Relative Air Density Research Articles

Positive corona inception voltages and corona currents for air at various pressures and humidities

The variation of positive dc corona characteristics with air pressure and humidity using a point/plane electrode system was studied in a perspex chamber allowing the pressure and humidity to be varied. A method of calculation was developed to determine the corona current for humid air over a range of pressures and humidities in which the effective ionization coefficient was calculated as a linear interpolation of the effective ionization coefficients for dry air and water vapor. The calculation method involved the solution of partial differential equations using a particle-in-cell algorithm. This also allowed the determination of the corona inception voltages for a range of pressures and humidities as the voltage at which the corona current first appeared. The validity of these calculations was confirmed by a series of experimental determinations of the corona inception voltages and of the variation of the corona current with the applied voltage for the same range of pressures and humidities. It was found that the corona current increased when the air pressure or humidity decreased (at constant applied voltage) mainly because of the rise in the space charge density or the positive ion mobility, respectively. The coefficient C, in the relationship between the corona current, the applied dc voltage and the corona inception voltage, declined with the increase of humidity. The rate of decrease of C with humidity became larger as the pressure was reduced. An approximately inversely proportional relationship was found to exist between C and the relative air density at constant humidity. This variation of C was principally associated with the variation in the positive ion mobility.

연소화염 존재 시 수직형 모델 전력선의 섬락 특성

전력선이 통과하는 지역에서 발생된 산불은 계통 섬락사고의 원인이 되고 있다. 본 연구에서는 연소화염에 의한 전력선의 절연내력 저하특성을 알아보기 위해 직류 및 교류 전압 인가 시, 연소화염과 고전압 도체의 수평거리(s)를 변화시켰을 때, 수직형 모델 전력선에 대한 섬락특성을 조사하였다. 실험 결과, 수평거리(s)가 작은 범위에서는 화염이 모델선로의 섬락전압을 크게 감소시킬 수 있는 것으로 나타났다. 이러한 섬락전압의 저하요인을 해석하기 위해 화염주변의 상대공기밀도가 고려되었으며, 화염이 존재하는 경우에는 그 영향이 매우 크게 나타남을 알 수 있었다.

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.

The effect of reduced air density on streamer-to-leader transition and on properties of long positive leader

New results of observations of the leader process in a pressure chamber are presented for reduced air pressures. The analysis of these data and observations of the leader discharge in peak regions shows that the length of the leader tip and some other characteristics vary by several times as pressure decreases from 1 to 0.3 atm, whereas, under the conditions considered, the leader velocity remains almost independent of air density, the leader current being the same. These data are used to extract relationships between discharge parameters. It is shown that, at reduced air densities, electric field in a ‘young’ section of the leader channel exceeds electric field in the streamer zone. Therefore, transition of the leader process to the final-jump phase is not inevitably followed by a breakdown of the gap for reduced pressures, as opposed to the discharge in atmospheric pressure air. The model suggested previously by the authors for the streamer-to- leader transition at atmospheric pressure is amended to take into account hydrodynamic expansion of the channel and used to simulate the process at a relative air density of 0.3. The calculated results are used to interpret the observations of the leader process at reduced air pressures.

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%

Positive corona inception in air at elevated temperatures

The conditions for the inception of non-uniform field corona in air are discussed. A model, that requires simple assumptions, is used to define the characteristics of the critical avalanches needed to initiate streamer corona as the temperature increases and the air density decreases. An iteration procedure that combines measured values of the inception voltages with simple criteria and published data on basic processes is used to define such avalanche characteristics as length, avalanche number and the positive ion space-charge field at criticality. The Meek criterion is invoked to equate the space-charge field to the local applied field, at which replication of the avalanche and streamer initiation is assumed to occur. The procedure is repeated using trial inception voltages to simulate the range measured experimentally at temperatures up to /spl sim/530 K, which corresponds to a relative air density of /spl sim/0.55. It is shown that for relative air densities </spl sim/0.8, the formation of critical avalanches follows a different law from that operating in the range between 1.0 and 0.8. The assumptions made in the calculations result in a good agreement between the calculated and experimental properties.

구-구갭의 섬락 특성에 미치는 연소화염의 영향

본 연구에서는 구-구갭을 수평 배치하였을 때, 연소화염에 의한 교류 및 직류 플래시오버전압 특성을 조사하였다. 화염에 의한 플래시오버 극성을 조사하기 위해 전압 및 전류파형을 측정하였으며, 플래시오버가 발생되기 전에 쿨롱력에 의한 화염의 형상 변화를 관찰하였다. 또한 플래시오버 전압의 저하 요인으로서 상대공기밀도의 저하 및 연소화염의 열전리 영향에 대해 고찰하였다. 실험 결과, 구-구갭에서 접지측 전극과 갭길이의 비를 k라고 할 때, 화염위치 k=0(접지측), k=0.5(중앙부) 및 k=1.0(전원측)인 경우 화염에 의한 교류 섬락전압의 평균값은 화염이 없을 때에 비해 각각 79.9, 82.9 및 87.5[%] 저하되었으며, 또한 화염의 높이 h=0, h=5 및 h=9[cm]일 때 교류 플래시오버전압의 평균값은 화염이 없는 경우를 기준으로 하였을 때 각각 85.0, 40.8 및 28.2[%] 저하되었다. 소규모 연소화염에서의 열전리 영향은 크지 않는 것으로 나타났다.

Quantitative spectrally resolved imaging through a spectrograph.

A grating spectrograph can be used for spectrally selective two-dimensional imaging if it is operated with a broad entrance slit. The resulting intensity distribution in its exit plane is a one-dimensional convolution of the spatial and spectral distributions of incident light. We present a dedicated deconvolution filter to reconstruct the spatial image from the spectrograph output. The algorithm is illustrated on Raman imaging of an underexpanded dry air jet. Recorded Raman images correspond to density maps convolved with the Raman spectrum of air; the latter essentially acts as a blurring function for the density map. The deconvolution filter combines the individual images recorded in the O2 and N2 Raman bands into a single image of relative air density.

Tests on the breakdown of air at elevated temperatures in non-uniform electric fields

Sparkover voltage characteristics have been obtained using a 20 cm rod–plane gap at temperatures between 294 and 770K. Lightning and slow-front impulse and alternating voltages have been applied; pre-breakdown corona has also been investigated under impulse voltages. The nature of the sparkover characteristics depends strongly on the type and polarity of the applied voltage. These are discussed in terms of the changes in relative air density with temperature and the relationship to the recommendations of IEC 60-1 for the adjustment of sparkover measurements to changes in atmospheric conditions.

Best-Range Altitude for Jet-Propelled Aircraft

Nomenclature CLopt = lift coefŽ cient for Emax c = speciŽ c fuel consumption Emax = maximum aerodynamic efŽ ciency for parabolic polar h = altitude, m S = wing area T = thrust t = dimensionless thrust, (TEmax=W / V = airspeed VR = reference or minimum drag airspeed, (2W=1⁄2SCLopt/ v = dimensionless airspeed,V=VR W = aircraft weight WF = fuel weight X = range X R = reference range, f8VRi0Emax[1i .1i 3 /]g=.c113⁄4 0:5 11 / y = dimensionless exponent for speciŽ c fuel consumption z = dimensionless exponent for thrust 3 = fuel-to-initial-weightratio,WF=Wi 1⁄2 = air density 3⁄4 = relative air density with respect to sea level, 1⁄2=1⁄20

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>

Examination of Grizzly Mountain database to determine effects of relative air density and conductor temperature on HVDC corona phenomena

A summary is presented of the results of an effort to determine the effect of relative air density (RAD) and conductor temperature on HVDC (high-voltage direct current) corona phenomena by statistically examining fair weather audible noise (AN), radio noise (RI), ion-enhanced electric field, and ion current density data in the 1986 database from the Grizzly Mountain HVDC Research Project. Since wind has such a strong effect on DC electric fields and ions, this comparison was made under the calm wind conditions of speeds less than 1 m/s. The results of this effort demonstrated that AN, RI, negative electric field, and negative ion current density increase with increasing ambient and conductor temperature and decrease with increasing RAD. However, positive electric fields and positive ion current densities showed essentially no correlation with either conductor temperature or RAD. >

DC Corona Discharge Characteristics and Ion-Flow Distribution for Several Types of Rods under Low Pressure

The dc corona discharge characteristics and ion flow distributions for differently shaped discharge rods in a model electrostatic precipitator unit at low relative air density & = 0.21-1 (0.21-1 atm at room temperature) are described. The purpose of this experiment was to clarify the electrical characteristics of hot-site electrostatic precipitators based on the similarity law of electrical gas discharges. Three types of discharge rods were used: 1-5-mm round rods, 4-mm square rods, and rods with attached pins. Results are reported for both polarities in terms of current-voltage characteristics, corona onset and flashover voltages, and ion-flow distributions.

Weather and its Effect on Air Insulation Specifications

Using weather statistics from 10 USA weather stations, the statistical distributions of relative air density, δ, humidity correction factor, Hc , and δ/Hc are determined to be best approximated by Gaussian distributions. The wind speed distribution is best approximate by a Weibull distribution. Regression of δand δ/Hc with altitude shows a significant correlation. Using these distributions and the parameters of these distributions, it is shown that the standard deviation of δand δ/Hc need not usually be considered. Reductions of BIL aid BSL for air-insulated stations and the required increase of strike distances for transmission lines as a function of altitude are presented. The effect of wind is shown to result in design swing angles considerably less than that given in standards.

High-Altitude Considerations for Electrical Power Systems and Components

The design and application of electrical systems at elevations in excess of 1000 m (3000 ft) requires knowledge of the effects of atmospheric conditions on each particular component. Failure to understand adequately and include the effects of high altitude in the design and application of the equipment may result in its poor performance, premature aging, and/or failure. The relationship of relative air density and altitude is discussed, followed by the effects of altitude on electric power system components. Along with the discussion of the effects of high altitude on each component are suggestions or solutions to the high-altitude problem. Although the subject deals with high-altitude applications of equipment, the performance of equipment from sea level to 1000 m may be affected by the relative air density. Since the relative air density decreases at a rate of approximately one percent per 100 m above sea level, the operation of any piece of equipment which is dependent on the air density will be different at 3300 ft compared to sea level. This subject is discussed so that independent conclusions may be drawn.

Phase-Phase Switching, Suite Flashovers: Weather Correction Factors

Weather correction factors for spacings of 3 to 12 feet have been obtained from phase-phase switching surge flashover tests. New statistical methods of analysis of flashover data were used to show that correction factors published for phase-ground flashovers are inadequate for phase-phase. Correction factors for phase-phase flashovers calculated from relative air density, humidity, and relative humidity are given.

A simplified approach to estimation of RIV levels from transmission system data

It is fairly well established that corona discharges from EHV transmission systems are dependent upon conductor surface and weather conditions. Several methods for estimation of radio-interference (RI) and radio-interference voltage (RIV) levels have been proposed in the past both for fair- and foul-weather conditions. The present paper aims at a simplified approach to the estimation of RIV levels based on “the ratio of working potential gradient to that of the general corona starting gradient”. Results of analysis for typical systems as influenced by the relative air density, conductor surface conditions and aging of conductors are presented.

Nonlinear self-consistent calculations of radiation induced cylinder skin currents

The particle method of numerical plasma simulation is applied to the calculation of surface currents on a metallic cylinder in air, exposed to an axially directed planar wavefront of gamma radiation. Assuming a collision-dominated plasma, the Compton electron trajectories, rate equations for the production of secondary electrons and the Maxwell equations are advanced self-consistently for each time step at all mesh cells in the finite difference solution. Numerical results presented include comparisons of self-consistent skin currents with results obtained by prescribing (no field reaction) the Compton current density and ionization rate proportional to the gamma flux. With a sin2 pulse of 40 ns duration and a relative air density of 1, the self-consistent surface current magnitudes lie considerably below prescribed source results for peak gamma dose rates ≥ 1011 Rad/sec. At 109 Rad/sec, the skin currents from the particle calculations are nearly identical with currents resulting from a prescribed source obtained from the electron dynamics in the absence of the Lorentz force.

Radio-Noise Levels of EHV Transmission Lines Based on Project EHV Research

The results of three years of experimental and theoretical research on transmission-line radio noise at Project EHV are presented. Computer programs were developed to process the data on a statistical basis, and as output provide both histograms and regression analyses for four test configurations operated at the 500-and 700-kV levels. Fair weather radio-noise levels were found to be significantly affected by relative humidity, relative air density, and absolute value of wind velocity, and these relationships are discussed. Wet-weather effects are presented and discussed. Both fair-weather and wet-weather experimental results are incorporated in a theoretical analysis of transmission-line radio noise resulting in a procedure which enables the transmission-line designer to predict, in advance of construction, average fair-weather and wet-weather radio-noise profiles for EHV transmission lines.