Short Air Gaps Research Articles

Breakdown in short rod-plane air gaps under positive lightning impulse stress

Prediction of withstand voltages in air-insulated systems are made on the basis of empirical models that are not sufficiently accurate for complex geometries. Better understanding of the spatiotemporal development of electrical discharges is necessary to improve the present models. Discharges in lightning impulse stressed 20–100 mm rod-plane gaps are examined using a highspeed camera, photo-multiplier tubes (PMTs) and a highbandwidth current measurement system. The images and measurements of gaps larger than 20mm show a fast initial streamer discharge with a current rise time of some tens of ns, followed by a dark period of a few μs and a propagation of a slower leader-type channel leading to breakdown. The breakdown mechanisms in the shortest gaps are faster and geometry dependent, probably occuring by heating of initial streamer channels. Different light filters used with the PMTs indicate that all parts of the leader-type discharge development emit light over a spectrum from UV to IR. The initial discharges emit low amounts of warm light and IR compared to the leader-type channel. Finally, it is suggested that empirical breakdown voltage prediction models should be interpreted in light of the leader-type breakdown mechanism.

Breakdown in short rod-plane air gaps under positive lightning impulse stress

<p>Prediction of withstand voltages in air-insulated systems are made on the basis of empirical models that are not sufficiently accurate for complex geometries. Better understanding of the spatiotemporal development of electrical discharges is necessary to improve the present models. Discharges in lightning impulse stressed 20–100mm rod-plane gaps are examined using a highspeed camera, photo-multiplier tubes (PMTs) and a highbandwidth current measurement system. The images and measurements of gaps larger than 20mm show a fast initial streamer discharge with a current rise time of some tens of ns, followed by a dark period of a few μs and a propagation of a slower leader-type channel leading to breakdown. The breakdown mechanisms in the shortest gaps are faster and geometry dependent, probably occuring by heating of initial streamer channels. Different light filters used with the PMTs indicate that all parts of the leader-type discharge development emit light over a spectrum from UV to IR. The initial discharges emit low amounts of warm light and IR compared to the leader-type channel. Finally, it is suggested that empirical breakdown voltage prediction models should be interpreted in light of the leader-type breakdown mechanism.</p>

Comparison of the performance of silicone and glass barriers under direct current and very severe conditions of pollution

The main objective of this paper consists in the comparison of the performance in DC voltage of a short air gap with non-uniform electrical field protected by silicone or glass barriers under very severe conditions of pollution. The choice of the investigation system is related to the rod and plane electrodes. The essential influence parameters on the performance of the system are summarized in the severity of the level of pollution of the barrier and the orientation of the axis of the basic electrodes of the system. These tests were supported by a visualization of the breakdown path followed by the electrical discharge on the barrier. An experimental device allowing simultaneously the orientation of the axis of the electrodes and the displacement of the barrier inside the air gap was made. The results show that under pollution conditions, the effective electrical protection offered by the silicone barrier is highly efficient than the glass one. Under the effect of the contaminated water droplets, the system with vertical or horizontal silicone barrier, floating above the energized rod is more rigid than when this silicone barrier is horizontal, and located below the energized electrode. The quantification of the voltages decrease is made possible by a mathematical model.

A method for breakdown voltage prediction of short air gaps with atypical electrodes

The most determinant factor for the dielectric strength of an air gap is the inhomogeneity of the electric field. This paper attempts to establish the internal relation between the electric field distributions of air gaps with different electrode geometries, and so as to predict the breakdown voltage of the air gap with atypical electrodes. A classification model is established based on support vector machine (SVM), which considers that the breakdown or withstand of an air gap under an applied voltage can be viewed as 1 or -1. 28 features are defined and applied to characterize the electric field distribution of an inter-electrode air gap. Taking these features as input parameters of the SVM model, the grey correlation between the air gap breakdown voltage and its electric field distribution can be trained by some training samples, which are selected from typical air gaps like sphere-sphere and rodplane gaps according to the electric field distribution similarity along the shortest discharge path. The SVM model is applied to predict the breakdown voltages of atypical air gaps including the serial gaps, the ring gaps and the stranded conductor gaps. The predicted results are in good agreement with the experimental data, and the mean absolute percentage errors of these gap arrangements are within 5.4%. This method provides an alternative for the breakdown voltage prediction of air gaps with complex geometries, and therefore helps minimizing the required test work.

Hybrid prediction of the power frequency breakdown voltage of short air gaps based on orthogonal design and support vector machine

This paper attempts to establish the grey correlation between the breakdown voltage of air gap and its energy storage status before discharge inception. A new method is presented for the breakdown voltage prediction of short air gaps. This method is based on an orthogonal design array and support vector machine (SVM). A hybrid model is established to predict the power frequency breakdown voltage of three typical air gaps including sphere-sphere, rod-plane, and rod-rod gaps. The input data are fifty parameters extracted from the static electric field distribution of the air gap, and the output of the model is whether the gap will breakdown under a given voltage. This model is trained by 9 samples selected by L 9 (33) orthogonal array and applied to predict the breakdown voltage of 90 test samples. With effective feature dimension reduction and parameter optimization of the SVM model, the predicted results coincide well with the experimental data, and the mean absolute percentage error (MAPE) is only 3.9%. The SVM model is also applied to predict the breakdown voltage of some atypical short air gaps including sphere-plane gaps, rod-sphere gaps, sphere-sphere gaps with different diameters, and rod-plane-sphere gaps. The MAPEs are respectively 4.3, 3.2, 4.2, and 6.4%, which verifies the validity and generalization performance of the proposed method. This model provides a new way to gain the critical breakdown voltage of air gaps, and therefore contributes to reducing the required test work.

Positive switching impulse discharge performance of rod‐plane short air gap under rain conditions

In China external discharge accidents caused by heavy rain is growing. However, the rain effect is not considered in atmospheric correction for external insulation design in IEC 60071-1(Edition 8.1, 2011) or domestic standard. Hence, a series of experiments are implemented with positive switching impulse, a typical kind of overvoltage of power grid, to analyse the discharge performance of rod-plane short air gap under rain conditions. The effects of rain and its intensity, rain conductivity and ambient temperature on the discharge voltage are explored, and the theory behind them, which can help in the development of discharge mechanism of air gap under rain condition, are respectively given. The results show that under the experimental rain condition, the discharge voltage can be up to −10.9, −9, −8.5, −3.2 and −2.3% lower than under dry condition for clearance distance of 0.1, 0.2, 0.3, 0.4 and 0.5 m, respectively. Besides, it decreases with increase in rain intensity, rain conductivity and ambient temperature, respectively. Among them, the rain intensity is the most significant one, next is temperature, whereas rain conductivity is the minimum and trifling. Finally, a correction formula for breakdown voltage (positive switching impulse) of rod-plane air gap under rain condition is proposed for reference.

Electric Field Measurement in Rod-Discontinued Plane Air Gaps Using Distributed Capacity Probe

The present experimental carried out investigations aimed at elucidating the effect of earth discontinuity on the breakdown voltage of short rod-plane air gap (80 to160 mm) under negative lightning impulses (-1.2/50ms).We also carried out investigations on electric field measurement on the surface of a discontinuous plane of a rod-plane air gap arrangement. For this purpose, we used a probe with distributed capacity, under negative lightning applied impulse voltage. The probe is incorporated on the same level of plane surface.The interface locally reinforces the electric field. The electric field increases at the interface may lead to a discharge between the high voltage rode and the interface. In the vicinity of the interface, we observe a kind of discontinuity in the evolution of the electric field intensity. This one becomes greater than the value obtained in the case of gaps with homogeneous plane earth. Keywords :Breakdown voltage; electric field; distributed capacity.

A prediction method for breakdown voltage of typical air gaps based on electric field features and support vector machine

Breakdown voltage of the air gap is of vital importance for the design of the external insulation in high-voltage transmission and transformation projects. In this paper, a new prediction method for the breakdown voltages of typical air gaps based on the electric field features and support vector machine (SVM) was proposed. According to the finite element calculation results of static electric field distribution, the electric field values in the whole region, discharge channel, surface of the electrode and the shortest path were extracted and post-processed, which constituted the electric field features characterizing the gap structure. Then, the breakdown voltage prediction model of the air gap was established by using electric field features as the input parameters to SVM, and whether the gap breakdown would happen as the output parameters of SVM, which changing the regression problem to a binary classification problem. This model was applied to predict the power frequency breakdown voltages of different short air gaps including sphere-sphere gaps, rod-plane gaps, sphere-plane gaps and sphereplane- sphere gaps. The power frequency breakdown voltages of longer air gaps which are affected by corona, and the 50% positive switching impulse breakdown voltages of long sphere-plane gaps and rod-plane gaps were predicted as well. The predicted results agree well with experimental values and simulated results of the published models, which validate the effectiveness of the proposed model. This method supplies a new possible way to obtain the breakdown voltage of air gaps.

Effect of water streams on the AC breakdown performance of short rod-plane air gaps

The ac breakdown performance of rod-plane air gaps in the presence of water streams has been explored. Experimental results show that the ac breakdown voltage of rod-plane short air gaps in the presence of a water stream is significantly lower than in the dry condition. In addition, the ac breakdown voltage of an air gap decreases with the increase of the water stream velocity, diameter and conductivity. The relative breakdown voltage of an air gap decreases with an increase of water stream velocity from 0.75 m/s to 1.79 m/s and nozzle diameter from 1.0 mm to 3.0 mm by 2.6% / 0.1 m/s and 0.8% / 0.1 mm, respectively. High speed images of the deformation of the water stream and the breakdown process of air gap are reported show that with the increase of the applied voltage, the critical break-up length and the size of main water droplets decrease. In addition, the effect of the electrical field on the break-up length of a water stream and the size of droplets are independent of the jet velocity.

Study on the effect of artificial radiation simulating terrestrial solar ultraviolet radiation on negative DC discharge performance of air gap

It is important to reveal the effect of terrestrial solar ultraviolet radiation (TSUVR) on discharge performance of external insulation for the design and operation of a power system in regions with strong TSUVR, especially in Tibet. However, no research was carried out in this field until now. In this study, an artificial UV lamp was used to imitate TSUVR in the relevant experiments to research on the effect of negative DC discharge. It was found that artificial radiation simulating terrestrial solar UV radiation (STSUVR) does not affect appreciably the negative DC corona characteristics and breakdown voltage of short rod-plane air gaps of several electrode materials. That is because the wavelength of the STSUVR is in the range of 290–400 nm, and its intensity is less than 200 W/m2. Thus, air, Cu and Fe cannot be photoionised by STSUVR. There is also no effect of the STSUVR on arc propagation because of energy. Although a part of the photons in the TSUVR has higher energy than the work function of Al, it is very easy for the Al to be oxidised, and the oxide film (Al2O3) can prevent Al from being ionised by TSUVR. Therefore the TSUVR does not have an obvious effect on negative DC discharge performance of air gap.

Dynamics of streamer‐to‐leader transition at reduced air densities and its implications for propagation of lightning leaders and gigantic jets

In this paper we present modeling studies of air heating by electrical discharges in a wide range of pressures. The developed model is capable of quantifying the different contributions for heating of air at the particle level and rigorously accounts for the vibration‐dissociation‐vibration coupling. The model is validated by calculating the breakdown times of short air gaps and comparing to available experimental data. Detailed discussion on the role of electron detachment in the development of the thermal‐ionizational instability that triggers the spark development in short air gaps is presented. The dynamics of fast heating by quenching of excited electronic states is discussed and the scaling of its main channels with ambient air density is quantified. The developed model is employed to study the streamer‐to‐leader transition process and to obtain its scaling with ambient air density. Streamer‐to‐leader transition is the name given to a sequence of events occurring in a thin plasma channel through which a relatively strong current is forced through, culminating in heating of ambient gas and increase of the electrical conductivity of the channel. This process occurs during the inception of leaders (from sharp metallic structures, from hydrometeors inside the thundercloud, or in virgin air) and during their propagation (at the leader head or during the growth of a space leader). The development of a thermal‐ionizational instability that culminates in the leader formation and propagation is characterized by a change in air ionization mechanism from electron impact to associative ionization and by contraction of the plasma channel. The introduced methodology for estimation of leader speeds shows that the propagation of a leader is limited by the air heating of every newly formed leader section. It is demonstrated that the streamer‐to‐leader transition time has an inverse‐squared dependence on the ambient air density at near‐ground pressures, in agreement with similarity laws for Joule heating in a streamer channel. Model results indicate that a deviation from this similarity scaling occurs at very low air densities, where the rate of electronic power deposition is balanced by the channel expansion, and air heating from quenching of excited electronic states is very inefficient. These findings place a limit on the maximum altitude at which a hot and highly conducting lightning leader channel can be formed in the Earth's atmosphere, result which is important for understating of the gigantic jet (GJ) discharges between thundercloud tops and the lower ionosphere. Simulations of leader speeds at GJ altitudes demonstrate that initial speeds of GJs are consistent with the leader propagation mechanism. The simulation of a GJ, escaping upward from a thundercloud top, shows that the lengthening of the leader streamer zone, in a medium of exponentially decreasing air density, determines the existence of an altitude at which the streamer zones of GJs become so long that they dynamically extend (jump) all the way to the ionosphere.

A chemical vapor sensor based on Rayleigh scattering effect in simplified hollow-core photonic crystal fibers

A novel optical fiber sensor for minor chemical vapor content detecting, based on Rayleigh scattering effect in simplified hollow-core photonic crystal fiber (HC-PCF), is proposed in this paper. Both ends of the simplified HC-PCF are aligned with standard single-mode optical fibers without splicing, leaving a very short air gap to allow molecules diffusing. As the concentration of volatile organic compounds (VOCs) in a chamber increases, molecules come into the air holes of the simplified HC-PCF so that the Rayleigh scattering is excited, resulting in an intensity loss of transmission light. We can detect the minor change of VOC content by monitoring the output transmission intensity using an optical power meter. The experimental results show that the ethanol concentration sensitivity is ~0.022dB/ppm, when a ~5.1cm long simplified HC-PCF is used. The sensitivity will be further improved when a longer simplified HC-PCF is used. When an optical power meter with 0.01dBm resolution is used, the ethanol concentration's resolution can reach ~0.45ppm.

AC breakdown performance and voltage correction of rod-plane short air gap under rain conditions

According to the operation experience and investigation in China, the insulation performance of transmission line can be reduced due to the effect of strong rain, which is not considered in the external insulation design. Meanwhile, there is no correction reference regarding the breakdown voltage of air gap under rain conditions in IEC standards. Therefore, a series of experimental tests were conducted in the artificial climate chamber and at Xuefeng Mountain Natural Site with the usage of rod-plane air gap to analyze the relationship between ac breakdown voltage and rain precipitation rate, rain conductivity and ambient temperature. It is shown that the breakdown voltage of air gap decreases with an increase of rain precipitation rate and rain conductivity, and increases with an increase of ambient temperature. In addition, according to the test results, the correction formula of ac breakdown voltage of rod-plane air gap under rain condition is proposed and verified in this paper.

DC positive discharge performance of rod-plane short air gap under rain conditions

To study the discharge performance of air gap under rain conditions and provide reference to the selection of clearance distance of external insulation in strong rain, this paper explored the influence of different environment conditions, rain intensity, rain conductivity, and ambient temperature on the dc positive breakdown voltage of rodplane air gap in the artificial climate chamber and at Xuefeng Mountain Natural Site. It is shown that the discharge voltage of rod-plane short air gap in wet condition is slightly higher than that in the dry condition; light rain has little influence on the positive dc discharge voltage of rod-plane short air gap. The relative discharge voltage of air gap decreases with an increase of rain intensity and rain conductivity with 2.2% to 0.2% / (mm/min), 1.4% to 0.1% / (100 μS/cm), respetively; and it is associated with the increase of ambient temperature with 0.3% to 0.1% / °C. In addition, several computations were carried out with different droplet sizes in rod-plane air gap. It can be indicated that the maximum electric field in the air gap increases with the increase of the radius and number of droplets.

Impulse behavior of dielectric-covered rod-plane air gaps

The impulse behavior of composite short rod-plane air gaps with a dielectric-covered rod under positive lightning and switching impulse voltages is investigated, by monitoring the electric field strength at the earthed plane, by still photographs of corona discharges and through electric field computations. Experiments were performed in normal laboratory air at pressure around 0.1 MPa, temperatures in the range 19 to 28°C and absolute and relative humidity varied naturally between 11-21 g/m3 and 60-90%, respectively. Breakdown probability distributions were obtained for composite gaps with a dielectric cover made of either PTFE or epoxy resin and for simple air gaps with a bare rod. Also, basic characteristic parameters of coronas preceding breakdown in composite gaps, namely initial corona in the vicinity of the covering tip, surface coronas emerging along the dielectric cover and developing mainly in air, and secondary corona in the vicinity of the cover upper end, were measured. Surface corona development is greatly affected by initial corona growth and facilitates secondary corona inception. Breakdown, occurring at higher voltages for composite gaps, is closely related with the secondary corona inception; a similar dependence upon impulse waveshape and cover material was seen to exist. At breakdown, the spark channel bridges the gap that includes the dielectric cover length. The spark channel develops either along the cover surface or partly or solely in free air, being affected by surface corona development.

Single-Longitudinal-Mode Fiber Ring Laser Stabilized by Tandem All-Fiber Fabry–Pérot Micro-Cavities

We demonstrate a stabilized single-longitudinal-mode fiber ring laser using tandem-Fabry-Pérot-microcavities (T-FP-MCs) as the all-fiber intracavity filter. The T-FP-MCs are formed with short adjustable air gaps between pairs of vertical cleaved single-mode fiber ends without mirror-coating deposition. The T-FP-MCs' spectral filtering effect is analyzed with the Rouard's method. Mode-hopping-free continuous-wave operation with 19-dBm power, 0.06-nm bandwidth and sidemode suppression ratio of 28 dB at 1565-nm wavelength is achieved using two T-FP-MCs. Q-switching pulsed operation of this fiber laser with 10-W peak power is also investigated and analyzed.

Prebreakdown stages of a barrier discharge in air

The initial stages of a barrier discharge in a short air gap at atmospheric pressure are investigated by means of numerical simulations. A highly nonuniform electric field caused by the residual surface charges on the dielectric surfaces was taken into account. The results of calculations of the two-dimensional dynamics of the discharge radiation are in good agreement with the experiment.

Design of Ultrahigh-$Q$ 1-D Photonic Crystal Microcavities

Waveguide based 1-D photonic crystal (PC) microcavities in silicon-on-insulator are investigated by 2-D finite-difference time-domain method. Values up to 6.7 times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> for the quality factor ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> ) are feasible if the cavities are properly designed. The factors that govern <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> are analyzed in both real space and momentum space. Etching down into the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer is found to give more than 20% improvement in <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> compared to the structure in which etching is stopped at the oxide layer. Short air gap mirrors are used to reduce the vertical scattering loss. The addition to the Bragg mirrors of tapered periods optimized to produce a cavity mode with a near Gaussian shaped envelope results in a major reduction in vertical loss. A new tapered structure with varying Si block width demonstrates an ultrahigh- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> and relieves the fabrication constraints compared to the conventional air slots tapered structure.

Effects of temperature and pressure change on positive corona and sparkover under direct voltage in short airgaps

Measurements are reported of breakdown voltage values and pre-breakdown voltage–current characteristics as a function of air density in a simple rod–plane electrode gap 50 mm in length. Pressure has been reduced and temperature raised from normal atmospheric values, independently of each other, to give the relative air densities in a common range less than unity. The effects of different diameters of rod have been studied. The results allow a comparison of the characteristics at equal relative air densities attained by either pressure or temperature adjustment. It is shown that at common relative air densities, breakdown is temperature- and geometry-dependent and that there is a temperature dependence of the pre-breakdown glow currents. The density-dependent relationship between the pre-breakdown glow discharge currents and breakdown voltages is discussed.

GAによる 0.1mm ギャップ長SRモータの最適設計

This paper presents an improved optimum design approach based on genetic algorithm for Switched Reluctance Motor (SRM) with 0.1mm airgap. A simple nonlinear magnetic analysis and an iron loss prediction employed in the previous approach had problems in terms of their analytical accuracies for SRM with short airgap. To improve the computational accuracy, it is reconsidered that how to determine the geometry of a saturable permeance tip in the simple nonlinear magnetic analysis and the coefficients of hysteresis and eddy current losses for iron loss calculation. As a result, it is demonstrated that the computational accuracy of the improved approach is as high as that of 3D FEM. Finally, a 400W SRM with 0.1mm gap is redesigned, by which high efficiency more than 88% at the rated operating point is realized.