Voltage Air Research Articles

Field optimisation of high-voltage electrodes

Stresses on materials for HV components have become increasingly large owing to the reduction of their size and the worldwide increase in voltage levels. Since the electrical stress depends heavily on geometric factors, critical domains of an arrangement cannot be represented by such simple bodies as rings, spheres or cylinders. The paper describes an algorithm based on the CAD concept for optimising electrode contours according to given field distributions on their surfaces. This algorithm strictly separates field calculations and geometric corrections. The curvature change necessary to alter the contour is obtained by the simultaneous displacement of all contour points. Using this method, for example, a constant and minimum field strength, or a maximum initial voltage in air or SF6, is achieved. Tests were run on several HV components designed for practical use. The experimental results demonstrated the improvements predicted by the computer calculations.

Onset or breakdown voltage reduction by electrode surface roughness in air and SF6

This paper describes investigations on the influence of surface roughness on the onset or breakdown voltage in air and SF 6 . Applying the streamer theorie of electrical discharges the role of ionization characteristic, gas pressure, shape and size of electrode surface protrusions is discussed. The maximum allowable field gradients of electrode surfaces with hemispherical, spherical, and filamentary irregularities were computed in air and SF 6 . Pictures of a sparked electrode surface, showing slze and shape of surface roughness, were made using an electron scanning microscope. A comparison between computed breakdown field gradients of rough electrodes and experimental results obtained by employing different electrode configurations in a wide pressure range confirmed the theoretical considerations.

Surface breakdown in silicon planar diodes equipped with field plate

The distribution of the electrical potential around the edge of the field plate in a silicon planar diode has been solved by means of a computer. The breakdown voltage related to this region has been derived as a function of impurity doping in silicon and of oxide thickness. If the oxide layer is not thick enough, avalanche breakdown voltage is often considerably lower than the value allowed by the material. The effect of Q ss has been found to be negligible. Breakdown voltage vs. gate voltage measurement on gated diodes and avalanche light emission support an edge breakdown mechanism. Breakdown voltage on field plated diodes matches rather well with the calculated values. It normally drifts towards higher values and during this walkout a hot carrier current is collected by the field plate. The field plate current is fairly proportional to avalanche current and increases with bulk doping. A new planar diode structure suitable for a BV up to 900 V has been designed. The discharge voltage in air between field plate and equipotential ring (EQR) is shown to be correlated with the actual field at the edge.

Secondary Leader Channels in the Impulse Breakdown of Air and Nitrogen

In impulse gaseous breakdown, it has never been quite established how the transition from the low ion density of the primary streamers to the high carrier density of the pre-spark channel occurs. This phase of development was investigated for air and N2 by employing the Lichtenberg figure technique. Gas pressure varied between 50 and 760 Torr. It was surprising to detect an ionization process, termed a ``secondary leader,'' that is distinctly different from the primary streamer. During this process a relatively highly ionized channel is formed. Similar channels were previously observed only under static voltages and in very long gaps. The paths of the secondary channels were found to be narrow and to follow a radial pattern; they terminate in ``threads'' of low intensity that have an irregular circular pattern. Measurements were made of their onset voltage and their rate of growth as a function of the voltage and pressure. Their onset voltage in air lies between 4 and 12 kV for 100 and 400 Torr, respectively; in N2 it is about 15% lower. Their length is linearly dependent upon the voltage, though the rate of growth varies with pressure.

The Role of Wire Size in Negative Electrical Discharge at High Temperature

Negative corona onset and sparkover voltages in air are shown to be affected by temperature, density, and the pipe-to-wire ratio of radii when using concentric cylindrical electrodes. Use of large wires at high temperatures demonstrates that onset of electrical discharge occurs at voltages significantly less than those predictable by earlier theory and that sparkover occurs at voltages considerably greater than those anticipated. Theory is presented to account for the results obtained.

The corona ignition voltage of an electrical discharge in a gas

The corona ignition voltage of an electrical discharge in air of atmospheric pressure depends on the presence of (moisture) particles, which may increase the corona losses. A relation between the corona ignition voltage and the particle size when tested shows unexpected results. With the corona ignition voltage in air as observed by Rose and Wood our calculations do not give particle sizes of 50 μm (as used by Rose and Wood), but sizes of about 1 A corresponding to the diameters of the molecules of the component gases in air. Our conclusion is that these molecules align in a conductive channel from the axial wires to the particle considered. In this way the charge transfer from the axial wire to the particles may be explained.

The time dependence of depolarization currents in diode structures of the metal-dielectric-metal type

The depolarization currents of Al-Al2O3-Au and Zr-ZrO2-Au diode structures with oxide thicknesses of D = 5600 A and 700 A were investigated. The depolarization currents were observed after switchoff of the polarizing voltage and removal of the geometric capacitance charges over a lengthy time interval. The direction currents were always opposite in direction to the currents which flowed through the structure under an applied voltage. The functions Idep =f (τ) were recorded at room temperature for various polarizing voltages in air under atmospheric conditions, in a ∼10-4 mm Hg vacuum, and in a vacuum after heating of the structures in a vacuum. It turned out that in the case of the structures investigated in a vacuum after heating in a vacuum, the time dependences of the current under an applied voltage and of the depolarization current were adequately explained by the mechanism of capture by the interior traps of the electrons injected under the action of the field [10]. The mechanism underlying the time dependence of the current in other cases was much more complicated.

Evaluation and Vacuum Stability Tests of Silicon Radiation Detectors

Tests have been carried out on a group of commercially available silicon radiation detectors of both diffused junction and surface barrier types. Measurements were made of reverse current and RMS noise level as a function of applied bias voltage in air and vacuum, alpha particle pulse height and energy resolution, and thickness of the sensitive surface dead layer. Long term stability was evaluated for samples in air and in vacuum by periodic measurements of reverse current, RMS noise voltage, and the pulse height and energy resolution for 5.8 Mev alpha particles. The samples in the vacuum tests were maintained at rated bias voltage and a pressure of approximately 10-6 Torr continuously for a period of six months. Only minor changes were observed in the performance characteristics of the devices. This work was sponsored by the Aeronautical Research Laboratory of Wright-Patterson Air Force Base, Office of Aerospace Research, United States Air Force.

Self-Propagating Intermittent Discharge

When a constant current pulse is applied to a pair of electrodes at small separations in air a series of electrical discharges takes place. These are characterized by a fixed distance between successive discharges. The intermittent nature of the discharge is determined by the local circuit and the spatial distribution of a series of discharges results from the blast wave which is propagated from each discharge. Measurements of the propagation distance as a function of the time interval between discharges and the energy released in each discharge are in qualitative agreement with those predicted from the theory of an intense explosion. The voltage transients across the electrodes show that during a series of discharges the breakdown voltages remain substantially constant and equal to the minimum breakdown voltage in air. This is attributed to the fact that the discharge can always find a low-density region which is quite near the front of the blast wave where the conditions for minimum breakdown voltage can be satisfied.

On the Fluctuation of Corona Onset Voltages in Air

On the Fluctuation of Corona Onset Voltages in Air

Calculation of spark breakdown voltages in air at atmospheric pressure

After a brief discussion of the different sparking criterions proposed during the last decades a new semi-empirical criterion for spark breakdown in air at atmospheric pressure is briefly mentioned. The new criterion depends on the ion density and is an extension of the quantitative criterion given by M. O. J orgensen.

Calculation of Initial Breakdown Voltages in Air

Calculation of Initial Breakdown Voltages in Air

Some developments in the routine pressure-testing of high-voltage cables

The difficulties experienced in connection with cable-ends when testing at high voltages in air are dealt with; and the reason for the preliminary sparking at the lead sheath and its tendency thereby to produce spark-over is explained. Various methods in normal use of overcoming this disadvantage are noted and are described in the figures. In particular, a suggestion is put forward for the use of semi-conducting material, as a temporary method of attaining greatly improved results for ordinary routine testing work. Comparative results are given both in tables and in curves, showing the advantages obtained by using semi-conducting carbon paper applied in various ways on or over the surface of the cable insulation.Theoretical lines-of-force diagrams are shown to explain these results and in what manner they are produced, and these are supported to some extent by practical measurements. Fusing-current tests are given for various samples of the material, and in view of these a possibility of the more permanent use of the principle is suggested in connection with the installation of insulators and bushings, etc.