Critical Flashover Research Articles

Electrical degradation of fiberglass distribution line pole

This paper presents the results of experiments conducted on fiberglass distribution line pole. First, the critical flashover (CFO) voltage of the fiberglass distribution line pole and its combination with polymer and porcelain insulators is given. Then, the data obtained from the study performed on accelerated aging of fiberglass distribution line poles are presented, followed by a discussion of the results

Effect of insulator diameter on AC flashover voltage of an ice-covered insulator string

The effect of the diameter of an insulator covered with ice on its flashover voltage was investigated. The insulator diameter was simulated and varied by controlling the width of a layer of ice artificially accreted on a short string of 5 IEEE standard units. The 50% withstand voltage (V/sub 50/) was experimentally determined using the method described in IEC 60507. The results show that the V/sub 50/ decreases as the width of the ice layer increases. Moreover, a mathematical model for predicting the critical flashover voltage of ice-covered insulators is proposed, and is validated against the experimental results. The model is then applied to ice-covered industrial insulators with different diameters yielding good concordance between the results from the model and the experimental ones.

Lightning Performance of 500-kV Double-Circuit Line Schemes for the Three-Gorge Project

A few problems in the present calculation methods in the aspects of insulation flashover criteria, influence of operating voltages, induced overvoltage component, and shielding failure in China are briefly analyzed and discussed. Lightning performance of 500-kV double-circuit line schemes (nine tower types and thirteen schemes of insulation disposition) for the Three-Gorge Project is computed and compared with the existing operation experience of double-circuit lines. Different schemes are compared with each other. The cause of the remarkable increase of the total lightning outage rate of the usual unbalanced double-circuit lines, with one circuit insulation reduced, in the operation is analyzed. Different from the usual unbalanced insulation lines, a one-circuit insulation enhanced scheme is proposed in this paper. It is emphasized in the paper that the key of the lightning protection of double-circuit transmission lines is to reduce the double-circuit simultaneous trip-out rate and to keep a reasonable total trip-out rate at the same time. It can be achieved by choosing the proper insulation disposition scheme referring to the order of the weighted trip-out rate values. The viewpoints and methods in the paper would be useful to others working in this field.

‘Bridge-mixing’ of Gasified Fuel Vapors and Simultaneous Ignitions of Different Combustibles at Flashover

‘Bridge-mixing’ of igniting fuel vapors gasified from different combustibles by thermal radiation during flashover is presented in this article. This explains why combustible items of different ignition temperatures appear to be ignited at roughly the same time in flashover. Sample cubes of polymethylmethacrylate (PMMA), polyvinyl chloride (PVC), polycarbonate (PC), and wood are tested in a cone calorimeter at high thermal radiative heat fluxes of 50 and 70 kW m 2. Two samples are tested each time. Different arrangements of the fuel tray are made to give different mixing of fuel vapors. It is observed that fuel vapors gasified from combustibles with different ignition temperatures are ignited simultaneously under some mixing conditions, depending on the geometry of the fuel tray under the cone. Mixing of gasified fuel vapors by the fire-induced airflow in rooms of different geometry is a key factor in igniting each combustible item. This releases more heat and generates flames to give conditions satisfying the flashover criteria.

Added CFO Voltage by the Fiberglass Distribution Line Pole

This paper presents the study results conducted on the critical flashover (CFO) voltages of the fiberglass distribution line pole and its combination with polymer and porcelain insulators. Special attention was made to evaluate the added CFO voltages by the fiberglass pole to the insulators when pole served as a second component in the distribution line structure. A discussion of the results follows the data obtained.

Experimental study and mathematical modelling of flashover on extra‐high voltage insulators covered with ice

AbstractUsing a test method developed at the high‐voltage laboratory of the NSERC/Hydro‐Quebec/UQAC Industrial Chair on Atmospheric Icing of Power Network Equipment (CIGELE), the relation between the minimum flashover voltage VMF and the insulator dry arcing distance for standard porcelain station post insulators, as typically used in Hydro‐Quebec substations, was investigated under icing conditions. The experimental results show that, under wet‐grown ice, known as the most dangerous type of ice for power transmission systems, the VMF increases nonlinearly with an increase in insulator length. Based on these results, an improved mathematical model for predicting the critical flashover voltage versus length of ice‐covered insulators is presented. This model is helpful for understanding the flashover phenomenon on ice‐covered insulators and presents a powerful tool for choosing the proper length of outdoor insulators in cold climate regions. Copyright © 2004 John Wiley & Sons, Ltd.

Electrical performance of ice-covered insulators at high altitudes

This paper presents results on the flashover of ice-covered insulators at low air pressure. The flashover performance of two different types of insulators covered with polluted ice was experimentally determined and the effects of several determining factors, such as the air pressure, ice severity, and pollution rate, were discussed. Based on the laboratory investigations, a mathematical model for predicting the critical flashover voltage of ice-covered insulators at low air pressure was established. There is optimal agreement between the calculated and experimental results.

Dynamic modeling of de arc discharge on ice surfaces

A dynamic model for predicting DC arc behavior and critical flashover voltage of ice-covered insulating surfaces is presented. The model takes into consideration insulating geometry, pre-contamination level, and characteristics of ice layers. Assuming arc behavior as a time dependant impedance, it is possible to determine various arc characteristics such as time histories of leakage currents, potential gradient, channel radius, trajectory, propagation velocity and the energy injected into the zones free of ice (also called air gaps). The simulated results provided by the model are in agreement with those obtained experimentally using a simplified ice-covered cylinder as well as a short string of five IEEE standard porcelain suspension units covered with artificial ice.

Comparison of Criteria for Room Flashover

In structural fires flashover is characterized by the rapid transition in fire behavior from localized burning of fuel to the involvement of all combustibles in the enclosure. Major parameters affecting flashover are firegrowth rate, ventilation opening area, and room area. A comparison of flashover criteria is undertaken using the Thomas, Babrauskas, and the CFAST/FASTLite criteria, concentrating on the similarities and differences between the criteria in their assessment of the major parameters affecting the time to reach flashover.

Propagation of ac and dc arcs on ice surfaces

Using a high-speed camera, the fundamental behavior of arcs on ice surfaces, including propagation velocity and root radii were studied under both ac and dc voltages. It was found that the arc may propagate randomly inside or outside of the ice with a relatively low velocity before the critical length is reached, and then at a much higher velocity. Moreover, the relationship between the radius of the arc root and leakage current was determined. The results obtained allowed us to improve our previous model dealing with insulators. There is a good concordance between the critical flashover voltage of ice-covered insulators calculated from the model and laboratory results.

Modelling of HVDC wall bushing flashover due to uneven wetting

An electric field based flashover model is proposed to predict the flashover voltage of an unevenly wetted HVDC wall bushing. Criteria for initiation and propagation of discharges have been incorporated in the model. The critical flashover voltage of a 600 kV wall bushing has been predicted and it is shown that the flashover stresses are very close to experimentally derived values, i.e. 60-70 kV/cm. Important aspects of wall bushing flashover such as critical dry zone length, the U-shaped characteristic of critical flashover voltage versus dry zone length, the effect of waterdrops, influence of rain conductivity and altitude are well reproduced and explained by the model.

Predictability of Flashover by Zone Models

Two additional flashover inspection criteria were established from a recent study on identifying routes to chaos in a compartment fire. There, phase space diagrams on fire temperatures and their time rates of change were analyzed. The proposal was applied to study whether flashover can be predicted by a two-layer zone model. Two popular zone models ASET and CFAST were se lected. Compartments of different sizes and fires with different heat release rates were considered. Transient values of the fire temperatures and their time deriva tives were studied. It is concluded that in addition to the current flashover crite ria based on gas temperature near to the ceiling, heat flux at the floor level and flames coming out of the openings, the rate of change of fire temperature should be inspected.

Multiple regression technique for estimating the insulation strength of series dielectrics on distribution systems: A statistical approach

A method of estimating the lightning-impulse critical-flashover (CFO) insulation strength of two components used on distribution construction is presented. The multiple regression technique (MRT) has been applied to comprehensive CFO data of two dielectric materials in series. General CFO populations of two-component models and sample models for different combination of two materials have been developed. A diagnostic correction test is performed for the general case and for each combination model, to decide which models fit well. Suggestions are made regarding the more accurate prediction model, and the main factors that might have affected predicted results are emphasised. Also, a procedure to predict values outside the experimental results range is described for other sizes and lengths of the tested components. This procedure may be a good tool for finding better insulation added in distribution systems.

Modeling of altitude effects on AC flashover of polluted high voltage insulators

The paper introduces a new physical approach to account for the effect of reduced air density on the flashover voltage and critical leakage current of polluted high voltage insulators. The analysis starts by updating the mathematical model, previously established, of power frequency flashover of polluted insulators at normal atmospheric pressure. It then proceeds to introduce the effect of ambient pressure on the physical parameters of the dielectric recovery equation. The effect of reduced pressure on the arc boundary radius is investigated. The combined effect of humidity and reduced air density on the dielectric strength at ambient temperature is also accounted for. The above analysis results in a new expression for the reignition voltage which includes ambient pressure effects. The analytical findings are then used to investigate the effect of reduced air density on the critical leakage current and flashover voltage of simple-shaped polluted insulators. The effect of more complex profiles is subsequently introduced. The model results are compared with experiments and the agreement established is quite satisfactory. Finally simple practical altitude correction factors for polluted insulators are proposed.

Estimation of lightning impulse insulation strength of two series dielectrics using the least square technique

In this paper, the least square (LS) estimation technique was used to estimate the total critical flashover (CFO) for combinations of two insulating materials from the experimental data obtained. This method had acceptable accuracy when applied to the total population of the two-component combinations but lost some accuracy for some of the sub-sets of the two materials.

Experimental verification of HVDC test sources requirements for pollution tests

This paper presents the results of tests conducted to verify the effect of the voltage source on the critical flashover voltage of DC insulators under pollution. Tests were performed under 3 pollution levels using voltage sources with different voltage drop x leakage current characteristics. A data acquisition system was used to record all current pulses above a certain pre selected current level and the corresponding voltage evolution during current pulse duration, which made possible a complete analysis of current pulses and voltage drops characteristics. The experimental results show that the present restrictions imposed on the DC voltage sources can probably be relaxed. Also from the experimental data collected it was found that the insulator tested presents a constant impedance x current characteristic. This characteristic can be useful in checking theoretical models for polluted insulator flashover. >

Polluted insulator flashover model for ac voltage

Considerable theoretical and experimental research on polluted insulators has yet to yield a perfect generalized mathematical model which can predict accurately critical flashover voltages and currents. In this paper an ac model has been proposed especially applicable to static arcs. The salient feature of the proposed model is that it takes into account the appropriate selection of arc constants for different chemical nature of the pollutants. The computed results closely follow the experimental curves and can predict rms value of critical flashover voltages with a mean absolute error (MAE) of >

Calculating the switching surge critical flashover voltage of phase-to-ground and phase-to-phase bundle conductor gaps

The paper presents a method for calculating the 50% breakdown voltages of conductor-to-plane and conductor-to-conductor air gaps for any bundle design, all the three phases of lines being taken into account. The method is developed for switching impulses with long time front (/spl tau/f=1000-4000 /spl mu/sec) which are the most typical for UHV lines. The results of calculations are extensively compared with previous experimental results for different bundles and over a wide range of gap spacings. The influence of bundle configuration, type of phases displacement (flat, delta or inverted delta) and voltage distribution among phases for UHV lines are analyzed. >

Switching impulse strength of compact transmission lines

This paper summarizes test results which were carried out to investigate the influence of several parameters in the switching impulse strength of phase-to-phase insulation. As a consequence, a new formulation for the phase-to-phase critical flashover voltage is presented. Test results are also compared with those available in the literature, taking into account the differences among the various configurations used.

AC and lightning performance of fiberglass crossarms aged in 115 kV transmission line

This paper presents the results of an investigation of the electrical performance of 115 kV transmission line fiberglass crossarms used by Mississippi Power and Light Company. A transmission line fiberglass crossarm removed from service and companion crossarms stored outdoors but not in service were examined. The evaluation of electrical performance was based on flashover voltage value at AC voltage and standard lightning impulse as well as under dry and wet conditions. The tests were performed in the Mississippi State University High Voltage Laboratory. The obtained flashover voltages show no large difference in electrical strength of fiberglass crossarms removed from service and those stored outdoors. The added critical flashover voltage by fiberglass crossarm to the porcelain suspension insulators is presented versus the length of the fiberglass crossarm for dry and wet conditions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>