Flashover Probability Research Articles

Characterising the discharge development due to surface contamination in GIS using the UHF technique

The effect of surface contamination on GIS spacers has been investigated using UHF partial discharge measurements. The discharge characteristics have been recorded, enabling surface contamination to be distinguished from other sources of partial discharge. In addition, a method of determining the flashover probability is discussed.

On the assessment of extremely low breakdown probabilities by an inverse sampling procedure (gaseous insulation)

First breakdown voltages obtained under the inverse sampling procedure assuming a double exponential flashover probability function are discussed. An inverse sampling procedure commences the voltage application at a very low level, followed by applications at stepwise increased levels until a breakdown occurs. Following a breakdown, the procedure is restarted at the initial level. The procedure is repeated until a predetermined number of breakdowns have occurred, and the average and standard deviation of the observed first breakdown levels are recorded. The authors derive the relation between the flashover probability function and the corresponding distribution of first breakdown voltages under the inverse sampling procedure, and show how this relation may be utilized to assess the single-shot flashover probability corresponding to the observed average first breakdown voltage. Since the procedure is based on voltage applications in the neighbourhood of the quantile under investigation, the procedure is found to be insensitive to the underlying distributional assumptions. >

Flashover vulnerability of transmission and distribution lines to high-altitude electromagnetic pulse (HEMP)

The vulnerability of transmission and distribution lines to flashover from the electromagnetic pulse generated by a nuclear detonation 400 km above the Earth is estimated. The analysis consists of determining the cumulative probability of induced voltage on three-phase lines, including shield and neutral conductors, for four operating voltages and then comparing these stresses to estimates of line insulation strength. The 12 kV line is shown to be vulnerable to HEMP-induced flashover. The 69 kV line is marginally vulnerable to HEMP-induced flashover, but only the minimum-insulation designs in current use are susceptible, and the probability of flashover for those designs is minimal. The 500 and 230 kV lines are invulnerable to HEMP-induced flashover for all HEMP field levels considered. >

Model experiment of upward leaders-shielding effects of tall objects

Discharges originating from laboratory upward leaders are studied experimentally with a configuration consisting of two parallel vertical rods at ground and an elevated plane. The flashover probability of each rod, the effect of gap configuration on that probability, and the shielding effect of the taller rod on the smaller one are investigated. Experimental results are analyzed, taking into account the deviation of time to flashover of the gap. Application of the results to the case of natural lightning is also discussed. >

Townsend coefficients for electron scattering over dielectric surfaces

A method for describing the probability of initiating flashover discharges across dielectric surfaces is presented in which we define a transport coefficient for electron multiplication similar to the Townsend coefficient used for gas discharges. The coefficient is a function of the scaling parameter (charge released from the cathode)/(cathode-anode separation) and is also a measure of the growth of the sheath on the dielectric surface resulting from electron scattering. We discuss results for when the source of seed electrons does not necessarily depend upon field emission at the cathode-vacuum-dielectric triple point. This may occur when the surface is illuminated by UV radiation. For these conditions, there is a different functional dependence of flashover probability on voltage and geometry (e.g., thickness of the insulator) than when field emission provides the seed electrons. As a result, criteria previously used to predict flashover discharges may not apply.

Characteristics of interelectrode flashover in air with the existence of a weakly ionized plasma channel induced by a KrF laser (248 nm)

The irradiation of a KrF laser (248 nm) into the interelectrode space affects the electric flashover characteristics there. The results obtained show that negative oxygen ions dominate formation of the interelectrode arc in air. A laser beam is introduced parallel to the interelectrode axis and the equipment is arranged to prevent the occurrence of photoelectric effects on the electrode surfaces. The time separation between laser firing and application of the interelectrode voltage is varied from 10 ns to 10 ms. The interelectrode spacing is set in the range 2–12 cm. The arc formation time and the flashover probability are measured. The initial ion number density is estimated experimentally to be on the order of 1011 cm−3 throughout the channel.

Probabilistic Short-Circuit Uprating of Station Strain Bus System???Probabilistic Formulation

This paper is the second in the series dealing with probabilistic station strain bus uprating. The paper describes the formulation, computational procedure, and analytical techniques for evaluating insulator assembly failures due to conductor bundle collapse and conductor swingout. In addition, the problem of conductor clearance has been formulated to determine the probability of interphase flashover. This paper provides detailed descriptions of all the steps involved in the comprehensive probabilistic analysis and presents an application of this technique to a real case of a transformer station (Middleport TS) in Ontario Hydro's power system.

Calculation of Minimum Safety Distances for Live-Line Maintenance A Statistical Method Applied to 765 kV AC Itaipu Lines

This paper presents a statistical method to calculate the minimum safety distance for live-line maintenance. The method takes into account the complexity of the tower gap configuration and the presence of the lineman working at the line potential. It includes the inherent dispersion of the dielectric characteristics of the air gaps such as the probability of flashover and the times-to-flashover. Laboratory tests were also performed to corroborate the results of the study, checking not only the performance of the various live-line tools but also the effectiveness of the by-passing insulators protective device and the occurrence of broken insulators in the string.

Calculation of Ninimum Safety Distances for Live-Line Mintenance - A Statistical Mehod Applied to 765 kV AC Itaipu Lines

This paper presents a statistical method to calculate the minimum safety distance for live-line maintenance. The method takes into account the complexity of the tower gap configuration and the presence of the lineman working at the line potential. It includes the inherent dispersion of the dielectric characteristics of the air gaps such as the probability of flashover and the times-to-flashover. Laboratory tests were also performed to corroborate the results of the study, checking not only the performance of the various live-line tools but also the effectiveness of the by-passing insulators protective device and the occurrence of broken insulators in the string.

Probabilistic Short-Circuit Uprating of Station Strain Bus System - Probabilistic Formulation

This paper Is the second in the series dealing with probabilistic station strain bus uprating. The paper describes the formulatlon, computational procedure and analytical techniques for evaluating Insulator assembly failures due to conductor bundle collapse and conductor swingout. In addition, the problem of conductor clearance has been formulated to determine the probability of Inter-phase flashover. This paper provides detailed descriptions of all the steps involved In the comprehensive probabilistic analysis and presents an application of this technique to a real case of a transformer station (Middleport TS) in Ontario Hydro's power system.

Modeling of fire spread through probabilistic networks

The postflashover fire spread from room to room is treated in a stochastic analysis beginning with the development of a probabilistic network, and followed by a method of solving the network for discrete probability distributions. The first step in this analysis is the construction of a graph representing a space in which the rooms are nodes, and the walls and other fire barriers are links between the nodes. The space network graph is then transformed into a probabilistic network by introducing one node for representing the preflashover state and another node for representing the postflashover state of each room with the link between them representing the probability of flashover and the time characteristic to flashover. Each link between a flashed-over room and an adjacent space has a probability of the barrier being breached and a characteristic time of fire resistance. The probabilistic network is then solved by creating an “equivalent network” which has multiple links between the nodes to represent the uncertainty intrinsic to fire spread. For instance, a door may be open or closed. This would be represented by two links, one with the probability of the door being open with a characteristic time of zero, and the other with the probability of the door being closed with the time associated with the fire resistance of the door. The analysis of the possible flow through equivalent networks is discussed and the probability of a source node connecting with the sink node as well as the expected shortest travel time are calculated. Finally a numerical example is solved in which the source node is the room of origin of a fire, and the sink node is a section of the corridor which is critical to the escape of the occupants in nearby rooms Two cases are developed, one with “5-minute” doors and the other with “20-minute” doors and automatic closures. A different fire scenario is shown to be represented by each path through the equivalent network, and the probability and characteristic time for each of these scenarios is also calculated. The consequence of the changing to a 20-minute door is presented in quantitative terms and the probability of the door being open is used as a sensitivity parameter.

Possible Precision of Statistical Insulation Test Methods

The report deals with insulation test methods where voltages of constant shape but different amplitudes are applied to a test object to determine the voltage which corresponds to a certain probability of flashover. A hypothetical test method is considered which gives the minimum statistical error in the determination of that voltage for any given number of voltage applications. It is shown that some existinig test methods give nearly this result. There is therefore no need for further development of such methods. Present methods to determine voltages corresponding to low probability of flashover are criticized.

Evaluation of a New Protective Coating for Porcelain Insulators

This paper describes the evaluation of a new elastomeric coating made from a silicone rubber compound for use on porcelain insulators. Field experience has shown that coated insulators have a lower flashover probability under wet contaminated operation than uncoated insulators.

Lightning Insulation Coordination for a ??600-kV DC Gas Insulated Cable

To determine the required BIL of a +600 kV dc gas-insulated cable, a probabilistic insulation coordination study was performed. An equation was developed to determine the maximum voltage on the cable as a function of directly connected highly nonlinear surge protective devices (MO). After estimating the distribution of surge voltage magnitudes and shapes which arrive at the line-cable junction, the probability of flashover within the cable was determined. The results, analyzed in terms of mean time between failures, MTBF, verified that a 1300 kV BIL was adequate.

Ligntning Insulation Coordination for a ±600 kV DC Gas Insulated Cable

To determine the required BIL of a +600 kV dc gas-insulated cable, a probabilistic insulation coordination study was performed. An equation was developed to determine the maximum voltage on the cable as a function of directly connected highly nonlinear surge protective devices (MO). After estimating the distribution of surge voltage magnitudes and shapes which arrive at the line-cable junction, the probability of flashover within the cable was determined. The results, analyzed in terms of mean time between failures, MTBF, verified that a 1300 kV BIL was adequate.

Flashover test of Artificially iced Insualtors

Test apparatus and procedures were developed to test energized, non-contaminated, 120 kV system insulators under simulated freezing rain conditions. The flashover performances were compared between non-ceramic composite transmission line insulators and standard porcelain 53/4 x 10 inch suspension insulators. Standard and long leak Multicone porcelain insulators were also compared in the icing tests to standard porcelain station post insulators. Icing tests at several voltage stresses provided the full flashover-range of the insulators from the minimum or zero flashovers in ten trials up to the maximum or 10 flashovers in 10 trials. Using the desired insulator voltage stress flashover curve for specific insulator types and the voltage stress applied to the insulator, the comparative probability of flashover due to ice storms on the system can be evaluated for 120 kV, 230 kV and 345 kV systems.

Flashover Tests of Artificially Iced Insulators

Test apparatus and procedures were developed to test energized, non-contaminated, 120 kV system insulators under simulated freezing rain conditions. The flashover performances were compared between non-ceramic composite transmission line insulators and standard porcelain 5 3/4 x 10 inch suspension insulators. Standard and long leak Multicone porcelain insulators were also compared in the icing tests to standard porcelain station post insulators. Icing tests at several voltage stresses provided the full flashover range of the insulators from the minimum or zero flashovers in ten trials up to the maximum or 10 flashovers in 10 trials. Using the desired insulator voltage stress flashover curve for specific insulator types and the voltage stress applied to the insulator, the comparative probability of flashover due to ice storms on the system can be evaluated for 120 kV, 230 kV and 345 kV systems.

Distributions of front time of switching overvoltages: their influence on line flashover probability

With the use of digital-simulation and hybrid-simulation techniques, investigations have been carried out on the distribution of front time of switching overvoltages caused by line energisation. The effect of front time on flashover probability of a line has been clarified. Further consideration is given to the variation of the front-time distribution due to various parameters such as line length, source inductance and so on.

Study on overvoltages due to nonsimultaneous closure by hybrid method

A new hybrid computer system for the statistical analysis of switching surge behavior consisting of the transient network analyzer (TNA) and a digital computer (used for data processing and control of TNA) was developed. The nonsimultaneous closure of the circuit breaker has a great effect on the maximum overvoltage and little effect on the average of overvoltage but it has a great effect on the standard deviation of overvoltage. The tower flashover probability is greatly dependent on the location of the steel tower. The line flashover probability is greatly dependent on the closing delay time of the circuit breaker. The distribution curve of the switching surge under nonsimultaneous closure is more similar to normal distribution curve than that under simultaneous closure.

A method to estimate the lightning multiline flashover on transmission line towers

Most transmission line faults are caused by lightning. In Japan under ordinary circumstances, two circuits of transmission lines are strung on one tower due to the limited right of way. Therefore it is not rare that the two circuits break down simultaneously. Since the simultaneous two-circuit fault has a very serious effect on the security of power systems, it is very important to predict accurately the probability of multiline flashover. Much useful data have already been accumulated about the potential distribution of the transmission tower subject to lightning stroke. These data are used to analyze the mechanism of multiline flashover, and a tower potential rise method (abbreviated as TPR method) for the analysis of multiline flashover phenomena is proposed. In this method, it is possible to take into account the surge impedances of the transmission line and transmission tower and the coupling factor between the power line and ground wire, as well as the potential distribution of the transmission tower. In the proposed method, the conventional formulas for lightning stroke current and the frequency distribution curve of lightning stroke current, which have been used widely so far in designing the insulation level of the transmission line, are used. Hence, itmore » is possible to calculate the multiline flashover probability without modifying the basic principle of conventional insulation design procedures.« less