model. The FOV and leakage current (LC) tests are carried out on one un-aged and three aged specimens under uniform and longitudinal non-uniform pollution circumstances. The proposed ANN model is designed based on equivalent salt deposit density (ESDD), pollution non-uniformity degree, aging time, LC first harmonic magnitude I 1 , and total harmonic distortion. Furthermore, a FOV gradient model is proposed based on ESDD, I 1 , and creepage distance. To validate the proposed models, the FOV and LC tests are conducted on two different types of SiR insulators. Then, the predicted FOV from the ANN model and calculated FOV from the voltage gradient model are obtained. The results indicate that the relative errors of the ANN model and FOV gradient fitting model are

The flashover voltage of high temperature vulcanised (HTV) silicone rubber will be decreased after corona ageing, and it further decreases under the effect of mineral oil. The mechanisms for the phenomena can be explained by considering the role of trap level distribution. In this study, the corona ageing of silicone rubber was conducted and the flashover voltage was tested. The isothermal surface potential decay was tested to explore the internal relationship between trap distribution and flashover voltage. Besides, the surface microtopography and chemical structure of silicone rubber were gained by scanning electron microscopy and Fourier infrared spectroscopy to explain the change of trap distribution. The test results show that the flashover voltage of HTV silicone rubber decreases with AC corona ageing time, the reduction of flashover voltage for an oil-impregnated sample is more serious. The density of shallow electron traps increases at first and then decreases, while the density of shallow hole traps increases with ageing time. The density of deep traps decreases after corona ageing. These trap level distribution characteristics are beneficial to promote the development of flashover, leading to lower flashover voltage. The effect of mineral oil can be regarded as an accelerated ageing effect.

One of the leading causes of power system outages is lightning surges. A precise calculation of lightning overvoltages across insulators has vital significance for the investigation of lightning outages. This study investigates the impact of different lightning surges for the analysis of transient frequency characteristics of line commutated converter (LCC) based ± 800 kV ultra-high-voltage direct current (UHVDC) converter station by developing a wideband (WB) model of the converter station. The influence of lightning flashover criterion (LFC) of double exponential and Heidler surges methods on insulator/gap for precise lightning withstand level is examined. An analytical comparison for LFC of insulator/gap installed at high altitude transmission lines by incorporating the IEEE volt/time curve, CIGRE leader propagation (LP) and improved LP methods is presented. The proposed WB model explores the novel attributes of high-frequency transfer characteristics and the coupling mechanism of transient frequencies passing through the converter station. Moreover, the effect of internal frequency dependant DC line faults on the WB model is examined. The analysis presented here aims to investigate issues like overvoltage, electromagnetic disturbance prediction, insulation design of the equipment and standardisation of sensitivity issues that are being faced by the present relay protection systems.

Repetitively pulsed gas volume and dielectric surface discharges have gained growing attention because of peculiar and exciting physics phenomena, high efficiency, high reactivity, and potential to obtain conventionally unachievable plasma properties. Nevertheless, incomplete understanding of fundamental mechanisms renders the repetitively pulsed discharge far less predictable and controllable, where the inherent memory effect and the discharge mode transition are universal challenges. In this topical review, the authors will explore the macroscopic characteristics of the gas gap breakdown and the surface flashover, state-of-the-art mechanisms and dominant agents of discharge memory effects, operation regimes and transitions of the discharge mode, and how waveform parameters affect the pulsed discharge properties. Challenges and potential approaches for further understanding the memory effect and the discharge mode transition in the repetitively pulsed discharge are discussed.

Based on a ±10 kV three-terminal MMC-MVDC (medium voltage DC distribution system based on modular multilevel converter) system, in this study, the authors mainly study on the influencing factors and protection schemes of the lightning intruding overvoltage (including direct lightning and back-flashover lightning). Firstly, a lightning intruding overvoltage simulation model of the MMC-MVDC distribution system using overhead lines is established based on the PSCAD/EMTDC program. The influence of steady-state operating voltage and the number of converter stations on the simulation results are simulated and analysed. Meanwhile, the proper simulation modelling method of the lightning intruding overvoltage for multi-terminal MMC-MVDC system is proposed. Secondly, the lightning intruding overvoltage is simulated when the line is struck by the lightning stroke current that occurs once in 20 years and the different lightning protection schemes are proposed. Finally, based on the lightning impulse withstand voltage, lightning trip-out rate and the cost of the protection schemes, the lightning protection schemes in different scenarios are proposed. The simulation results show that if the cost of the system is prioritised, the protection scheme of increasing the number of parallel stacks of the DL arresters can be selected, and if the power supply reliability of the system is prioritised, the protection scheme of installing the overhead ground wire as well as reducing grounding resistance can be selected.

Due to the differences in typical pollution sources, the soluble salt composition deposited on the naturally contaminated insulators varies with the region. As different soluble salts have different effects on the pollution flashover voltage of insulators, it is necessary to investigate the spatial and temporal distribution of the soluble salt composition deposited on the insulator surface to make the external insulation design more scientifically and reasonably. In this study, an artificial contamination test platform was built to collect the contamination naturally deposited on the specimens. The test results showed that the contamination composition deposited on the specimen was similar to that in the atmosphere, and the inclination angle of the specimen and the duration of pollution deposition had little influence on the contamination composition. After examining the industrial post insulators from actual substations, it was found that the contamination composition was almost the same for different insulator surface materials, insulator shed types, and voltage polarities. It was also found that the deposited contamination was mostly influenced by pollution sources. The results can provide a reference for the studies of pollution distribution of natural insulators.

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 C(3)F(7)CN/CO(2) gas mixtures have become a potential alternative to SF(6) gas due to its excellent performance. This study reviews the basic physical properties of C(3)F(7)CN 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 C(3)F(7)CN/CO(2) gas mixtures.

This study presents a probabilistic approach, based on the finite element method (FEM), to model insulator flashover and calculate the failure risk of insulators under contaminated conditions. In the proposed method, the voltage distribution along the insulator surface is estimated by FEM and the random turn wheel is utilised to describe the stochastic propagation of arc along the insulator surface as well as in the air. This basis is used to obtain the probability of insulator flashover for different contaminations, separately. Then, from the simulation results, the probability density function of flashover in terms of pollution can be determined based on the Kolmogorov–Smirnov test. The proposed model is verified by comparing the associated results with those of the literature, in which the reliability and accuracy of the presented method are approved. Finally, having the cumulative distribution function of insulation strength and probability density function of pollution stress, the failure risk is calculated, in which the results can be used to predict the annual outage rate of transmission lines.

In this study, to improve the sensitivity and accuracy of the detection of surface partial discharge (PD) in gas insulation switchgears (GISs), a combination of ultra-high frequency (UHF) and optical methods is proposed based on an integrated sensor. The PD triggered by a fixed metal needle defect attached to the spacer is detected from PD inception to flashover in a real GIS. The characteristics of UHF and optical signals under different applied voltages are compared and analysed in terms of PD waveform, Phase Resolved Partial Discharge (PRPD) pattern, PD count and amplitude. The results show that the surface discharge process can be precisely divided into three stages according to the combined characteristics of the UHF and optical signals. The comparison of the two detection methods shows that the optical method can detect the weak avalanche PD under low voltage while UHF cannot, so it can detect more PDs than UHF method in an AC cycle. In addition, with the deterioration of solid insulation material, the amplitude of the optical signal increases significantly while the amplitude of the UHF signal only increases slightly.

Multiple-level and up-and-down test methods are commonly employed in laboratories to obtain the breakdown voltage distribution of self-restoring insulation, which is applied for equipment evaluation and for risk calculation in insulation coordination studies. This study investigates the reliability of the methods and proposes a methodology for evaluating their performance. The accuracy and precision of the estimated distribution parameters were analysed considering the variation of the test parameters. Algorithms that simulate the random nature of the tests were developed and applied in simulations. Then, a case study of risk calculation was carried out. The results indicated that both methods estimate the critical flashover voltage with acceptable accuracy and precision, even for the minimum parameters defined by standards. However, estimating the standard deviation is the critical step of the tests. The proposed methodology allows parameter definition for a given reliability requirement in the case of the constant voltage method. In the case of the up-and-down method, however, high dispersion was observed for all analysed cases, which indicates that this method presents low reliability for calculating flashover risk and, therefore, its use for insulation coordination studies is not recommended.

Atmospheric snow accretion along the insulators can eventually bridge adjacent shed spacing, which pose severe risks to the security of power networks. Aiming to investigate the influence of snow accumulation on the electrical performance of various types of insulators, the flashover tests of snow-accreted insulators were conducted. The effects of various factors on the electrical characteristics of insulators were analysed based on the experimental data. Results show that AC arc flashover gradient of the insulator covered with nonuniform snow accumulation is lower than that of the uniform snow-covered insulator. The former is as low as 22.5% lower than the latter. Moreover, the AC arc flashover gradient of the porcelain and glass insulators is 0.27–6.27% higher than that of the DC. For a silicone rubber insulator, this gap is greater than 20%. The flashover gradient of the glass insulator is the minimum at the snow thickness of 10 mm. The flashover process of snow-covered insulator is neither similar to that of the contaminated insulator nor different from that of the iced insulator but a combination of the two. The presented work helps to better understand the flashover mechanism of snow-covered insulators, and take efforts to mitigate its effects.

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.

The vacuum discharge along the dielectric surface, also called surface flashover, brings significant damages to the vacuum-solid insulation system. Here the authors implement shape-flexible, complex surface groove microstructures on the dielectric to mitigate the initiation of vacuum flashover. A particle-in-cell simulation is employed to reveal the real-time discharge development considering the blockage of the multipactor propagation as well as the space field distortion in the presence of specific surface microstructures. Electron trajectories within barriers are theoretically analysed to present how the barrier shape affects the discharge process, showing consistent results with the simulation. By analysing three parameters, namely the anode current, the surface average charge density and the flashover threshold, it is found that the effect of suppressing surface flashover remarkably augments when the groove number and depth rise up, while such effect gradually saturates when the groove depth reaches a critical value. Theoretical analyses are also provided for the decay factor as well as the optimal shape and trapezoid grooves are proved as the optimal shape distribution of microstructures. Furthermore, different secondary emission yield (SEY) distributions are considered with two kinds of structures, including the material with low-SEY inside and on the top surface of grooves.

This study presents a dynamic arc model of a polluted insulator based on the formulation of the Obenaus model and the Hampton criterion for discharge propagation. An experiment was conducted on a practical glass insulator installed in the Algerian network for the purpose of measuring the flashover voltage and estimating the arc parameters (A and N) using a genetic optimisation algorithm. The parameters used in the dynamic model were the arc current, arc length, and arc resistance calculated using MATLAB. Moreover, an attempt was made with finite element software to calculate the voltage and electric-field throughout the insulator surface with and without the presence of contaminants. Finally, the obtained simulation and experimental results showed high performances under operational conditions and respect IEC 60060-1 standard recommendations.

The composition of metallurgical contamination is rather special, which has a great influence on the contamination accumulation and flashover characteristics of the insulators. In this study, the 500 kV AC line insulators in metallurgical industry area of central China were sampled. The ion composition, element composition, micromorphology and particle size distribution of contamination at high and low end of insulator string were tested, respectively. The equivalent salt deposit density (ESDD) and non-soluble deposit density (NSDD) of high-voltage end, low-voltage end and middle segment of three-phase insulator strings were tested. The results indicate that the contamination particles are irregular in shape and dispersed with each other. The contamination particle size of high-voltage end is larger than that of low-voltage end. The main components of the contamination are Fe2O3 and CaSO4. The ESDD and NSDD of upper surface of upper phase is lower than that of middle phase and lower phase with the cleaning of rainwater. Due to the subsidence of metallurgical contamination from high altitude, the ESDD and NSDD of lower surface of the upper phase is higher than that of middle phase and lower phase. The ESDD and NSDD of lower surface of three phases reduce with the decrease of height.

The porcelain ceramic insulators have been used for outdoor insulation applications for a long time due to their impressive electrical and mechanical properties. The lightning impulse withstand voltage and flashover voltage characteristics of the insulators are important for insulation coordination of transmission lines and substations. This study presents a computer-based simulation methodology to determine the positive polarity lighting impulse flashover voltage of porcelain suspension and station post-insulators. The flashover voltage is determined by evaluating the streamer inception, propagation and sustenance criteria along the suitable paths based on finite-element-method-based electric field computation. The streamer propagation path is found through air along the dry arcing distance rather than along the creepage path of the insulator. The lightning impulse flashover voltage of suspension and post-insulators are computed based on simulation. The comparison of computed results with the experimental results gives a good agreement between them.

The arrangement of insulator strings has an effect on its pollution flashover characteristics. A new arrangement of insulator strings, inverted T-type arrangement, is proposed in this study. The electric field distribution is simulated and the pollution flashover characteristics are tested in an artificial climate chamber. The test results indicate that the inverted T-type arrangement can greatly improve the flashover voltage of polluted insulator strings, the ‘7 + 2’ inverted T-type arrangement can increase the flashover voltage of each insulator by 8.5% on average, and the ‘3 + 3’ inverted T-type arrangement can increase the flashover voltage of each insulator by 13.4%. The discharge process is different from that of the normal suspension string when the insulator string is inverted T-shaped, i.e. a local arc is produced at the low voltage end of the insulator string and develops downward along the suspension part. While as for the horizontal tension string parts, it is difficult to generate arc during discharge development.

The pollution on insulators is composed of various chemical components, which threaten the stability of external insulation. However, the existing insulation designs have not taken this issue into consideration. This work aimed to study the influence of pollution chemical components on flashover performance of insulators with various materials. Flashover tests of porcelain, glass and silicone rubber insulators polluted by eight types of common contaminants were conducted. Based on the test data, the influence of chemical components and their solubility on equivalent salt deposit density (ESDD) and insulator flashover performance were analysed; and the electrical properties of different types of insulators were compared. Results show that the ESDD contribution rate changes with chemical components. The dissolution characteristic has a significant influence on the measurement of ESDD; slightly soluble contaminants need to be considered separately. The corresponding insulator flashover gradient for ions can be summarised as Cl^−

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.

Umbrella-type structure, the material of insulators and surface positions influence the wetting process of polluted insulator surfaces tremendously. Using a custom-made phase-angle-difference (PAD) detection device, the PAD of the different positions of different polluted insulators is measured, and the PAD curves and partial pollution surface electrical conductivity γ 20 varying with time are plotted. The results show that there is a certain difference in the wetting characteristics of different positions. The saturated wetting time of different positions of LXAY-70 insulators is 12.5-15 min; that for LXHY5-70 insulators is 12-25 min; that for XWP2-70 insulators is 8-14 min; and that for FXBW-10/70 insulators is 18-26.5 min. The umbrella-type structure is shown to have a direct effect on the wetting process of polluted insulators. The complicated umbrella-type structure, especially the existence of umbrella ribs, causes an uneven degree of wetting between the upper and lower surfaces, and leads to the increase of the difference in wetting state between the different positions. Furthermore, there is a certain difference in the wetting speed of insulators comprised of different materials. Hydrophilic porcelain and glass insulators are more easily wetted than composite insulators, even though the hydrophobicity class of composite insulators is 7.

The modelling of tower grounding device is the key for calculation of lightning strike response and lightning withstand level. In order to solve this problem, this study proposes a method to calculate the lightning response of transmission line grounding device by combining the advantages of CDEGS and EMTP software. Firstly, the authors calculate the frequency domain response of the tower grounding model in CDEGS; then the result is fitted according to modified vector fitting method to obtain the rational approximation expression of the impedance function. Furtherly, the corresponding equivalent circuits of tower grounding device is established in EMTP to realise accurate simulation of grounding device lightning strike response. The results show that the equivalent circuit model is more accurate than non-linear resistance model. Then, the factors that influence the lightning response and lightning withstand level are analysed based on the above method according to an actual transmission line model, which can provide reference for the lightning protection and grounding design.

Pollution flashover performance of insulators is one of the key factors to be considered in the selection of external insulation for transmission lines at high-altitude areas. In pollution laboratories at Tibet high-altitude test base and Beijing ultra high voltage direct current (UHVDC) test base, artificial pollution tests of insulators adopted in AC 500 kV transmission project are performed both in sea-level region and real high-altitude regions, through which flashover voltages at typical equivalent salt deposit density are obtained. The relationship between altitude and flashover voltage is acquired as well.

Based on completely mastering ±800 kV transmission technologies, the first ±1100 kV direct current (DC) transmission demonstration project is being constructed in China. Combining theoretical analysis and a large number of experiments, the margin of switching overvoltage in converter stations, the configuration scheme and performance of lightning arresters, the shielding angles of ground lines under different geographical conditions, and the maximum air gap of lines have been determined. The switching impulse flashover characteristics of equipotential sphere and typical rod-plane air gaps are also provided. The external insulation is designed differently for light, medium, and heavy pollution areas. For the ±1100 kV project, if the 8 × 1250 mm^2 conductors are applied, the pole gap is 26 m and the height of the line is 25 m, all the electromagnetic parameters will meet the requirement of international electromagnetic standards. The key design points of ±1100 kV converter transformers, smoothing reactors, converter valves, and wall bushings are researched and the 75 mH smoothing reactors, ±1100 kV/5000 A converter valves, and ±1100 kV/5523 A wall bushings are successfully made. The optimised hierarchical connection modes and coordination control measure of the ±1100 kV project are studied and the results can provide sufficient technical support for the demonstration project.

Polymer composite insulators acquire the position of their counterpart ceramic and glass insulators in high voltage overhead transmission systems due to their remarkable pollution performance. They have different potential and electric field distributions compared to that of ceramic insulators and the absence of intermediate pin and cap metal fittings makes them compact in size by reducing the distance between energised and grounded ends. This study presents a simulation-based approach to compute the streamer propagation characteristics in polymer suspension insulators under positive polarity lightning impulse voltages. The streamer inception, propagation, and sustenance criteria are evaluated using finite element method-based electric field computations. The significance of insulator geometries on streamer inception and propagation phenomena is well established. The insulator geometries such as weather sheds, location of the first shed, and shed radius have shown a strong influence on streamer inception and propagation path, which govern the flashover characteristics of the insulator. The lightning impulse flashover voltages of different insulators were determined experimentally to validate the computed results. A good agreement is found between simulation and experimental results.

The authors investigated the effect of the insulator hydrophilic fraction on the pollution flashover performance by the up-and-down method. The hydrophobicity of parts subjected to a high electric field intensity such as two ends of an insulator, may be weakened by corona discharge. In this study, they simulated the behaviour of composite insulator surfaces at six hydrophilic fractions. Furthermore, arc discharge was recorded using a high-speed camera and the probability of arc discharge was analysed. They found that as the hydrophilic fraction was increased, the 50% probable flashover voltage first decreased and then increased. The wetting process and discharge type could be classified into two types and the flashover process could be divided into two stages by a clear hydrophilic fraction boundary through leakage current characteristic, discharge probability, and arc shape analysis. The results indicated that uneven hydrophobicity loss might lead to further flashover voltage reduction, up to 26.5% lower, than that with uniform hydrophilicity. This should be considered in transmission line design and maintenance of composite insulators.

This work attempts to clarify the potentials of hybrid model based on least-squares support vector machine (LS-SVM) and a novel meta-heuristic algorithm called Grey Wolf optimiser (GWO) in high-voltage applications, considering several kernel functions. The selection of the suitable kernel function and its parameters play an important role in the performance of LS-SVM. For this purpose, GWO is proposed in this study as an efficient optimisation approach to adjust the parameters of various kernel functions such as linear kernel (Lin), radial basis function kernel, polynomial kernel (poly) and multi-layer perceptron kernel. Afterwards, the LS-SVM with the most appropriate kernel function is designed to model flashover voltage of polluted high-voltage insulators. The performance of the developed model is compared with the previous works. The results confirm high capabilities of the proposed hybrid model for the prediction of the flashover voltage of polluted insulators.

Based on the analysis of the principle and structure of the convolutional neural network (CNN) model in a deep learning theory system, an intelligent method for judging the flashover of a porcelain insulator with ultraviolet discharge is proposed. In this method, the porcelain insulator chip was subjected to power frequency flashover testing, and the ultraviolet spectra of different discharge states without discharge, weak coronal discharge, and strong spark discharge were captured by FILIN UV imager. The Alexnet deep convolution neural network model was used to predict the discharge state of the UV spectrum for classification training and identification assessment. The new method doesn't use UV imaging to detect flashover warnings. It is necessary to extract the characteristics of the UV spectra and leakage current parameters manually. The multi-layer combination of UV imaging method with end-to-end autonomous learning with deep learning training and the adopted test method provided classification identification, through a large number of UV images in the deep CNN training. This allowed flashover evaluation of abstract feature parameters of the independent extraction. The results show that this method has the advantages of high accuracy, and provides a new idea for the intelligent detection of UV flashover.

Anticorrosion coatings are commonly applied to prevent rust-based corrosion in transmission towers. Unfortunately, anticorrosion coatings may drip onto the surface of composite insulators during the construction process, which is likely to do harm to operation characteristics of composite insulators. This Letter was conducted to determine the influence of anticorrosion coatings on composite insulator performance. The mechanical properties, hydrophobicity, hydrophobic transfer, and flashover voltage of high-temperature vulcanised (HTV) silicone rubber specimens with and without the coating were investigated in detail. Appropriate solutions for avoiding unwanted effects on insulator performance are proposed accordingly. The influence of coating on the pollution flashover voltage of the composite insulator was also determined by changing the type, area, and shape of coating attached to the insulator surface. Finite element software was also used to analyse the effects of coatings on the electric field distribution of the composite insulator. The coating slightly affects the mechanical properties of HTV silicone rubber, but significantly impacts its hydrophobicity and hydrophobicity transfer. The pollution flashover characteristics of test composite insulators covered with the anticorrosion coating are also worst than those without coating.

In recent years, abnormal heating of composite insulators has become a serious problem, especially in the coastal areas of southern China. In this paper, an improved multi-stress aging test for composite insulators was proposed based on the environment of the coastal areas in southern China, and 5000 h aging test was carried out. The test results showed that the wet flash-over voltage decreased linearly, while the hydrophobicity of the shed showed a nonlinear aging characteristics, whose change rate increased gradually. The multi-stress aging condition had little influence on shed hardness for the change of the shed hardness remained within 1%. The aged composite insulator showed significant temperature rise and the temperature rise concentrated on the high voltage end at high humidity, while it showed little temperature rise at low humidity. The dielectric loss of silicone rubber after moisture absorption was tested. It could effectively reflect the moisture absorption characteristics and the dielectric properties of silicone rubber after moisture absorption, and was closely related to the wet flash-over voltage and temperature rise of the composite insulator. The dielectric loss of silicone rubber after moisture absorption could be used as a new method to evaluate the operating state of composite insulators.

Surface charge accumulation on spacer is an important factor to cause surface flashover. An experimental set-up is established to measure surface potential over a truncated cone-type spacer under DC voltage in SF 6 gas at atmospheric pressure using a Kelvin vibrating electrostatic probe. The surface charge density is calibrated based on multipoint measurement technique, which takes the influence of electrostatic probe into account. The experimental results show that surface charge distribution patterns can be divided into three classes including centrosymmetric distribution, spot-like distribution, and cloud-like distribution. The symmetrically distributed charges are injected by electrode, which migrate along spacer surface through the surface conductivity. While the charges showing spot-like and cloud-like distribution are generated by the ionisation of SF 6 due to local electrical field concentrated. Besides, the hetero-charges are more easily to be saturated with different charging durations and the polarity of the net charge is the same as that of applied voltage.

Due to excellent anti-pollution flashover performance, a composite insulator has become the most frequently and widely used insulator product in transmission lines . Sheath hydrophobicity is the core factor that determines the anti-pollution flashover performance of the composite insulator. To study the change rule of insulator sheath hydrophobicity under the long-term operation condition, more than 390 samples produced by the same manufacturer that had operated for 3–22 years were extracted from the adjacent lines to eliminate the impact of the running environment and manufacturer formula. To study the reasons for hydrophobic fluctuations, surface energy tests and Fourier transform infrared spectroscopy tests were conducted on the superficial layer materials based on a two-droplet method. The change rule of the material physical and chemical properties with operation time was obtained. Next, the relationship between the surface microstructure of the material and operation time was determined by laser scanning confocal microscopy and scanning electron microscopy . Finally, based on the analysis results of surface energy and surface topography, the physical model of shed material hydrophobic variation in the operation process was obtained.

Most of the HV power equipment with gas-solid insulation suffers a lot from the surface charge accumulation due to corona discharge. The existence of surface charge distorts the local electric field and leads to surface flashover faults in extreme situations. As a result, it is important to figure out the mechanism of surface charge accumulation process. In this study, a simulation model combining both the plasma hydrodynamics and charge trapping-detrapping process was built. The simulation results have a good agreement with the experimental data, the main summary is as follows: in the surface charge accumulation process, the corona discharge intensity increases first and then decreases with time. The curves of the surface potential distributions have different shapes at different times, the central value goes up rapidly with time in the beginning and finally reaches saturation. Surface charges exist in the skin layer of epoxy insulator, some of them may be captured by traps while transporting away under built-in electric field.

The possibility of insulator flashover would be higher with the increase in icing degree. Therefore, the method of monitoring the icing weight of the insulator is of vital importance. In this study, the estimation method of icing weight on composite insulator in rime icing was proposed. By quite a few experiments under different rime icing conditions, results showed good agreement between estimated value and measured icing weight. Therefore, the estimation method of icing weight proposed by this study was effective and may be used for predicting icing weight. Based on the estimation method, this study discusses the effect of key structural parameters of composite insulator under different rime conditions on icing weight. In order to better apply the composite insulator to electric transmission line in icing area, several suggestions on design optimisation of structural parameters are proposed.

Epoxy disc-type spacer, as the major insulator of gas insulated transmission line, plays a significant role in the reliability and safety of the entire power grid. While surface charge accumulation on the spacer could induce flashover and accelerate degradation of the insulator, which threats the operation of high-voltage DC transmission and grid. In this study, a corona discharging system was employed to charge the epoxy samples before the charge dissipation measured, and the surface flashover voltages of samples with different modification time under DC voltage were also measured, the results are obtained from the research. It is found that the carrier mobility and surface flashover voltage of samples are affected by modification time, and maximum value of both can be obtained when the sample is modified for 60 min. Under the combined voltages, the initial surface charge density and carrier mobility are affected by both pulse voltage and modification time. It is indicated that surface modification is an appropriate method which can significantly inhibit the surface charge accumulation, and improve the flashover characteristics of epoxy sample by increasing carrier mobility. The trap distribution characteristics suggested that the modification treatment and charging condition have a significant effect on the depth and density of trap.

In recent years, the frequent occurrence of fog and haze in China has attracted increasing attention in the field of electrical insulation. Fog–haze weather is an extreme weather condition with high humidity, high concentrations of suspended particulate matter, and sometimes high conductivity. This exacerbates insulator surface contamination and moistens the contamination layer, which may lead to flashover incidents. Here, an experimental system was established to simulate the fog–haze environment under controlled and tunable conditions. Simulated contamination tests were then conducted using three typical insulators. The influence of insulator type and environmental parameters was investigated. Test results showed that higher humidity led to faster pollution accumulation for all three insulators. Moreover, equivalent salt deposit density (ESDD) on the surface increased linearly with increasing fog conductivity, and that on the upper surface rose faster. The ESDD increased significantly under AC voltage, especially on the lower surface. Finally, insulator contamination was studied in a real-world fog–haze environment with periodic measurement of the ESDD and environmental parameters. The ESDD showed a good correlation with the PM2.5 and PM10 concentrations. This work should have relevance for the external insulation of the power grid to resist the effects of fog and haze.

The study presents novel solutions and risk mitigation strategies to reduce internal arc and accidental electrical contact injuries hazard and health consequences. Motorised closed-door racking in–out operational mode enables to modernise the installed base equipment and to keep personnel at a safe distance: by an innovative hard-bus retrofill design concept, enabling to install a standard motorised-racking withdrawable new breaker into a wide range of existing panels, or by adding on existing panels an external motorisation system applicable to roll-in retrofit. Both solutions improve operational procedure safety on installed base, keeping personnel out of arc flash boundaries and reducing personal protection equipment requirements.

Given that currently available lightning protection measures cannot effectively protect overhead lines from lightning strikes, this study develops an active and fast arc-extinguishing lightning protection gap to address this issue. When a power frequency arc is formed within the protection gap due to a lightning strike, the lightning current immediately ignites the device, while strong airflow is synchronously produced. In this manner, the follow-up power frequency arc can be extinguished within an arc duration that is considerably shorter than the action time of relay protection; thus, relay protection can be avoided, and the lightning trip-out of overhead lines can be effectively prevented. A test of insulation coordination for impulse flashover and a test of power frequency arc extinction are performed to validate the effectiveness of the device. The arc-extinguishing test proves that the strong airflow can extinguish the arc within 0.4 ms. The devices have been widely applied in China and have been extremely successful.

Polypropylene (PP), with low loss and high dielectric strength, is widely used as dielectric for the metallised film capacitor (MFC). The electric ageing and dielectric breakdown of the dielectrics under high-voltage stress is a major concern in the research on polymer materials. Surface discharge and flashover may happen on the PP film between the metallised segments in MFCs, which can influence the dielectric property and cause the breakdown of insulation. Fluorination is effective on modifying the surface characteristics of the polymers, meanwhile retaining the bulk properties. The effects of direct fluorination on the surface charge and flashover characteristics of PP films under pulse voltage are experimented in this study. The samples with different fluorination time were prepared. Surface charging was performed by corona discharge using repetitive pulse power, which was applied to measure the surface flashover voltage as well. The experimental results indicated that the surface charge and flashover voltage were strongly correlated with the charge polarity, pulse frequency and fluorination time. It was suggested that the fluorination of certain time could significantly improve the decay rate of the surface charge as well as the surface flashover voltage of PP samples.

This study presents the improved flashover dynamic model based on both, impedance criterion for the discharge propagation and energy balance criterion for estimating the discharge elongation. One main contribution is to study the electro-thermal model of flashover polluted insulators in DC case enabling to predict the instantaneous changes of the discharge characteristics such as leakage current, discharge temperature, arc resistance and the temporal evolution of their analytics parameters (A, N). To demonstrate the efficiency of the developed model, the authors applied it for flat plat insulators polluted only with NaCl and with mixed pollution (NaCl with insoluble materials). The recommended dynamic approach includes directly the temperature in the dynamic study of the flashover phenomenon since it is an essential parameter involved in the development of the discharge which has not been treated in the earlier studies for dynamic model. Of particular interest, it is important to note that both critical electrical parameters current (I cr) and voltage (V cr) calculated using the presented model are in good agreement with those of the experimental one.

Polluted insulators frequently face flashover accident and it threatens the safety and stability of power system. In order to help to reduce the occurrence of flashover, it is necessary to study the contamination characteristics of insulators and its influence factors. In this study, the contamination of insulator is simulated by the Eulerian multiphase model in computational fluid dynamics. The flow field around insulators is calculated and the volume fraction of the particle phase is used to characterise the pollution degree on insulator surface. Results show that the pollution is mainly distributed on windward side and leeward side of insulator. The crosswind side of insulator is slightly contaminated. The four influence factors, wind velocity, particle concentration, particle diameter and flow angle have different effects on insulator contamination. Among these factors the particle concentration contributes most to the whole contamination degree of insulators. A wind tunnel contamination test on insulator is carried out to verify the feasibility of the numerical simulation. It can be found that simulated contamination distribution and variation of contamination degree with wind velocity well coincide with the results of wind tunnel test. The numerical simulation in this study is practical in studying the insulator contamination.

This study presents a novel technique for estimation of contamination level as well as flashover voltage of outdoor insulators based on dielectric dissipation factor evaluated from the applied sinusoidal excitation and corresponding surface leakage current. Existing literature shows that surface leakage current of insulator is infested with odd harmonics though the applied voltage is sinusoidal in nature. The presence of harmonic contents in leakage current is due to non-linear characteristics of the insulator surface owing to formation of dry bands. The nonlinear characteristic of the insulator surface depends on its contamination level which in turn affects the corresponding flashover voltage. Therefore, the harmonic configuration in leakage current can be correlated with the contamination level on insulator surface and corresponding flashover voltage. Considering the fact, in this contribution, a non-linear equivalent circuit of insulator has been proposed based on harmonic components of the leakage current. For each harmonic component of the leakage current flowing through the insulator system, energy lost and energy stored in the system is calculated, which eventually enables to calculate dielectric dissipation factor. The proposed technique is applied for experimental investigation on 11 kV disc porcelain insulator artificially contaminated in the laboratory under controlled environmental conditions.

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.

Affected by long-term crosswind, insulator surface will get coated with fan-shaped non-uniform pollution, which lowers insulator flashover voltage as previous study revealed. In this study, a further research in specific to HVDC line insulation is presented. Taking typical dc insulator as the sample, its dc flashover performance under fan-shaped non-uniform pollution was tested. Then the influence of that kind non-uniformity was investigated through analysing the pollution layer residual resistance, the flashover process and critical flashover voltage. Finally the relevant HVDC insulation selection was made. Research results indicate that: under fan-shaped non-uniform pollution, insulator flashover voltage ( U 50) gets impaired, and is more obviously impaired by ratio J of windward to leeward side salt deposit density (SDD) than leeward side area proportion T ; under various T and J , there is still an exponential relationship between U 50 and SDD; under fan-shaped non-uniform pollution the pollution layer residual resistance decreases, which theoretically promotes the arc propagation and flashover process, and thus finally lowers flashover voltage; for the transmission line in area with fan-shaped non-uniform pollution, adding insulator units to strengthen the withstand property are needed.

The occurrence of large-scale haze–fog weather in some developing countries brings a new threat to the safe operation of outdoor insulation. In order to study the effect of the haze–fog on the flashover performance of insulators and the flashover mechanism, kieselguhr, calcium sulphate (CaSO­ 4) and ammonium sulphate ((NH4)2SO4) in different particle sizes were mixed with ultrasonic fog to simulate the haze–fog environment and quantitative ac flashover experiments on the standard suspension porcelain insulator were carried out. The test results indicate that the flashover voltage is related with the pollution degree, particle component, particle size and fog-water conductivity, and may decrease by more than 20% under haze–fog environment. The effect of salt aerosols on flashover voltage is determined by the solubility and larger than that of non-soluble mineral aerosols or organic carbon aerosols. The flashover voltage increases with the increase of the particle size, but decreases with the increase of fog-water conductivity. The flashover of insulator under haze–fog condition is a special kind of pollution flashover in essence.

The electrical characteristics of ice-covered insulators in coexisting conditions of high altitude, icing and pollution are a basis for external insulation selection of transmission lines in icing regions. Ice accretion on insulators has been studied for decades, while few studies focus on the flashover performance of ice-covered insulators polluted by two types of contamination, that is the solid layer method and icing water conductivity method, simultaneously. On the basis of the investigations on DC flashover performance of ice-covered insulators in the artificial climate chamber, the influence of different parameters, such as insulator string length, icing water conductivity, salt deposit density as well as atmospheric pressure on DC icing flashover voltage of insulator strings is studied. According to the experimental results, the relationships between the flashover voltage and each of such factors are determined and analysed. Moreover, the correction formula for DC flashover voltage of ice-covered insulators in complex ambient conditions is proposed.