Flashover Research Articles

Influence of a shield wire flashover on the indirect lightning performance assessment of distribution lines

This paper presents a new aspect of shield wire (SW) modeling for assessing the indirect lightning performance of overhead medium-voltage distribution lines: a flashover (FO) between an SW and the reinforcing bars of a distribution pole or a crossarm (SWFO). In general, an SW is periodically grounded approximately every four to ten poles and not grounded at the other poles. However, owing to lightning-induced overvoltages, the voltage difference between the SW and the distribution pole may even exceed 100 kV, and the SWFO can occur. In this paper, we evaluate the effect of the SWFO on the number of FO occurrences of phase-conductor insulators by the Monte Carlo method using a 2D finite-difference time-domain-based indirect lightning surge analysis program. The effect of the SWFO is more significant in lines with high soil resistivity (a soil resistivity of 1000 Ωm was assumed) regardless of the installation of surge arresters: the total number of FO occurrences markedly differs by up to 50% between cases in which the SWFO is considered and not considered. The analysis presented in this paper will assist the formulation of lightning protection measures particularly, in regions with high-resistivity soils.

Experimental Study and Modeling of the Effect of ESDD/NSDD on AC Flashover of SiR Outdoor Insulators

Pollution flashover occurs when soluble and nonsoluble materials cover the surface of an insulator, and this may ultimately cause a reduction in its performance. In this paper, the common type of sodium chloride (NaCl) was used as a soluble pollutant (ESDD) and kaolin as a nonsoluble pollutant (NSDD). Samples of silicone rubber (SiR) insulators were selected for this study and fabricated at the Advanced High Voltage Engineering Research Centre (AHIVEC) at Cardiff University. The samples were preconditioned and polluted according to standard specifications. Additionally, the AC voltage ramp technique was used to achieve flashover (FOV) voltage with different pollution levels. The aim of this work was to investigate the effect of nonsoluble materials on flashover characteristics to understand their interaction with dry-band arcs by using FOV electrical equations and experimental data. The test results show that the FOV voltage of the silicone rubber insulator substantially decreased with the increase in both ESDD and NSDD values. It was also identified from these results that the dry-band arcs were considerably influenced by both ESDD and NSDD levels. This impact can be quantified by determining the variation of discharge parameters (N, n). Based on the FOV equations and experimental data, a mathematical model was suggested, taking into account the effect of both ESDD and NSDD.

Study of Oil/Pressboard Creeping Discharges Under Divergent AC Voltage—Part III: Effects of Temperature and Stress Type

The present article studies the effects of temperature and stress type on the oil/pressboard creeping discharge under divergent ac voltages. A ramp-stress test determines the short-time electric strengths of the oil/pressboard interface. Subsequently, long-term tests are carried out at different temperatures using constant and step stresses, respectively. Afterward, a novel methodology is proposed to detect the developing fault using synchronous partial discharge (PD) and online dielectric measurements. The interface short-time electric strengths correlate positively with the temperature. In long-term tests, the creeping discharges can generate three failure modes to oil/pressboard insulation: interface flashover (FO), surface tracking (ST), and internal treeing (IT). Different failure modes vary significantly in the discharge characteristics and the severities of damage to the pressboard. Temperature plays an important role in determining the failure modes of creeping discharge, and its influence mechanism is expounded in terms of the change of material properties, the interface of electric and dielectric phenomena, and their sophisticated interplays. The influence of stress type is related to the cumulative effect of stressing and the voltage rising steepness. The proposed methodology proves to be a potent tool in detecting the developing IT.

Insulator nondestructive testing based on VGGNet algorithm

In the power system, it is difficult to detect the insulator's deterioration in operation. Aiming at this problem, this thesis applies the convolution neural network algorithm to evaluate the insulator's deterioration degree based on the deep analysis of the principle and structure of the convolution neural network model. Firstly, the power frequency flashover test was conducted on the insulator to produce three states as follows: no discharge, weak discharge, and strong discharge. Moreover, the Ultraviolet imager was applied to collect the insulator's ultraviolet images in different discharge state to establish the ultraviolet images sample library. Subsequently, the VGGNet framework neural network algorithm was applied to perform the classification training and the state-prediction evaluation on the samples so as to eventually achieve the purpose of judging whether the insulator is degraded. From the experimental results, it can be seen that the accuracy rate of the algorithm is as high as 98.4%, which has broad application prospects in the insulator's degradation detection. Furthermore, it provides a mentality for the reliability detection of other power equipments.

Analysis on the Characteristics of Short-tail Voltage Wave of Insulator on the Double-circuit Transmission Line in Back Flashover and Design of the Experiment Circuit

Analysis on the Characteristics of Short-tail Voltage Wave of Insulator on the Double-circuit Transmission Line in Back Flashover and Design of the Experiment Circuit

Analysis of Flashover Accident and External Insulation Characteristics of AC Filter Circuit Breaker in ±500 kV Convertor Station

Analysis of Flashover Accident and External Insulation Characteristics of AC Filter Circuit Breaker in ±500 kV Convertor Station

Pollution Flashover Characteristic of Composite Insulator with Non-uniform Hydrophobicity Distribution on Surfaces

Pollution Flashover Characteristic of Composite Insulator with Non-uniform Hydrophobicity Distribution on Surfaces

AC Suspension Insulator’s Ice Flashover Based on Characteristics of Leakage Current

AC Suspension Insulator’s Ice Flashover Based on Characteristics of Leakage Current

Surface Flashover Characteristic of EP/MWCNTS in Vacuum Under DC Voltage

Surface Flashover Characteristic of EP/MWCNTS in Vacuum Under DC Voltage

Influence of High Conductivity Fog on AC Flashover Characteristics of Suspension Porcelain Insulator

Influence of High Conductivity Fog on AC Flashover Characteristics of Suspension Porcelain Insulator

Wormholes Effect in DC Flashover Process of Alumina Filled Epoxy Resin Surfaces in SF6

Wormholes Effect in DC Flashover Process of Alumina Filled Epoxy Resin Surfaces in SF6

Influence of Different Parameters on Flashover Propagation on a Solar Panel

This paper first presents a short review of the recent experiments performed on large test fixtures, whose aim was to evaluate the flashover (FO) propagation during an electrostatic discharge. Then, the model of plasma bubble expansion is presented and compared with the results collected during the review. We will show that the model can represent qualitatively and quantitatively most of the results. The parameters of the model are mainly the ion velocity and the backscattered electron emission yield. The other experimental parameters influencing the FO propagation will be discussed.

Analysis of vacuum insulator stack flashover probability with statistical model

The statistical model is used as a typical method for vacuum insulator stack flashover probability calculation. We have calculated the stack flashover probability and the g factors by which a flashover will change the voltages on the each of the insulator rings with different conditions. The calculated results show that the real performance of insulator stack characteristics with the multi-stage and large circumferential transit time. The matrix a in the statistical model was predigested to reduce the g calculation time and make the probability calculation accurate. Finally, analysis based on fixed distance between insulation rings showed that there were some optimized ring numbers with maximum voltage amplitude and electrical field amplitude. This conclusion could be used for evaluation of insulation ring number in insulator stack design.

Surface flashover characteristics of insulating materials under hundred-nanosecond pulse

In order to research the surface flashover characteristics of two typical insulating materials (nylon and synthetic glass) under hundred-nanosecond pulse in nitrogen gas, experimental device of surface flashover under hundred-nanosecond pulse was designed. The variation laws of the flashover voltage of the insulating material with air pressure and flashover distance have been researched via experimentation. The experimental results showed that the flashover voltage increased with the increasing of the air pressure. However, the flashover electric field intensity decreased with the increasing of the distance. In the same condition, the flashover voltage of PMMA is higher than that of nylon. The flashover voltage of the materials with different gas pressure and distance under hundred-nanosecond pulse were obtained via the surface flashover experiment.

Analysis of a back flashover across insulator strings on a 115 kV transmission line tower by PSCAD

Lightning striking on a transmission tower induces high ground potential rise and high voltage at tower arms in which potential is normally at ground level, and subsequently causes overvoltage across an insulator string. If this overvoltage is higher than the withstanding voltage of the insulator string according to the v-t (voltage-time) curve, back flashover phenomena will occur and this event may cause outage. The main objective of this paper is to study the factors influencing the back flashover phenomena. The computer program PSCAD/EMTDC (Power System Computer Aided Design/Electromagnetic Transients including DC) is used to simulate lightning striking on a transmission tower 115kV. Lightning current, transmission towers, ground resistance, insulator strings and back flashover phenomena are modeled. Main simulations are lightning striking on different towers, different soil resistivity, different lightning current magnitudes and wave shapes, different locations, and different phase angles of source voltage. Simulation results show that the higher tower encounters higher induced voltage. A back flashover occurs at the top tower arm easier than at the middle and lower arms. The higher soil resistivity induces higher voltage. The larger lightning current magnitude impacts on higher induced voltage. The longer rise time of lightning current generates lower induced voltage. Lightning strikes directly on tower generate higher voltage than that of striking on overhead ground wires.

Diagnostic of Neutralization Current for Arcs on Satellite Solar Panel Coupons

Risk of arcing in space plasma environment on satellite solar panel has always been an issue of major concern in the aerospace community. The outcome of round robin tests performed in various laboratories has resulted in finalization of the guidelines for the ground test of satellite solar panel coupons (SSPCs). This is documented as ISO-11221 standards. Authenticity of arc plasma flashover (FO) parameters are still under debate. Some experimental findings that support the perimeter theory of arc FO on geostationary (GEO) satellites are discussed in this paper. It has been observed that characteristics of an the arc are dependent on its location on the solar coupon, bias charge (charge due to bias voltage and external capacitance), and the total charge dissipated during the arc discharge. It is also observed that the duration between the two consecutive arcs is directly proportional to the dissipated charge or energy of the latter arc. A combined dielectric consisting of solar cell, adhesive, Kapton, and Carbon Fiber Reinforced Polymer sheet holds charge during irradiation by the electron gun. This resembles surface FO capacitance. Arc energy is governed by the combined effects of a surface flashover capacitor (C-sf) and an external capacitor. It is observed that if intermittent arc duration is sufficiently longer than the gradual accumulation of charge on the SSPC, body may lead to a major arc. This would increase the probability of secondary arcs when the sustained arc environment is experienced by the satellite. To understand primary arcs on dielectrics, an artificial triple junction is prepared and exposed under adverse GEO-like space conditions. Various signatures identified from the arc current and arc neutralization current waveforms specify the behavior of these arcs at different locations on the coupon surface. In this paper, current waveforms of arcs captured on different types of solar panel coupons and their significance are studied. Information about arc energy, arc velocity, and neutralization velocity are extracted from the measured waveforms.

EXPERIENCES IN NUMERICAL ANALYSES OF SURFACE CHARGE ON INSULATORS EXPOSED TO HIGH VOLTAGE IN VACUUM

This paper deals with the electrical insulation of a gap bridged by a solid dielectric between a pair of electrodes in a vacuum, where the dielectric is a model of an insulating spacer holding electrodes in various vacuum devices. Such gap configuration is typical as a model for studying physical mechanisms of an electrical breakdown or a flashover in a vacuum, in which the insulation ability is considerably lower than a gap without any insulating spacers. Many researchers have long investigated the mechanisms of flashover, and it is believed that the charging of the insulator surface is the primary drive of such weakness. One of the effective approaches for studying mechanisms is to analyze the characteristic of charge accumulated on the insulator surface both by experiments and by numerical analyses. This paper reviews previous works conducted by the author’s research group and demonstrates the difficulties we have encountered upon conducting the numerical analyses.

Flow analysis and validation of numerical modelling for a thin walled high pressure die casting using SPH

High pressure die casting (HPDC) is an important process for high throughput manufacturing of complex shaped metallic components. The flow involves significant fragmentation and spray formation as the high pressure liquid jets into the die from the gate system. An important class of die cast components is one with large areas of thin walls. An example of this is the chassis of the laptop computer. Computational modelling provides an opportunity to both better understand the filling process and to optimize the runner, gates, flash overs and venting systems for the die. SPH has previously been found to be very suitable for predicting HPDC for bulkier automotive components. The modelling challenges arising from the very thin sections and the many flow paths in a laptop chassis require careful validation. A water analogue experiment is used to validate the predictions of the SPH model for this representative thin walled casting. SPH predictions are used to understand and characterise the filling process. Finally, comparison of flow lines visible in an etched finished casting with the high speed flow paths in the final filled SPH model show very strong agreement. Together these demonstrate that such an SPH model is able to capture substantial detail from both the water analogue system and the actual casting process and is very suitable for simulating these types of complex thin walled castings.

Characteristics of grooved insulator flashover under pulsed voltage

The characteristics of grooved insulator flashover under pulsed voltage are discussed in this paper. The groove widths are determined by the secondary electron emission trajectory. The experiments results show that the flashover voltages increased while the insulator samples with 0.05 mm, 0.1 mm, or 0.7 mm wide grooves were used and the biggest voltage multiple was 1.4. The samples with 1 mm wide grooves had the same flashover voltage level as the flat ones. It is concluded that proper grooves design could increase the insulator flashover level by preventing the development of the secondary electron emission avalanche. Mechanism of inhibition was analyzed with calculation of electric field strength on the surface of insulator. Wider grooves, would trap the electrons and narrower ones would inhibit the motion of the initial electrons. Comparing the samples surface micrograms, a conclusion was given that the surface micro-flaw would drop the insulator flashover level.

Evaluating Current Fire Test Methods for Determining Flammability Performance of Ceiling Materials

Many fire tests have been developed to evaluate the flammability performance of a lining material. According to the test methods, specimens are usually mounted vertical as a wall or horizontal as a floor. The only exception is the ISO 9705 room corner test in which ceiling material is installed beneath a ceiling. This study was accordingly designed to discuss the test results of ceiling materials in the ISO 9705 room corner test with the testing capacity of the other commonly-used tests to evaluate the feasibility of the other tests to rank materials mounted beneath a ceiling. Materials used were gypsum board and particle board, which were ranked in the best and the worst classes by the cone calorimeter, a commonly used testing apparatus. Our results showed that the fire behaviors associated with materials tested in the other tests can not completely perform those tested in the ISO 9705 room corner test. A penetration occurred in the gypsum test and led to a severe fire although flashover was not observed. The results from the other tests primarily considered the potential of a material leading to flashover. The penetration of flames through ceiling materials cannot be assessed in the other tests. A modification of an existing test is recommended when ceiling materials are tested.