Flashover Probability Research Articles

Research on the Inner Surface Discharge of the Insulation Sheath of Electric Locomotive Cable Terminals

Adding an insulating sheath to the exposed metal part of the outer insulation of a roof cable terminal can extend the creepage distance of the leakage current and significantly reduce the probability of pollution flashover on the outer insulation of the equipment. However, during the actual operation of the locomotive, the inner surface of the insulating sheath is discharged, resulting in cable insulation breakdown, the mechanism of which is unclear. This paper establishes a cable terminal–sheath electric field simulation model and studies the interface air gap, the interface with wet pollution, and the distribution of damp pollution on the outer surface and other factors on the electric field distribution of the cable terminal–sheath structure and the interface discharge, revealing the mechanism of discharge on the inner surface of the cable terminal’s insulation sheath. A voltage of 25 kV rms is applied, and the simulation results show that when the outer surface of the cable terminal is clean and there are air gaps and wet dirt on the inner surface, the maximum distortion electric field at the inner surface is 0.8 × 105~3.4 × 105 V/m, and the value of the electric field at this time is not enough to cause a partial discharge; when there is a uniform layer of wet dirt on the outer surface of the cable terminal, the electric field on the inner surface averages 1.5 × 105 V/m, which is about 275% higher than the average electric field when the outer surface is clean; when there is wetting or an air gap on the inner surface at the same time, the maximum aberration electric field on the inner surface is 1.8 × 105~1.9 × 106 V/m. The wetting on the outer surface of the cable terminal strengthens the non-uniformity degree of the distribution of the electric field, and when there is wetting and an air gap on the inner surface, the over-voltage on the cable terminal inevitably leads to a discharge phenomenon in the air gap. This provides a theoretical basis for optimizing the insulation sheath structure to solve the discharge problem.

Coordination of the back flashover probability of HVAC and HVDC on a hybrid transmission tower

The demand for power transfer has increased throughout the green transition, especially as power generation is becoming increasingly decentralized. However, constructing new transmission lines and towers has met with public opposition. One option to increase transmission capacity while minimizing the erection of new towers is to use hybrid HVAC-HVDC multi-circuit towers. If not carefully designed, considerably more back flashovers could occur to the more vulnerable DC circuit. This paper investigates the effect of a change in insulator size on the ratio of back flashovers between AC and DC circuits on the same hybrid tower by using a PSCAD model with HVAC-HVDC hybrid towers. A back flashover ratio calculation method is developed and used in combination with an algorithm to provide a DC insulator length based on a predetermined back flashover ratio. The algorithm allows for faster design of the insulator sizes for a multi-circuit tower, coordinating the ratio of back flashover probabilities to either the HVAC or the HVDC circuit.

Repetitively pulsed streamer discharge with laser-induced surface trapped electron desorption to exploit residual charges in situ

Positive streamer behaviors under repetitive pulses are predominantly dependent on the availability of free electrons. If surface residual electrons stored from previous discharges could be intentionally released and involved into the next discharge, an alternative control freedom is provided apart from voltage waveform tailoring methods that mainly attract or repel gaseous residual charges. Evolutions of repetitively pulsed surface streamers in compressed (0.2 MPa) air were investigated after low-photon-energy pulsed visible (532 nm) and infrared (1064 nm) laser irradiations. Pulse-sequence and temporally resolved diagnostics were implemented to investigate effects of laser parameters (irradiation moment, wavelength, energy) and gas composition. A 2D surface streamer fluid simulation was performed to qualitatively unveil impacts of localized plasma patches. The surface streamer morphology and emission light are significantly and repeatably affected by the laser irradiation before the streamer inception, while, variations totally disappear without the solid surface. The secondary streamer is prolonged accompanied by a higher flashover probability after the pulsed laser irradiation in compressed air. Intriguingly, influences of the infrared laser persist for tens of microseconds before the next voltage pulse. Residual charge dynamics under the laser irradiation are analyzed, where the additional increase of O2− of low electron bound energy is emphasized. The laser induced surface trapped electron desorption is achieved through the direct or the step-wise process, dependent on the laser energy and the surface trap state distribution.

Externally gapped line arresters placement effect on transmission line lightning performance including the frequency response of the grounding system

AbstractThis paper conducts the frequency‐dependent grounding system impact on externally gapped line arresters (EGLAs) operation and their placement effects on the lightning performance of a transmission line (TL). The study involves placing EGLAs at different phases of a 400‐kV double‐circuit TL with two downstream shield wires and selecting the optimal state with fewer back‐flashover incidents. The results show that using a non‐linear grounding system leads to a higher back return current from insulators or EGLAs, resulting in a higher flashover rate compared to the frequency grounding system (FGS). Installing four EGLAs in the middle and upper phases of the TL provides satisfactory lightning protection, and the flashover probability is zero and 4.2% for the FGS and the non‐linear grounding system, respectively. A sensitivity analysis including soil resistor variation, economic analysis, and grounding electrode configuration has been performed that confirms installing downstream shield wires and EGLAs is an effective and economical technique for lightning protection.

Flashover Risk-Based Probabilistic Design of Transmission Line Insulators under Contamination Conditions

In the traditional designs against contamination of transmission line insulators, the maximum insulator contamination severity and insulator surface wetting are assumed as the most severe conditions. Then, the required number of insulator units per string is determined, where a safety margin of insulator unit(s) is usually allowed based on knowledge obtained at sites and in laboratory tests. Other insulator designs may be created by considering the probabilistic distributions of factors affecting insulator flashover, such as contamination severity, surface wetting, and flashover voltage. Based on this concept, flashover risk, which indicates flashover probability in a certain period, is used in this study as an evaluation function of reliability. The characteristics of flashover risk are discussed, and the proposed probabilistic method is applied to the insulation design of a 500kV ac transmission line. Assuming a certain acceptable flashover risk, the number of cap-and-pin insulator units per string is calculated and compared with that obtained using the conventional deterministic method. When the number of insulator units per string is fixed, flashover risk can determine the number of flashover events in a certain period, such as a year. Findings suggest that the probabilistic approach is a potential insulation design method under contamination conditions.

Design Optimization and Flashover Probability Evaluation of a 3 MV, 3.4-m-Diameter Four-Level Axial Vacuum Insulator Stacks for a 15 MA Z-Pinch Accelerator

A four-level axial insulator stack is designed for a 15 MA Z-pinch accelerator. The height and diameter are 1.5 m and 3.4 m. The two upper levels operate at 2.5 MV, while the two bottom levels operate at 3.0 MV. The insulator ring stages in series, the vacuum flares, and water flares are optimized in details. By reiterative optimizations, the two upper levels consist of five 5-cm-thick insulator rings and four 0.8-cm-thick grading rings. While the two bottom levels consist of six insulator rings and five grading rings with the same individual insulator and grading ring. The inductance of each level is 14.02 nH, 15.48 nH, 18.71 nH, and 20.88 nH, respectively. The equivalent inductance is 4.21nH with the four levels in parallel. The voltage variations from average for the four levels are within ±4.9%, ±4.9%, ±2.1%, and ±2.2%, respectively. At the average operating electric field of 100 kV/cm, the predicted flashover probability for the two upper level and the two bottom level insulator stacks are about 0.29% and 0.53%, respectively.

Experimental Study On Bird Streamer Flashover for V-Type Insulators of DC Transmission Line in High Altitude Area

In order to obtain the bird streamer flashover characteristics in high altitude areas, the discharge of DC V-type string insulators was studied in high-altitude test base at 4300 m in Tibet. The study results show that medium viscosity bird streamer is easy to form a continuous channel and cause flashover. The flashover probability of positive polarity transmission lines is slightly higher than that of negative. The conductivity of bird excrement almost has no effect on the flashover probability in the range of 1.9∼20.9 mS/cm. To increase the vertical air gap distance can reduce bird streamer flashover probability. Based on the experiment results, the recommended vertical air gap distance values of ±400 kV DC transmission line were proposed. Considering the influence of wind and bird streamer falling angle, the protection radius was set 3.5 m at 3500∼5000 m altitude area. Bird streamer flashover is a mixed discharge of air gap breakdown and the surface discharge along the bird streamer channel, at the altitude of 4300 m area, the minimum breakdown potential gradient is about 2.1 kV/cm, and the discharge between bird streamer and shielding ring can be approximately equivalent to rod-rod gap discharge.

Metal Particle Movement and Induced Insulator Flashover Under Impact Vibration Generated By Switching Operation in GIS

Gas insulated switchgear (GIS) insulation failures occur frequently in field operations, seriously affecting the reliability of power systems. Fault statistics show that nearly half of the insulation flashover faults in field operation are strongly related to the switching operation, and metal particles are considered to be a crucial cause of flashover. The impact vibration generated by the switching operation may activate the metal particles and make them lift and jump in the GIS, reducing the insulation performance. Therefore, in this study, an experimental platform was established to simulate the impact vibration generated by the switching operation. The flashover characteristics induced by metal particles under impact vibration were studied, and the movement behavior of metal particles was also analyzed. The results show that metal particles lying on the enclosure lift under the impact vibration and then keep jumping in the GIS. These particles can jump to cause insulator flashover with high probability after the impact vibration. The longer the metal particles, the higher the probability of insulator flashover, and the shorter the average jump duration of the metal particles before the flashover. Moreover, compared with the flake particles, linear particles are more likely to cause insulator flashover after the impact vibration. This study demonstrates the mechanism of insulator flashover caused by metal particles under impact vibration, which provides a significant support for understanding the insulation failures strongly related to the switching operation in practical applications.

Back Flashover Probability of the Overhead Power Line Insulation Taking into Account the Soil Ionization and Frequency Response Features

Back flashovers caused by a lightning strike on an overhead power line tower or overhead ground wire give rise to voltage surges dangerous for the insulation of power substation electrical equipment. A pulse back flashover turns into a short-circuit arc leading to the power line tripping. Calculations of voltage surges in a single–circuit 110 kV power line are carried out using the methods of the grounding devices theory, and the back flashover probability is estimated based on the curve of dangerous currents. The insulation back flashover probability for a power line tower with a ground wire in areas with high-resistance soil is determined by the grounding conductor surge impedance. It is shown that the analysis carried out with taking into account the soil ionization and frequency response features yields an essentially lower back flashover probability. The second ground wire placed under the power line wires reduces the inductance of the ground wires, thereby intensifying the spread of current through the grounding conductors of neighboring towers. This leads to a decreased tower voltage and an increased voltage induced on the wires, thus resulting in a decreased voltage across the insulation voltage and, hence, lower back flashover probability. The factors governing a positive effect from using grounding counterpoises of an overhead power line in areas with high-resistance soil are shown, and the conditions under which this effect is achieved are determined.

DC Breakdown Characteristics of Needle–Needle–Plane Electrodes With Insulation Barrier and Thermal-Induced Bubbles in Liquid Nitrogen

Insulation barriers are an effective way to improve the insulation strength of the resistive-type superconducting fault current limiters (R-SFCLs). The objective of this article is to obtain the dc discharge characteristics of composite insulation systems including two liquid nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) gaps in series, but separated by a solid insulation barrier mixed with vapor bubbles. The needle–needle–plane stainless steel electrodes with the insulation barrier and bubbles generation systems are used to simulate the electric strength under quenching conditions of the R-SFCLs. The results show that the composite systems’ breakdown characteristic switched from insulation barrier breakdown to surface flashover with the insulation barrier approaches the needle electrodes side. The breakdown voltage of the composite systems decreased due to the thermal induced bubbles. The presence of the bubbles increases the probability of the surface flashover of the insulation barrier. The results have guiding significance for the design of the high-voltage R-SFCLs.

Simulation of surface flashover probability distribution with gas-solid charge transport/multiplication coupling of epoxy resin

<p indent="0mm">The insulation failure of key equipment in power systems caused by gas-solid interface flashover of basin insulators is a key scientific problem that must be solved urgently. The time and space scales of gas electron multiplication and solid-surface charge evolution span multiple orders of magnitude, resulting in an unclear physical mechanism of the gas-solid coupling interface discharge of a basin insulator. A gas-solid coupling surface flashover model was established considering three junction point charge injection, solid surface charge transport, and gas collision ionization. The violent collision ionization in the gas caused by electrons emitted from the solid was taken as the surface flashover criterion. The key parameters characterizing surface charge transport and gas collision ionization were extracted based on gas-solid coupling surface flashover, and a model considering random variables was established to simulate the flashover voltage distribution of epoxy resin. The flashover voltage obeys the Weibull distribution when the charge transport parameters are Gaussian random variables, and the simulation results are consistent with the experimental results. By comparing the simulation and experimental results, it is found that the characteristic flashover voltage and distribution parameter are mainly controlled by the gas and solid media, respectively, and each charge transport random variable has a different effect on the distribution shape parameter. The Weibull distribution of flashover voltage consistent with the experimental results can be obtained by adjusting the standard deviation of charge transport random variables. This research explains the relationship between the Weibull distribution of flashover voltage and the charge transport and gas collision ionization parameters, providing an effective method to simulate the probability distribution characteristics of flashover voltage, which can also be used to evaluate the failure probability of the gas-solid interface under a certain voltage.

Waveform Equivalence Method for Non-Standard Impulse Voltages Based on Gas–Solid Interface Flashover Probability

The characteristics of failure of a dielectric subjected to measured impulse voltages are significantly different compared with those under the double-exponential impulse voltages (DEIVs). This poses a daunting challenge to the coordination and design of insulation. The key issue in responding to this difficulty is identifying the relationship of universal equivalence between the nonstandard impulse voltages and the DEIV. This study uses this idea to establish a test platform for the damped alternating impulse voltages (DAIVs) and DEIV to experimentally examine the characteristics of flashover probability on the surface of samples of silicone rubber under two types of 207 waveforms. This is used to determine the influence of waveform parameters on the flashover probability. The results indicate that the flashover probability on a gas–solid interface under the DAIV is generally higher than that under the DEIV. Based on the error backpropagation algorithm and a large amount of experimental data, models to predict the flashover probability of the DEIV and DAIV are established and yielded a maximum relative error of lower than 7.5% in experiments. In light of the assessment of the quality of images, a method to assess the equivalence of the waveforms of the DAIV and DEIV is also proposed by using the flashover probability as the object of evaluation. The results of experiments show that the overall error in equivalence is lower than 1.5%. The proposed method, thus, provides a feasible route for statistically coordinating insulations.

Performance Evaluation of Outdoor High Voltage Glass Insulators in High Pollution Industrial Areas Using Simulated Acid Rain Contamination

A suitable, non-standard method for artificial pollution testing using simulated acid rain pollution in a laboratory was developed in this work. The method is applicable to a wide range of climatic conditions and rain acidities. The established method adopts the solid layer principle in the preparation and application of acid rain of various pollution severities to test insulators. Glass cap and pin insulators were contaminated, assembled into strings of two discs and flashover tests carried out on the strings at different orientation angles. The frequency of flashover occurrence, maximum withstand degree of pollution, effect of pollution severity, and effect of orientation angle were investigated. Test results showed that the orientation angle plays little or no role in the flashover of the insulators under dry conditions. Under wet conditions, the insulators performed better when inclined. Flashover voltage was observed to be inversely proportional to time while the time to flashover was inversely proportional to the pollution severity. The probability of flashover increases with an increase in voltage for a fixed pollution severity or increase in pollution severity for a fixed voltage level. This indicates that the probability of flashover is a function of the product of pollution severity and voltage.

Experimental characterization of a novel ventilation system for multi-story buildings with continuous curtain walls

PurposeThe purpose of this paper is to present the results of an experimental campaign conducted on a recently developed fire protection system (FPS), specifically designed for installation on continuous glass curtain walls systems typical of multi-story buildings.Design/methodology/approachThe authors will first present the theoretical derivation of the relevant parameters to characterize and predict the fire evolution and probability of flashover, according to existing codes and standards. Then, the results of two full-scale tests will be presented in terms of temperature fields, thermal gradients and position of the neutral plane.FindingsThe experimental evidence shows how the proposed system is able to dramatically reduce internal temperatures in the rooms interested by the fire, also allowing for safer evacuation procedures by increasing the height of the neutral plane.Originality/valueThe novel window frame element comprises an automatic doubly convergent aperture system that induces ventilation in the compartment by increasing internal convection in the rooms subject to the fire. This allows for an efficient dispersion of hot gases and fumes and a drastic improvement in safety for both the occupants and firefighting operators. The theoretical results are then compared to the experimental evidence to evaluate the performance of the proposed ventilation system in the context of existing standards and design procedures.

Initiation and Growth of the Secondary Electron Avalanche along High-Gradient Insulators in Vacuum

High-gradient insulators (HGIs) are materials created by alternating layers of insulating and conducting materials, which can result in a higher holdoff voltage than a monolithic insulator of the same length. We model the HGIs periodic electric field and use both a particle-in-cell direct simulation Monte Carlo code and a test particle approach to parameterize the growth of the secondary electron emission avalanches (SEEAs). To determine the effect of various HGI design parameters on secondary electron trajectories and SEEA growth, the following factors are investigated: layer width, edge layer materials, secondary electron yield, cell coupling, and surface defects as an electron source. We propose an alternative mechanism for initiating surface flashover, which considers surface defects on the downstream side of insulating layers, which differs from the current theory of cell coupling between adjacent insulating layers. Our results indicate surface conditioning is necessary because surface defects are a significant electron source in many cases. Our results reinforce the theory that thin layer widths are better at minimizing the growth of a surface avalanche, even though they will be more susceptible to cell coupling. If an HGI will not be conditioned prior to use, then thin layers are essential because of the significant avalanche growth capable along thicker layers. Finally, a metal layer adjacent to the cathode and an insulating layer adjacent the anode seems the optimal configuration to minimize the probability of flashover.

High-risk region of bird streamer flashover in 110 kV composite insulators and design for bird-preventing shield

Bird-related flashover is a major factor of transmission line outages according to IEEE Guide for Reducing Bird-related Outages. In recent years, with the improvement of people's awareness of environmental protection, the probability of bird streamer flashover increases especially in 110 kV transmission in China. The high-risk region of bird streamer flashover is generally believed as a circular area centred on the suspension point of the insulator at present, and many bird-preventing installations such as shield and baffle are designed according to it. However, bird-related flashover still occurs protected by such installations, the current high-risk region of bird streamer flashover and the performance of these installations may not be clear needing further research. In this paper, a simplified platform of 110 kV transmission line tower was built to simulate the process of bird streamer flashover at different dropping positions on the cross arm. The result shows that the protection radius R changes with the dropping position of bird simulation excrement. Electrostatic simulation was carried out to study the distributions of electric field during the flashover process and the experiment results are verified. High-risk region of bird streamer flashover in 110 kV composite insulators is quantitatively obtained through emulation, and the shape is more like a “flying saucer”. A new bird-preventing shield with the function of “drainage” is designed accordingly, whose effect is verified and evaluated by comparison experiment.

Probability Calculation of Pollution Flashover on Insulators and Analysis of Environmental Factors

Insulator pollution flashover occurs frequently and is harmful to the power transmission and distribution network. But the randomness of the pollution flashover and the complex influencing factors make it difficult to be predicted. The objective of this paper is to study the mechanism of the discharging on insulator surfaces and design a the calculation method which derives flashover probability. By calculating the distorted electric field in the initial streamer discharge quantitatively and combining it with the equivalent circuit of the insulator surface, the initial discharge voltage is derived. By analyzing the change in power input and dissipation in the process of the arc discharge, we propose the criterion on arc development and extinction, and the critical flashover conductivity of the insulator is obtained. Based on the probability density function, the behavior of the factors on the critical flashover conductivity is described, and the probability of insulator pollution flashover is calculated under different environmental conditions. In the validation experiment, the measured initial discharge voltage and pollution flashover ratio are close to the calculated values. The methods proposed in this paper could be used in the quantitative calculation of the initial discharge voltage and the prevention of the pollution flashover on insulators.

Influences of low temperature on internal gas-filled cavity discharges in EPR cable termination and its feature factors analysis

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 &gt;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.

Probabilistic assessment of insulator failure under contaminated conditions

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.

Bulk FDTD Simulation of Distributed Corona Effects and Overvoltage Profiles for HSIL Transmission Line Design

Power system load growth and transmission corridor constraints are driving industry activity in the area of high surge impedance loading (HSIL). Examples include compact structure design and uprating existing transmission lines. Recent research relating electric field uniformity to transmission line capacity and critical flashover voltage underscored the need for better overvoltage data to quantify insulation margins for HSIL design. To that end, this work extends the finite difference time domain (FDTD) method with distributed corona losses to transmission lines with bundled conductors. The model was adapted for practical use in high-volume statistical transient simulation and applied to an example 500 kV line. Transients included line energization and trapped charge reclosing. Overvoltage profiles and statistical distributions were generated from 9500 simulations obtained by random breaker close timing and variation in line length and altitude. Distributed corona losses reduced 98th percentile line-to-ground switching overvoltages by 4%–14% of nominal. The estimated line-to-ground switching surge flashover probability was 54%–80% lower with corona loss. Corona had less impact on line-to-line overvoltages, but the effects were still notable. Results highlight the importance of considering detailed overvoltage profiles and accounting for corona loss attenuation when seeking to carefully quantify insulation design margins.