Lightning Impulse Flashover Voltage Research Articles

Application of a SVR model to predict lightning impulse flashover voltages of parallel gaps for insulator strings

Discharge voltage prediction of engineering gap configurations is a great challenge for external insulation design in high‐voltage engineering. This paper presents an air gap discharge voltage prediction model based on the support vector regression (SVR). A series of features are defined on the shortest interelectrode path of an air gap, and they are extracted from the electric field calculation results using the finite element method. These electric field features are considered as the input parameters of the SVR‐based model, and the output is the air gap discharge voltage. The genetic algorithm is used for parameter optimization of the SVR model. Trained by a few experimental data of the Institute of Electrical and Electronics Engineers standard rod–rod gaps, this model is applied to predict the standard lightning impulse flashover voltages of parallel gaps for insulator strings under different gap configurations. The prediction results coincide well with the experimental data collected from previous publications. The mean absolute percentage error of the eight test samples is within 4.07%, which is acceptable for engineering applications. The SVR‐based model may be helpful to guide the structure design and insulation coordination of parallel gaps and insulator strings, thus reducing the required full‐scale tests. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Flashover voltage computation of porcelain insulators under positive polarity lightning impulse voltages

The porcelain ceramic insulators have been used for outdoor insulation applications for a long time due to their impressive electrical and mechanical properties. The lightning impulse withstand voltage and flashover voltage characteristics of the insulators are important for insulation coordination of transmission lines and substations. This study presents a computer-based simulation methodology to determine the positive polarity lighting impulse flashover voltage of porcelain suspension and station post-insulators. The flashover voltage is determined by evaluating the streamer inception, propagation and sustenance criteria along the suitable paths based on finite-element-method-based electric field computation. The streamer propagation path is found through air along the dry arcing distance rather than along the creepage path of the insulator. The lightning impulse flashover voltage of suspension and post-insulators are computed based on simulation. The comparison of computed results with the experimental results gives a good agreement between them.

Influence of the insulator geometry on the streamer propagation characteristics in polymeric insulators under positive polarity lightning impulse voltages

Polymer composite insulators acquire the position of their counterpart ceramic and glass insulators in high voltage overhead transmission systems due to their remarkable pollution performance. They have different potential and electric field distributions compared to that of ceramic insulators and the absence of intermediate pin and cap metal fittings makes them compact in size by reducing the distance between energised and grounded ends. This study presents a simulation-based approach to compute the streamer propagation characteristics in polymer suspension insulators under positive polarity lightning impulse voltages. The streamer inception, propagation, and sustenance criteria are evaluated using finite element method-based electric field computations. The significance of insulator geometries on streamer inception and propagation phenomena is well established. The insulator geometries such as weather sheds, location of the first shed, and shed radius have shown a strong influence on streamer inception and propagation path, which govern the flashover characteristics of the insulator. The lightning impulse flashover voltages of different insulators were determined experimentally to validate the computed results. A good agreement is found between simulation and experimental results.

Analysis of the dielectric behavior of distribution insulators under non-standard lightning impulse voltages

Overhead distribution lines are often exposed to lightning overvoltages, whose waveforms vary widely and can differ substantially from the standard impulse voltage waveform (1,2 - 50). Different models have been proposed for predicting the strength of insulation subjected to impulses of non-standard waveforms. One of the most commonly used is the "disruptive effect model", but there are different methods for estimating the parameters needed for its application. This paper aims at evaluating the dielectric behavior of medium voltage insulators subjected to impulses of non-standard waveforms, as well as at evaluating two methods for predicting their dielectric strength against such impulses. The test results relative to the critical lightning impulse flashover voltage (U50) and the volt-time characteristics obtained for the positive and negative polarities of different voltage waveforms are presented and discussed.

Surface flashover along polymeric rods partially immersed in LN/sub 2/

Surface flashover phenomena along polymeric rods partially immersed in liquid nitrogen were investigated between parallel electrodes and compared with those fully immersed in liquid nitrogen and those in gaseous nitrogen at cryogenic temperature. It was shown that ac flashover voltage along polymeric rods half immersed in liquid nitrogen was almost the same as that of fully immersed rods when the length of the rod was <10 mm. In the case of 20 mm long rods however, ac flashover voltage was close to that obtained in gaseous nitrogen at cryogenic temperature. Contrary to ac flashover, regardless of the length of a rod to 20 mm, lightning impulse flashover voltage along the rod partially immersed in liquid nitrogen lies between that in liquid nitrogen and that in cryogenic gaseous nitrogen.

Flashover characteristics of vapor‐mist dielectrics under lightning impulse and means to increase the flashover voltage

AbstractIt has been reported that the flashover voltage of gaseous dielectrics may increase greatly when they are mixed with carbon‐halide mist. However, the factors which bring about the increase of the flashover voltage still are not clarified fully. Based on the study on the basic flashover characteristics of vapor‐mist dielectrics, the authors have suggested the scarcity of initial electrons as the predominant factor.This paper investigates further the effect of UV light irradiation, flashover characteristics under forced supply of initial electrons, and for lightning impulse superimposed on dc voltage. These results have verified that the scarcity of initial electrons is the decisive factor in increasing the lightning impulse flashover voltage of vapor‐mist dielectrics. It is clarified also that the scarcity is attributed principally to: (1) the reduction of effective period (lifetime) of initial electrons, and (2) the suppression of photoelectric emission from electrodes.

Withstand voltage characteristics of insulator string covered with snow or ice

The authors describe the results of an investigation into the withstand voltage characteristics of an insulator string covered with snow or ice and the application of these results to the insulation design and maintenance of a transmission line in a heavy snow area. It was found that the characteristics of withstand voltage were affected by the specific gravity of snow, the conductivity of water melted from snow, the height of snow, and the ratio of snow-covered length. The lightning impulse flashover voltage of insulator string covered with snow is much lower than that of nonsnow-covered insulator, indicating that the efficiency of lightning protection greatly decreases under snowing conditions. When transmission lines pass through a heavy snow area, an extension of the transmission system fault is feared in 54 and 275 kV transmission lines because the existing insulator string length does not have sufficient dielectric strength for temporary AC overvoltage and switching surge. >

Influence of impulse voltage waveshape on the flashover of clean and particle-contaminated spacers in SF/sub 6/ gas-insulated system

The influence of the impulse-voltage waveform on the flashover voltage of an epoxy resin disk spacer in a SF/sub 6/-gas-insulated system was studied. The results refer to voltage waveforms for which the two limiting cases are standard lightning impulse and standard switching impulse. It is shown that there is an essential difference between the influence which the impulse waveform has on the flashover voltage of clean and particle-contaminated insulators, respectively. For clean spacers the standard lightning impulse flashover voltage is the highest, while for particle-contaminated spacers it is the lowest. The reduction of the spacer flashover voltage caused by an increase in the particle length is different for impulse voltages of different waveform. For example, for a 14 mm-long particle, the standard lightning impulse flashover voltage is reduced by 70%, while the standard switching flashover voltage is reduced by 35%.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effects of Glow Discharge Conditioning and Addition of Gases on Surface Flashover

The dc, ac (60 Hz), and lightning impulse (1.2/50 ?s) flashover voltages of solid insulators in high vacuum at a pressure of about < 10-6 Pa have been measured before and subsequent to conditioning with an ac (60 Hz) glow discharge at a pressure of 130 Pa. Sulphur hexafluoride, air, nitrogen, and hydrogen have been used in the glow discharge conditioning. Quartz, Pyrex glass, and Teflon insulators in the form of right angle cylindrical spacers sandwiched between a pair of planar stainless steel electrodes are employed. It has been found that the dc, the ac, and the lightning impulse flashover voltages of the insulators in vacuum are higher after conditioning with a glow discharge. The flashover voltage is also measured in the pressure range 10-6 - 5xl0-4 Pa prior to and after glow discharge conditioning. The flashover voltage increases in the pressure range 10-2 - 10-1 Pa only when sulphur hexafluoride is used. No pressure dependence is found in the range 10-6 - 10-1 Pa in air, nitrogen, and hydrogen when using a pair of stainless steel electrodes bridged by any one of the three dielectric materials. SF6 gives the highest surface flashover voltage compared to air, N2, and H2.

Improved Performance of Cylindrical Solid Insulators with Concave Curved Edges in Vacuum

The withstand voltage capability of a cylindrical solid insulator can be improved considerably by incorporating a concave geometry. in the insulator ends at the contact region with the conductors. It is found that the higher the value of the angle γ between the curved edgèof the insulator and the ambient field the lower is the value of the tangential field at the triple junction. The influence of the relative dielectric constant of the material and the length of the insulator on the field at the triple junction is found to be negligibly small. Measurements of the. flashover voltage in. ultrahigh vacuum (< 10-8 Torr) are conducted for cylindrical rods made of glass-ceramic with concave curvature at both-ends. γis varied in the range 0-65°. The DC, AC and 1.2/50 Ps lightning impulse flashover voltages are between 20 to 30% higher than the respective voltages of a straight cylindrical insulator of the same thickness. The DC pre-stress has negligible influence (< 10%) on the subsequent combined DC + impulse and DC + AC flashover voltages, of a concave insulator made of Macor glass-ceramic.

Effect Of Test Geometry, Permittivity Matching And Metal Particles On The Flashover Voltage Of Oil/Solid Interfaces

The 60 Hz and lightning impulse flashover voltages of various pressboards, polyethylene and polypropylene in transformer oil were studied. Except for one discrepancy the two widely different test geometries used generally grade the materials in the same order. It is concluded that the difference between materials is due to different discharge propagation properties, and that the propagation is assisted by permittivity mismatch between liquid and solid. Surface structure can also affect the flashover, especially at impulse voltages. At 50 Hz the introduction of solid surfaces can increase the flashover voltage. Metal particles on the interface can reduce average values of the flashover voltage by at least 20 percent and minimum values even more.