Sparkover Voltage Research Articles

Characteristic of AC surface spark voltage in LN/sub 2/ and LHe

The AC surface spark-over voltages on solid insulators with a back electrode in liquid N/sub 2/ (LN/sub 2/) and liquid He (LHe) were measured and discussed. The larger gap length did not give a higher spark-over voltage. The spark-over voltage in LHe was about two-thirds of that in LN/sub 2/. Experimental studies were carried out on how bubbles in liquid influence surface spark-over voltage. The result shows that it does not influence the voltage.

Air breakdown in the conductor–rod gap under composite slow-front impulse/alternating voltages

The dielectric strength of a conductor–rod gap under combined alternating and impulse voltage is discussed. The effect of the alternating voltage on the impulse profile, under withstand and under the breakdown condition, is described in terms of the angle in the cycle at which the impulse is triggered. Sparkover data is presented for composite voltages using three switching impulses, which have times to peak of 119, 241 and 342 µs in the absence of pre-stress voltage. Results are given for triggering angles 0°, 90°, 135°, 180° and 270° and it is shown that significant variations occur in the composite sparkover voltage depending on both the amplitude of alternating voltage and the triggering angle. The results are discussed in terms of the influence of the corona set up by the alternating voltage.

Power frequency AC sparkover voltage measurements of small air gaps

This paper present results of tests conducted in the High Voltage Laboratory of the Ohio State University and deals with power frequency AC sparkover voltage measurement of small air gaps. Emphasis was placed on obtaining breakdown characteristics of electrodes and air gap separations that are representative of power lines 69 kV and below. Besides the customary rod-rod, rod-plane, and sphere-plane arrangements, other electrode configurations included various conductor sizes with conductor-rod, -plane, and -sphere arrangements and conductor-hot stick configurations with hook, puller, and wire brush accessories. It was found that the conductor size has a significant effect on the sparkover voltage. In addition, for the more practical electrode arrangement involving conductors, the sparkover voltages were greater than the rod-rod results in all but a few instances.

Sparkover in the conductor–rod and conductor–plane test gaps under composite slow front impulse/direct voltages

Measurements of sparkover voltages and of corona in 0.5 and 0.8 m conductor-rod and conductor-plane gaps are described. Where positive pre-stress voltages are used, sufficiently great to produce corona, significant increases in positive composite sparkover voltages are found in the conductor-rod gap, but effects in the conductor-plane gap are insignificant. Sparkover voltage variations are discussed in terms of corona measurements made in the same series of experiments; it is concluded that corona from the rod is dominant in determining conditions for breakdown.

Sparkover in the rod–plane gap under composite slow front positive impulse/alternating voltage

Problems are discussed which relate to the measurement of the sparkover voltage of an airgap under a composite voltage comprising an impulse superimposed on an alternating voltage. The rod–plane gap has been taken as an example. Sparkover values have been measured with the impulse generator fired at angles 0°, 90°, 180° and 270° in the alternating cycle. Tests have also been made with firing at random angles. Sparkover voltages do not differ significantly from those for simple impulse alone, except for the case of random firing, where a small apparent increase occurs. It is shown that the ‘up and down’ method of measurement is satisfactory where specific firing angles are used, but is unsatisfactory and misleading where firing is at random angles.

Sparkover in the rod–plane gap under combined direct and impulse voltages

Experiments are described in which composite voltages, produced by superimposing positive and negative switching impulse voltages on pre-stressing positive or negative direct voltages, are applied to a rod–plane gap of lengths up to 1 m. Changes in 50% sparkover voltages are described over a range of positive and negative pre-stressing voltages; it is shown that negative pre-stressing always reduces the composite sparkover voltage, but that positive pre-stress increases the positive composite sparkover voltage and reduces the negative one. Recordings of photomultiplier observations and ‘still’ photography of pre-breakdown corona have been made at the 50% sparkover level and the results related to the sparkover characteristics. It is shown that the latter depend on the way in which leader initiation is affected by the preceding DC corona.

Tests of the ‘early streamer emission’ principle for protection against lightning

Experiments are described which are designed to test two devices based on the 'early streamer emission' (ESE) principle, for lightning protection, against the traditional Franklin rod. In all three cases, the device was subjected to a steady negative electric field from a sphere, simulating the field beneath a thundercloud, prior to application of a superimposed negative impulse field, simulating the field due to the downward leader. The first device consisted of a vertical rod to which a subsidiary 1/50 /spl mu/s positive impulse voltage, variable up to 40 kV peak, could be applied with varying delays from the start of the negative impulse field. Energising of the rod was thus independent of the applied negative field. The second device was a commercial product, energising of which was controlled by its own power supply. Sparkover voltages in the sphere/device gaps and times to breakdown were measured. It is shown that the ESE devices showed a small advantage, in time to breakdown, over the Franklin rod.

Influence of irradiation on impulse breakdown of sphere gaps and sphere–rod gaps

The influence of irradiation on the breakdown of sphere and sphere-rod gaps for short gap spacings, has been investigated under standard impulse waveshapes of both polarities. It is known that, for short gap spacings, in nonirradiated sphere gaps and sphere-rod gaps the values of the standard deviations of the breakdown probability distributions are relatively large, and the same is true for the influence of humidity on the sparkover voltage. However, the experiments showed that irradiation acts interactively with other discharge parameters, i.e. impulse shape, gap spacing, polarity, absolute humidity, diameter of the spheres and the shape of the earthed electrode. It was found that the breakdown mechanism of small gaps is influenced by irradiation and that the irradiated gaps in most cases become more reliable for measuring and/or calibrating purposes.

Spark breakdown of sphere gaps stressed by standard impulse voltages

Sparkover voltages were measured for quasiuniform field sphere gaps consisting of 75 cm diameter spheres stressed by standard waveshapes of 1.2/50 /spl mu/s and 250/2500 /spl mu/s of both polarities. The work was undertaken with the aims of comparison with existing work with 25 cm spheres, and of investigating the breakdown mechanism of such gaps and the effect upon them of the applied impulse voltage, with emphasis on the waveshape, the gap spacing, the polarity, the diameter of the spheres and the absolute humidity. The factors that influence the breakdown mechanism have been studied systematically, in order ascertain their reliability as measurement standards and, hence, their effect on the sparkover voltage values proposed by the IEC.

Analysis of sphere–rod gaps under standard lightning and switching impulse voltages

The relative low cost of sphere–rod gaps compared to sphere gaps makes it worthwhile to study the breakdown mechanisms of the former, if only to find whether they could be used for calibrating or measuring purposes. Having this in mind, the influence of impulse shape, gap spacing and humidity on the sparkover voltage of sphere–rod gaps always in comparison with sphere gaps has been investigated under standard impulse waveshapes. It is found that breakdown is influenced by humidity, waveshape and especially by polarity. With these factors in view, the breakdown mechanism and the feasibility of using them as measuring substandards is discussed.

Assessment of air‐density correction for practical electrode systems

AbstractA survey is given of available data on the effects of air density upon pre‐discharges and sparkover for nonuniform field geometries under a variety of voltage waveshapes of both polarities. Semi‐empirical relationships describing the trends of the experimental data are presented. The correction methods set up by the IEC are reviewed and some emphasis has been given to the need to distinguish between the separate influences of pressure and temperature in the overall air‐density correction factors. The limits of applicability for density correction are also discussed. Finally, a generalised semi‐empirical equation taking into account the main atmospheric variations is proposed. This is shown to offer a good accuracy for the calculation of sparkover voltages over a wider density range than has been hitherto been possible.

Very high frequency overvoltages at open air EHV substations during disconnect switch operations

EHV open air substations experience flashovers produced by very high frequency overvoltages (VHF OV) during disconnect switch operations. It was found, during field tests, that the highest measured value of sparkover voltage across the EHV switch gap between contacts during its operation was 1.9 p.u. The most unfavourable scheme, theoretically, occurs when a bus is fed from a single transformer, without outgoing lines. Calculations showed that in 787 and 525 kV substations with branched buses, VHF OV could result in flashovers on a bus open end with unfavourable ratio of bus lengths. For transformer insulation protected with an arrester, VHF OV are not dangerous. Some protective measures are recommended.

Sparkover voltage of sphere gaps under standard lightning and switching impulse voltages

The influence of impulse shape, gap spacing and of humidity on the sparkover voltage of sphere gaps consisting of 25 cm diameter spheres has been studied for both polarities under standard waveshapes of 1.2/50 /spl mu/s and 250/2500 /spl mu/s. Results are compared with the values proposed by the IEC standard and an attempt to explain the breakdown mechanism and especially how it is influenced both by humidity and the waveshape has been made. Further suggestions are given concerning their use as calibrating gaps, especially under switching impulses. Also, under certain conditions, the inclusion in the calibrating gaps of 15 cm gaps is proposed.

A 60 Joule, 600 kV, 500 ps risetime, 60 ns pulse width Marx generator

The experimental and computer modelling of the performance of a 24-stage coaxial line Marx generator are presented. Internal inductance, stray capacitance, coupling capacitance between the stages and switch characteristic are considered. The detailed geometry of each stage and of the total system is included in the computer simulation. The UV pre-ionized spark gaps forming the switches in the system are characterized by the spark overvoltage. The transmission line effects are also observed. The output characteristics are given at different overvoltages.

Characteristics of discharge induced by laser‐generated plasmas in atmospheric air

AbstractThe possibility of laser‐induced lightning has been investigated for the future application in the protection of electric power systems from lightning strokes. This technique induces lightning strokes artificially along a laser‐produced breakdown plasma channel and conducts them to a harmless place on the ground.To clarify the plasma formation mechanism and the discharge (sparkover) induction process, sparkover induction experiments are performed for two types of long‐gap electrode configurations, plate‐to‐rod and plate‐to‐plate, with a 50‐J TEA CO2 laser.In the present paper, the sparkover voltage characteristics and interferometric measurement on laser‐produced plasma mainly are described, together with sparkover induction for a series of metal beads arranged equidistantly which simulate a plasma channel.

Critical switching impulse strength of long air gaps: modelling of air density effects

A physical modeling approach to investigate the effect of reduced air density on leader inception and sparkover of long air gaps under positive switching impulses with critical front time is presented. The model accounts for the effect of air density on continuous leader inception voltage, leader length, and sparkover voltage. The results of the model provides critical positive switching impulse air density correction factors for rod-plane, rod-rod, conductor-plane, conductor-rod, and conductor-tower window gaps over a wide range of gap distances and relative air densities. The model findings were extensively checked against previous experimental results with quite satisfactory agreement.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Minimum sparkover voltage characteristics in air, sulphur-hexafluoride, and some solid dielectrics: A macroscopic approach

Using a small number of plausible assumptions regarding the nature of the leader channel and the corona cloud in an electric discharge in a continuous medium, it is shown how a nonlinear first-order differential equation of the d’Alembert type can be derived which describes the history of the leader and corona cloud development in nondimensional terms. It is found that this solution can be used to derive a breakdown criterion which depends only on two material parameters, i.e., the minimum energy per unit length of the elongating leader channel and the average electric field in the corona cloud. These can be combined to form a single nondimensional parameter (Kekez number). The theory also shows how, for large gaps, the discharge must necessarily lead to the stepped-leader phenomenon. Comparison with experiments of other authors shows good agreement even for dielectrics of other than gaseous states of aggregation, where the concept of corona cloud requires some reinterpretation.

Applying metal-oxide surge arresters on distribution systems

The primary advantages of a metal-oxide surge arrester over a silicon-carbide surge arrester are identified as the extreme nonlinearity (flatness) of the metal-oxide voltage versus current characteristic and the elimination of the requirement for a series gap and its associated gap sparkover voltage. Without a gap, the metal-oxide valve element must support system voltage continuously, necessitating the consideration of thermal stability. Maximum continuous operating voltage and temporary overvoltage capabilities are the important parameters for selecting a metal-oxide surge arrester for a particular application.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Self-similar properties of minimum sparkover voltage in air for both polarities in rod-plane (0.1-30 m) gaps

It is suggested that the sparkover voltage can be described in terms of two parameters: (1) the minimum energy per unit length, W, supplied to the leader channel to enable the channel to elongate and (ii) the corresponding (average) value of the field, E, in the corona (streamer) cloud. For positive polarity W=80 J m-1 and E=5 kV cm-1, and for negative polarity W approximately=118 J m-1 and E=16 kV cm-1. The formulation can be extended to cover other electrode configurations and dielectrics.

Negative Corona in a Multiple Interacting Point-to-Plane Gap in Air

Negative corona discharge from multiple interacting points has been investigated with emphasis on the role of negative streamers in causing spark breakdown. The interaction among the evenly spaced bright discharge spots existing along a negative corona wire was simulated with a multiple point electrode whose point-to-point spacing was adjustable. The points were arranged in a circular and linear pattern. Although the current-voltage characteristic curves for the multiple point electrode-to-plane geometry are different from that for a single point-to-plane, the sparkover voltages are the same for both geometries. Additionally, sparking was always observed between the central discharge point and the ground plate. The dependence of the corona current on the configuration of the cathode, the effects of streamer branching on the sparkover, and the breakdown voltage versus gap length relation will be discussed. The experimental results confirm that negative streamers are less dominant than positive streamers in affecting spark breakdown.