Flashover Probability Research Articles

Concurrent Flame Spread Over Two-Sided Thick PMMA Slabs in Microgravity

Spacecraft fire safety is an important consideration when designing future exploration missions. Concurrent flow flame spread experiments were carried out aboard the Cygnus spacecraft while in orbit to address current knowledge gaps related to solid fuel combustion in microgravity. The experiments used PMMA (polymethylmethacrylate) samples that were 50 mm wide and 290 mm long with two variations—one sample was a 10 mm thick flat slab, while the other was a 10 mm thick flat sample that had a 4 mm thick grooved center section. The thickness variation had a major impact on the flame shape, and the grooved sample developed a deep inverted-V shaped notch as the thin center section burned through. This notch enhanced heat transfer from the flame to the solid through an effectively wider flame base, which is the part of the flame with the highest temperature. Unlike in normal gravity (and also likely in partial gravity), where buoyant flow promotes acceleratory upward flame growth, the microgravity flames reached a steady size (limiting length) for a fixed forced convective flow in agreement with theory (i.e. there is a zero net heat flux at the flame tip). The limiting length implies that the spread rate of the flame will be controlled by the regression rate (burnout rate) of the material because the flames remains anchored to the upstream end of the fuel samples. This is a significant finding for spacecraft fire safety, and makes the probability of flashover in a spacecraft unlikely as the flame size will be small for low convective ventilation flow environments typical in spacecraft. On the other hand, long-burning flames in small vehicles will generate significant quantities of fuel vapor that may reach the lean flammability limit and cause a backdraft. The rapid extinction of the flame when the flow was turned off also supports the existing fire mitigation strategy on the ISS to deactivate the ventilation system in the event of fire alarm. These findings can be applied to improve the safety of future space exploration missions.

Analysis of Switching Overvoltage and Suppression Measures in UHV Transmission Lines

As the ultrahigh-voltage (UHV) projects are rapidly developing, it is necessary to investigate the problems involved in overvoltage and insulation, thereby providing a technical basis for future UHV projects. The switching overvoltage of closing no-load lines was simulated and analyzed here using two kinds of typical UHV network structures: the power plant to the grid system and the grid-to-grid system. Different factors, such as line length, mode of operation on the power plant side, and short-circuit capacity, are considered. It is possible for lines shorter than 60 km to eliminate the closing resistor, determined by calculating the probability of flashover. We analyzed the phase selection closing in depth to inhibit the switching overvoltage and considered the factors such as phase deviation and the length of the line. Finally, the applicability of phase selection closing is put forward for lines of different lengths.

Effects of the External Series Impedance on Concurrent Flashover Probability in a Parallel Gap Configuration

Effects of the External Series Impedance on Concurrent Flashover Probability in a Parallel Gap Configuration

Icing Condition Assessment of In-Service Glass Insulators Based on Graphical Shed Spacing and Graphical Shed Overhang

Icing on transmission lines might lead to ice flashovers of insulators, collapse of towers, tripping faults of transmission lines, and other accidents. Shed spacing and shed overhang of insulators are clues for evaluating the probability of ice flashover. This paper researches image-processing methods for the natural icing of in-service glass insulators. Calculation methods of graphical shed spacing and graphical shed overhang are proposed via recognizing the convexity defects of the contours of an icing insulator string based on the GrabCut segmentation algorithm. The experiments are carried out with image data from our climatic chamber and the China Southern Power Grid Disaster (Icing) Warning System of Transmission Lines. The results show that the graphical shed overhang of insulators show evident change due to icing. This method can recognize the most serious icing conditions where the insulator sheds are completely bridged. Also, it can detect bridging positions including the left side, right side, or both sides of the insulator strings in the images.

Estimating the Flashover Probability of Residential Fires Using Monte Carlo Simulations of the MQH Correlation

An important indicator of fire hazard in residential fires is the occurrence of flashover in the room of fire origin. Since the variability of residential fire scenarios is large, many different cases must be considered to evaluate the hazard of a given flammable product. Efficiently predicting the occurrence of flashover of a naturally ventilated compartment is possible using the correlation of McCaffrey, Quintiere, and Harkleroad (MQH). The large variability in United States (U.S.) living room fire scenarios is characterized from available data and propagated through the MQH correlation using Monte Carlo (MC) simulation. For the parameters, for which no relevant data was found, uniform probability distributions were assumed. The scenarios sampled in the MC simulations generally fell within the range of scenarios for which the MQH correlation has been validated. Flashover probabilities were estimated for fires up to 5 MW in heat release rate (HRR) and up to 8 min in duration. It was found that fires with HRRs less than 400 kW have a flashover probability of less than 0.01% regardless of their duration. Typical furniture fires were used as example cases, and it was predicted that a three seat upholstered sofa with a peak HRR of 2.15 MW has a 90% chance of flashing over a randomly chosen U.S. living room. The results of a global sensitivity analysis indicates that the fire location parameter and the vent opening width are the most important parameters affecting the prediction of the occurrence of flashover in U.S. living rooms. The methodology presented is generalizable, and the results can be readily improved by the collection of more data and the use of higher fidelity fire models.

Flashover of insulators in a wet environment

This paper describes a method based on electric field calculation and random walk theory for evaluating insulator performance in a wet environment. Arc bridging of sheds via water particles suspended in close proximity to the insulator has been considered. The role of insulator dimensions (shank diameter, shed diameter and number of sheds) on the flashover performance has been discussed. It is shown that the increased probability of flashover due to the presence of water particles when compared to insulators with only wet surfaces can be quantified. With low to moderate levels of contamination commonly encountered in service, the relative increase in flashover probability is higher due to the presence of suspended water particles than that obtained under heavy contamination.

Evaluating flashover performance of insulators under fire fighting conditions

EHV and UHV high voltage transmission lines frequently pass through high vegetation areas as well as national forests where there is a significant risk of wildfire by natural or manmade causes. Firefighters use aerial vehicles to douse fires by dumping large amount of fire retardant liquid that is conductive, thus increasing the risk of insulator flashover and power interruptions. In this paper, an ammonium polyphosphate based fire retardant liquid was evaluated to understand its effect on insulator flashover properties. Composite and ceramic insulators, suitable for 115 kV transmission lines, were studied by the solid layer test method. Tests were conducted with and without external wetting by fog. Results indicate that there is a high probability of flashover if the entire insulator is coated with the fire retardant liquid. Statistical methods were applied to extend the validity of testing results to predict flashover voltages at higher voltage levels of 230, 345 and 500 kV. It is shown that standard insulator dimensions used for EHV and UHV lines in fire prone areas could be deficient and additional insulation required to improve the flashover performance to acceptable levels in such locations has been determined.

Protection radius of bird-preventing installation design for 110 kV suspension insulators based on withstand tests

According to IEEE Guide for Reducing Bird-Related Outages, several types of transmission line faults could be induced by birds. The number of birds increases as human awareness of environmental protection increases. Thus, the probability of suspension insulator string flashover induced by bird streamer would also increase. Analysis of transmission line faults in Yunnan, China, shows that many transmission line faults occurred in the winter bird migration zone, and most of the towers affected are 110 kV transmission line towers. Hence, investigating the flashover process of the 110 kV transmission line suspension insulator strings induced by bird streamer and designing a well bird-preventing installation for 110 kV transmission line suspension insulator strings is necessary. In this study, a simplified, self-designed 110 kV transmission line tower is utilized to experiment on the three types of 110 kV transmission line suspension insulator string with bird simulation excrement. The analysis of withstand test results has revealed the mechanism of flashover caused by bird streamer of the three types of suspension insulator string, and the protection radius R of the three types of insulator string has been presented. The mechanism of flashover and test results have been testified by means of the distributions of electric field and potential calculated by FEM method. Thus, the bird streamer caused flashover could be classified as the shortest air gap breakdown. Hence, it is valuable for designing or selecting a well bird-preventing installation.

Experimental investigation on pollution flashover performance of multiple parallel suspension insulators

In order to meet the requirement of mechanical properties, multiple parallel suspension insulators will be used in UHV lines. In this paper, pollution flashover performance of multiple parallel insulators has been investigated based on artificial contamination tests under both AC and DC voltage. Four types of multiple parallel suspension mode are used in the tests, which has 2 strings, 3 strings, 4 strings and 6 strings, respectively. Experimental results indicated that multiple parallel insulators’ pollution flashover voltage was lower than that of single string. Study showed that the result is not only affected by the multiple string leads to reduced pollution flashover probability, but also affected by the ionization of air. When the parallel number was less than 4, pollution flashover voltage of multiple parallel porcelain insulators was about 4% lower than that of a single string. If the parallel number was greater than or equal to 4, the value of flashover voltage was reduced by more than 10%. For composite insulators, the voltage drop of the multiple parallel insulators is not as obvious as that of porcelain insulators. In actual project application of the multiple parallel strings, the number of porcelain insulators and the insulation length of composite insulator was proposed to increase. The conclusion of this work can provide technical reference for the external insulation design and construction of UHV transmission lines.

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.

Source strength impact analysis on insulator flashover under contaminated conditions

A mathematical model that considers the source-insulator interaction is presented for modeling flashover of outdoor insulators under contaminated conditions. The model is applied to suspension and post type insulators with wettable surfaces. Several IEEE test cases of practical power systems have been evaluated using PowerWorld® to estimate the source strength. Results included in this paper show that significant difference exists in the probability of flashover in actual power systems when compared to laboratory results using sources that satisfy international standards. It is found in this paper that flashover in service can occur at much lower levels of contamination than predicted by international standards.

Choosing configurations of transmission line tower grounding by back flashover probability value

There is a considerable number of works devoted to electrical characteristics of grounding. These characteristics are important in general. However, in application to grounding of transmission line towers they are not enough to determine what grounding construction is preferable in some particular case, because these characteristics are calculated or measured apart from the grounded object, and only limited number of current (or voltage) source waveforms is used. This paper indicates reasons in favor of the fact that to choose the optimum design of grounding, the calculation model should include the tower as it is. The probability of back flashover, which provides both qualitative and quantitative estimate of the grounding structure efficiency, can be taken as the criterion for the grounding design. The insulation flashover probability is calculated on the basis of engineering method, which evaluates breakdown strength of insulation for nonstandard waveshapes, and probability data on lightning currents. Different approaches are examined for identifying the back flashover probability, as not only amplitudes but also other parameters can be taken into account. Finite-difference time-domain method is used for calculations of transients. It is found that lightning current waveform can greatly influence calculated back flashover probability value.

Flashover Probability Distribution and Volt-Time Curves of Medium Voltage Overhead Line Insulation Under Combined AC and Lightning Impulse Voltages

Lightning overvoltages are considered as one of the major causes of insulation failure in medium voltage overhead lines. Therefore, the evaluation of the dielectric strength of overhead line insulation is of great importance for insulation coordination studies. This paper presents the flashover probability distributions and the volt-time characteristics of different types of insulator gaps subjected to both positive and negative polarity standard impulse (SI) and short tail lightning impulse (STLI) voltages. In practice, the transient overvoltages generated by a lightning stroke are superimposed over the power frequency AC voltage. Accordingly, the flashover characteristics of the insulator gaps under combined AC and lightning-induced overvoltages are also presented in this paper. The results indicate that superimposition of the impulse voltage on the power frequency AC voltage changes the flashover characteristics of the insulation gaps. Therefore, it is recommended that the effect of power frequency AC voltage should be considered in the risk assessment of insulation for the insulation coordination studies.

Probabilistic Risk Assessment of MV Insulator Flashover Under Combined AC and Lightning-Induced Overvoltages

The effect of lightning-induced overvoltages is more profound in overhead distribution lines due to their limited height and low insulation level. Consequently, there is a high risk of line insulation flashover when exposed to lightning-induced overvoltages. This paper presents a probabilistic method to assess the risk of insulator flashover in medium-voltage overhead lines due to lightning-induced overvoltages. In order to accomplish this, a modified Gaussian cumulative distribution function has been used to predict the probability of single-phase, two-phase, and three-phase flashover of insulators under combined ac- and lightning-induced overvoltages. The validity of the modified probabilistic model is confirmed through experiments carried out in the high-voltage laboratory. Next, Monte Carlo simulations were performed on the simplified Rusck's model to generate the distribution of peak lightning-induced overvoltages. Finally, the risk of insulator flashover is calculated based on the distributions of lightning-induced overvoltages and insulator flashover voltages. The proposed procedure could be considered beneficial to select the optimum insulation level required against lightning-induced overvoltages by distinguishing between single-phase and multiphase flashover faults.

Effect of combined AC and lightning-induced overvoltages on the risk of MV insulator flashovers above lossy ground

Soil resistivity is one of the prevailing parameters associated with the flashover of line insulation in medium voltage (MV) overhead lines due to nearby lightning strikes. This paper investigates the influence of lossy ground to determine the level of lightning-induced overvoltages and the corresponding risk of insulator flashovers. Experiments are performed in the high voltage laboratory to evaluate the flashover characteristics of a 24-kV pin-type insulator under combined AC and lightning-induced overvoltages. Based on experimental results, a modified Gaussian cumulative distribution function has been employed to estimate the probability of single-phase, two-phase and three-phase flashover of insulators based on the assumption of ground resistance close to zero. Monte Carlo simulations are performed on Rusck's and Darveniza's models to obtain the probability density function of peak lightning-induced overvoltages for both ideal and lossy grounds, respectively. The results indicate that lossy ground increases the risk of insulator flashovers. The risk indices are further aggravated with increasing soil resistivity and become significantly high for soil resistivity above 3000Ωm. The advantage of the proposed method is its simplicity and ability to distinguish between single-phase and multi-phase flashover faults with increasing soil resistivity. Thus, the proposed model can be successfully employed for insulation coordination studies of MV overhead lines.

Effects of electric field distribution and water drop ejection on flashover of icicles in plane-to-plane gaps

Ice morphology was established from field failure observations on electrical insulators, considering partial and full bridging conditions. Small scale laboratory simulations showed an important effect of droplet ejection on the probability of flashover. During the experiments, factors including applied water conductivity, icicle length, combinations of air gap and icicle length and duration of melting period were investigated using typical in-service stress levels. According to the experimental results, applied water conductivity and icicle length affected the residual ice layer resistance. Flashover voltage decreased by 33% as the conductivity increased from 150 μS/cm to 750 μS/cm and reduced by 10% as icicle length declined from 6 to 4 cm for constant 2-cm air gap. Fully bridged icicles required extra energy for ice melting, leading to higher flashover voltage compared to partially bridged icicles of the same morphology. A single 3-cm air gap had 14% higher flashover voltage than a pair of 1.5 cm air gaps.

Probability of Concurrent Flashover in a Parallel Gap Configuration

Probability of Concurrent Flashover in a Parallel Gap Configuration

The Influence of Main Components in PM2.5 on Composite Insulator Contamination Flashover Characteristics

This paper studies about the influence of main components in PM2.5 on insulator pollution flashover characteristics. A paper written by Institute of Atmospheric Physics Chinese Academy of Sciences presents the main components in Beijing’s PM2.5. Basing on that, an artificial pollution test in our lab. The test result shows that components in PM2.5 have no significant effect on insulator pollution flashover characteristics. But, in haze weather, insulators are easier to be polluted. As a result, the probability of insulator flashover become higher.

Wind-Induced Clearances Infringement of Overhead Power Lines

This paper presents a methodical approach, suggestions and information to design 400 kV transmission lines, check which types of towers, conductors and insulators are suitable to use based on wind velocity. as well as calculate the effect of wind on L12 tower top geometrics, and keep the probability of flashover very low. In this project, weather and climate are considered in the design of overhead lines. The purpose of this paper is to determine the clearances necessary for a 400 kV line to withstand high wind velocity, and ensure that conductors have to maintain the clearances under lightning, switching and TOV (temporary over voltage). Excel and Visio are used to analyse the results and plot them, in order to find the swing angle of movement of the insulator and conductor, and the minimum clearances produced under certain wind speed. Finally, the return period per year and the probability of conductor infringes the clearances per year are calculated.

Comparative study on field collected samples of aged silicon rubber composite coatings for high voltage insulators

Abstract : Pollution of high voltage (HV) insulators is a phenomenon with a considerable impact to the performance of transmission and distribution electrical networks. The use of composite materials and especially Silicone Rubber proved to be an efficient improvement, capable of suppressing the problem and diminishing the flashover probability. As a result ceramic insulators in transmission lines are replaced by insulators with composite housing, either HTV Silicone Rubber or LSR. In the case of HV substations however, the replacement of insulators is rather difficult, due to the complexity of the equipment and the corresponding financial cost. In this case the application of RTV Silicone Rubber is an equivalent alternative. The ceramic insulators are covered with a 0.5 mm RTV SIR coating which provides the advantages of composite insulators on a ceramic substrate. After installation the possible material lifetime, which is determined by the service conditions and the material formulation, is of primary concern. In Crete, a large scale application exists and coatings that exceed a service period of 10 years are still in operation. The present study focuses on the structural and morphological characterization of field collected composite insulators of various ages so that the degradation degree can be correlated with their service.