Rod-plane Gap Research Articles

Breakdown voltage of compressed SF6 at very low frequency/low frequency (VLF/LF)

SUMMARY The breakdown voltage (BDV) of compressed sulfur hexafluoride (SF6) in the pressure range of 1–5 atm at 30 kHz, the frequency used by Navy for communications with vessels, is presented. Experiments were conducted in both uniform field (plane–plane gap) and non-uniform field (rod–plane gap). The impact of pressure on the BDV is quantitatively analyzed. The pressure correction factors for calculating the BDV of SF6 at 30 kHz have been developed from knowledge of breakdown characteristics of air and from regression models. Copyright © 2010 John Wiley & Sons, Ltd.

Negative impulse flashover along cylindrical insulating surfaces bridging a short rod-plane gap under variable humidity

Negative impulse flashover along insulating surfaces bridging a short rod-plane gap is investigated under variable humidity. The specimens, cylindrical in shape and with a smooth surface, were made of PTFE, silicone-rubber, nylon and glazed porcelain and were bridging the gap, which was stressed by standard lightning and switching impulse voltages. Breakdown probability distributions were obtained and the breakdown voltage and time to breakdown were measured. The gap was overstressed by applying voltages higher than that causing 100% breakdown so as to study the effects of the applied peak voltage on breakdown characteristics. When breakdown occurs over an insulating surface the breakdown voltage is significantly lower compared to that obtained for air alone, especially at lower breakdown probabilities and under switching impulse voltages. It is also lower for lightning than switching impulse voltages, this being less marked with increasing material permittivity. Breakdown is closely related to negative corona growth. The effect of absolute humidity on breakdown voltage is minimal. The IEC atmospheric conditions correction procedure yields satisfactory results when breakdown occurs both in air alone and along an insulating surface.

Interception Probability and Proximity Effects: Implications in Shielding Design Against Lightning

The effects of a neighboring object on the interception probability distribution of an air terminal are investigated through scale model experiments. The discharge interception probability of an earthed rod is influenced by the presence of a neighboring shorter one when both inserted in a rod-plane gap. This depends on the polarity of the impulse voltage, the height of the earthed rods and their displacement with respect to the energized rod. The experimental results are interpreted in terms of the breakdown mechanism and electric field calculations; their possible extension to shielding design is discussed. Proximity effects may result in a reduction in the striking distance and interception radius of an air terminal, especially at the lower interception probabilities. A new approach for shielding design is introduced, which considers, besides lightning parameters and height of the prospective struck objects, interception probability and proximity effects.

Lightning impulse behaviour of short rod–plane gaps with a dielectric-covered rod

The breakdown mechanism of dielectric–covered rod/plane air gaps under positive lightning impulse voltages is investigated. Several gap lengths, varying between 2.5 and 15 cm, were employed and the simple air gaps were regarded as reference. The basic characteristics of coronas, namely inception time and voltage, were measured at applied voltages just sufficient for their inception up to voltages causing breakdown. Multiple level tests were conducted; thus, corona inception and breakdown probability distributions were obtained and time to breakdown was measured at several voltage levels. Interpretation of the results was made based on oscillograms of the electric field strength at the earthed plane, which was monitored through a capacitive probe, geometric field calculations and still photographs of the discharge at breakdown. In the dielectric-covered rod/plane gaps, breakdown occurs at higher applied voltages as a result of increase in both corona inception voltages and discharge path. The development of the discharge depends on the electric field distribution as modified by both space and surface charges associated with coronas preceding breakdown. Surface charge accumulation may affect the discharge path at breakdown.

Positive lightning impulse discharges along cylindrical insulating surfaces bridging a short rod-plane gap

The salient characteristics of the predischarge phenomena and flashover along cylindrical insulating surfaces bridging a short rod-plane gap under positive lightning impulse voltages were measured in a wide range of applied peak voltages. Materials used in the investigation were polytetrafluoroethylene (PTFE), silicone-rubber, nylon and glazed porcelain; the case of air alone was regarded as reference. Corona inception is favoured by the presence of an insulating surface, this being more evident for materials of high permittivity and for initial than secondary corona. At relatively high applied voltages, the initial corona characteristics are directly related to the field enhancement at the tip of the stressed electrode. However, near threshold inception conditions there is a marked reduction of the inception field with material permittivity; an assisting mechanism of initiatory electron production was put forward to explain this behaviour. Initial corona streamers require higher applied voltages to arrive at the earthed plane when propagating along an insulating surface than in air alone. Flashover, closely related to secondary corona characteristics, is always preceded by continuous corona growth, occurring mainly in free air away from the insulating surface because of possible positive surface charge deposition by the initial corona; the latter is characteristic for polymeric insulating surfaces. There is a reduction of the dielectric strength of the gap in the presence of an insulating surface, more pronounced for higher material permittivity, related to the facilitating action of the field enhancement close to the rod tip on continuous corona growth preceding flashover.

Positive Switching Impulse Discharge Performance and Voltage Correction of Rod-Plane Air Gap Based on Tests at High-Altitude Sites

The Qinghai-Tibet Railway is the highest railway in the world. Up to now, there were no test and service data for the external insulation of the power-supply project of the railway system above 4000 m above sea level (a.s.l.). The ldquogrdquo parameter method recommended by IEC Publication 60.1 (1989) has a limited applicable range. Therefore, based on the former tests carried on the artificial climate chamber (ACC), in this paper, a series of test investigations is conducted on the positive switching impulse (PSI) discharge performance of rod-plane air gaps with gap spacing of 0.25 to 3.0 m at the six high-altitude sites along the Qinghai-Tibet Railway with altitudes of 2820 to 5050 m. With analyses of the mathematical optimization method on the test results, the new correction method of discharge voltage is proposed. They are also checked and compared with the test results obtained from the simulation tests carried out in the ACC. It is indicated that the 50% PSI discharge voltage U 50 of the air gap at high altitude is a power function of gap spacing d, also a power function of relative pressure of dry air and absolute humidity. The influence law of atmospheric parameters on U 50 obtained at high-altitude sites is the same as that obtained in the ACC. U 50 . obtained in the ACC, is about 8.15% higher than that obtained at high-altitude sites due to the influence of nonsimulated factors, such as ultraviolet ray and cosmic radiation. The ldquogrdquo parameter method is not applicable to the regions with an altitude above 2800 m.

Striking Distance and Interception Probability

The relation between the striking distance and interception probability is experimentally investigated under standard lightning impulse voltages of both polarities. By using the rod-plane gap as a reference case, results are presented relating to the discharge interception probability of an earthed rod inserted in the gap. There is a critical separation distance between the stressed and earthed rods at which breakdown occurs either to the plane or to the earthed rod with the same probability. Besides the impulse voltage polarity, the length of the reference gap and the earthed rod height are considered as parameters influencing on interception probability. Results are discussed on the basis of the breakdown mechanism and electric field calculations. The striking distance is defined with the aid of an expression that takes into account, besides lightning peak current, the interception probability and the effects of lightning polarity and struck object height. The results introduce the design of lightning protection systems on the basis of lightning interception probability.

Thermal bubble motion in liquid nitrogen under nonuniform electric fields

This paper describes an experimental study into the influence of a non-uniform electric field on bubble motion and behavior in liquid nitrogen. The electric field effect on bubble motion as it rises due to buoyancy within applied dc electric fields is quantitatively investigated using a rod-plane gap. Thermal bubble motion and bubble collision with the plane electrode processes were observed in these experiments. The experimental results show that dc non-uniform electric fields have an obvious effect on bubble behavior; bubbles move closer to the plane electrode away from a higher field region to a lower one, irrespective of electric field direction. A model based on analysis of the forces acting on the bubble has been developed. This set of differential equations describes the motion of a spherical bubble in the rod-plane gap with voltage and can be solved numerically to determine bubble trajectory. Compared to experimental data, the theoretical prediction is in very good agreement.

Flashover along cylindrical insulating surfaces in a nonuniform field under positive switching impulse voltages

Results are presented concerning discharge development and breakdown in air and along cylindrical insulating surfaces in a 12 cm long rod-plane gap under positive switching impulse voltages. Discharge development and breakdown probability curves were obtained. The times taken for the discharge to cross the full gap and for breakdown were measured and the corresponding instantaneous voltages were calculated. The gap was over stressed so as to study the effect of the applied peak voltage on the discharge characteristics. In the presence of an insulating surface the discharge consists of a 'surface' and an 'air' component; the former hinders the development of the latter. At voltages causing 50% breakdown probability only the 'air' component traverses the full gap and breakdown occurs always in free air. For higher applied voltages the 'surface' component may become able to cross the full gap and breakdown may occur also along the insulating surface; under these conditions both the arrival of the discharge at the plane and breakdown occur at shorter times and at lower voltages than in air alone, depending on insulating surface material.

Flashover Characteristics of Long Air Gaps with Negative Switching Impulses

The electrogeometric model (EGM) proposed by Armstrong and Whitehead has been used to calculate the lightning shielding features of transmission lines. While the striking distance equation in the EGM uses the 50% flashover voltage characteristics of negative switching impulses for rod-rod gaps, those characteristics for long air gaps have not been sufficiently examined. The present study has therefore focused on clarifying the 50% flashover characteristics of negative switching impulses for long air gaps. Consequently, saturating characteristics have been obtained for rod-plane gaps compared with almost linear characteristics for rod-rod and rod-conductor gaps. Furthermore, new knowledge was obtained indicating that, for air gaps of around 4 to 5 m, the 50% flashover voltage may possibly be lower for rod-plane gaps than for rod- rod and rod-conductor gaps.

Modeling of Positive Leader Speed Under Slow Front Voltages—Part I: Long Air Gaps

This paper introduces a new mathematical model of positive leader propagation speed based on intense ionization in a thin boundary layer ahead of the tip of the leader channel. The model explicitly accounts for the effect of overvoltage and the resulting increase in leader current on leader speed. The model was applied to large rod-plane gaps under critical switching impulses. Numerical assessment of the model findings is included and whenever possible comparison with experiments has been found satisfactory.

A Test Study of 50% Lightning Impulse Breakdown Voltage on Rod-Plane Gap with Two-Phase Mixture of Gas and Solid Particles

A test study on 50% lightning impulse breakdown voltage in two-phase mixture of gas and solid particles has been carried out in a specially designed discharge cabinet. A mechanical sieve is set up for sifting different solid particles into the discharge space uniformly. The lightning impulse voltage according with international electro-technical commission (IEC) standard is applied to the electrodes inside the discharge cabinet by the rule of up-down method in a total of 40 times. The results showed that the 50% lightning impulse breakdown voltage in two-phase mixture of gas and solid particles has its own features and is much different from that in air.

Study on Detection of Negative Corona Discharge Generated in Rod-Plane Air Gap by Using External Electrode Method

A detective method of a negative corona discharge by means of an external electrode is presented. The relationship between an area of the external electrode and a detected voltage waveform is examined experimentally. This experimental study is carried out with the use of a rod-plane air gap. The results obtained will be applicable to problems associated with silos, ducts, and high-voltage equipment.

Influence of absolute humidity on the electrical breakdown in air

One of the most important insulating gases in high-voltage technique, the air, is varying with climatic conditions. The most important parameters are the pressure, temperature and the absolute humidity. The influence of humidity on the discharges is investigated in this report. First, the physical processes on the discharge development are discussed in respect to the influence of water vapour in air. Experimental results of the breakdown voltage with variable absolute humidity are explained with these processes. The experimental investigation was done with a rod-plane gap of 20 cm to 100 cm distance and with different voltage wave shapes. The different influence of water vapour on different predischarge phenomena can be seen in different dependency of the breakdown voltage on the absolute humidity, for each type of predischarge dominating before breakdown.

Factors affecting the discharge parameters of small positive rod-plane gaps in atmospheric air

Rod-plane gaps below 55 cm with various terminations of the rod were tested with positive impulse voltages of various forms. The complete breakdown probability distributions as well as the histograms of the time-to-breakdown were deduced. The results indicate that while for gaps longer than 30 cm the specific breakdown voltage Em stabilizes around the value of approximately 4.5 kV/cm, below this length Em increases rapidly as the gap length decreases. Non-Gaussian distributions of the breakdown voltages were detected and factors leading to non-Gaussian distributions were investigated. The implications of a non-Gaussian distribution of the breakdown in the design of air insulation of electrical installations of lower voltages were also examined.

Computation of inception voltage and inception time of positive impulse corona in rod–plane gaps

A method is proposed for computing the inception voltage and time of the corona in air in rod-plane gaps under a positive switching impulse and ramp-shaped voltages. The method is based on the rate of natural production of free electrons in the atmosphere as a result of cosmic ray activity, local radioactivity or ultraviolet radiation from the sun. The computed inception voltages agree reasonably with those measured experimentally for different values of the steepness of the applied switching impulse and ramp-shaped voltages and also with those measured experimentally for a rod gap stressed by a switching impulse voltage. The method is applied to different rod-plane gaps with varying rod radius and gap spacing.

Positive Switching Impulse Discharge Performance and Voltage Correction of 1-m Rod-Plane Air Gap

Switching impulse discharge performance of air gaps at high altitudes, which is a fundamental in the extremely high voltage external insulation design, remains an unsolved key technology across the world. Due to the limited applicable range of g parameter method recommended by IEC Publication 60-1(1989), this paper aims to explore the relationship between positive switching impulse 50% discharge voltage U 50 of rod-plane air gaps and atmospheric parameters, and proposes two new voltage correction methods based on the parameter g and the atmospheric pressure ratio, respectively. The two methods have been proved by the experimental investigations carried out on 1-m rod-plane air gap both in the artificial climate chamber and at three spots of different high altitudes in this study. The results show that, if the additional influence of absolute humidity is considered, the positive switching impulse 50% discharge voltage U50 of rod-plane air gaps is a power function of relative pressure and relative air density; the g parameter method in IEC Publication 60-1(1989) is not applicable to the regions with an altitude above 2000 m or the condition of h/delta>15 g/m3; compared with the correction method based on the g parameter method, the correction method based on relative atmospheric pressure has a wider applicable range and less error; when the absolute humidity increases 1 g/m3 , U50 varies within the range of 1.0%-1.15%

On the fluctuations in positive glow corona

Current knowledge of fluctuations in the steady glow current of the positive rod-to-plane corona at normal air density is reviewed. New work has been extended to investigate how the frequency and amplitude of these fluctuations change with the air density and humidity. The air density has been changed both by decreasing the pressure and by increasing the temperature. The pressure has been decreased down to 0.6 bar, and the temperature has been increased up to 200 degrees C. A method is proposed for calculating the frequency of fluctuations in any rod-plane gap with known field distribution. The method is based on assessment of the space charges developed in a positive glow corona that fill not only the ionisation zone around the rod but also the inter-electrode spacing. These space charges result in an electric field that affects the frequency of these fluctuations. The calculated frequency values agree reasonably with those measured for different rod diameters.

Microstructure of the current channel of an atmospheric-pressure diffuse discharge in a rod-plane air gap

The multichannel structure of the current channel of an atmospheric-pressure diffuse discharge excited in a 10-cm rod-plane air gap was investigated using the imprint technique. A voltage pulse with an amplitude of 240 kV, a duration of 180 ns, and a rise time of 14 ns was applied to a 1-cm-diameter bullet-shaped cathode with a tip curvature radius of 0.2 mm; the discharge current reached 350 A. It is found that the diameter of the discharge channel in the anode plane varies in the range 2.5–9.7 mm from shot to shot. The overall imprint of the current channel is formed of 170–9500 imprints of microchannels with an average diameter of 5–20 μm. The parameters of the observed microstructure do not correlate with variations in the main electric characteristics of the discharge and the parameters of the generated X-ray pulse. It is shown that the formation of the microstructure is related to the onset of cathode-directed plasma structures developing from the anode. It is suggested that the same mechanism is responsible for both the formation of the current channels structure and the anode microstructure of diffuse nanosecond discharges developing in atmospheric-pressure air gaps with a highly nonuniform electric field.

Onset voltage of positive glow corona in rod-plane gaps as influenced by temperature

The postulate put forward by Hermstein to account for the positive glow corona was widely accepted at first and later criticised. Loeb's alternative postulate has not been fully satisfactory. In this paper, a method is described for calculating the onset voltage of positive glow corona in rod-plane gaps in air at room and high temperatures. The method has a flavour of both the Hermstein and Loeb postulates, but is based on the physics of air discharges occurring in the ionisation zone around the stressed rod. The threshold voltage of onset streamers and their development at higher voltages are prerequisites for calculating the onset voltage of positive glow. The threshold and onset voltages increase with the gap spacing at room temperature and decrease with the increase of air temperature for the same gap geometry in conformity with previous findings. The calculated threshold voltage and repetition rate of onset streamers, in addition to the onset voltage of the positive glow agree reasonably with those measured experimentally.