Essam Nasser

Digital computer calculation of the electric potential and field of a rod gap

The electric field and potential distribution in the gap between a cylindrical rod having a hemispherical tip and an infinite plane perpendicular to the cylinder axis was determined using a charge simulation technique. This method assumes a charge at the center of the hemisphere and a finite number of semi-infinite axial charges in the cylindrical portion of the rod electrode. Boundary conditions in the cylindrical and spherical portions enabled the formulation of simultaneous equations whose digital solution yielded the assumed lumped charges. The digital computer program provided values of the potential and both field components anywhere in the gap with an accuracy of about 2 percent.

Mathematical‐physical model of the streamer in nonuniform fields

The initial electron avalanche created in the high nonuniform fields near an anode leaves behind it a space charge of positive ions that enhance the field farther away from the anode and, hence, produce the necessary conditions for successive electron avalanches to produce the streamer. A mathematical model of these processes has been derived and the minimum anode voltage required to satisfy these equations has been computed for different geometrical parameters. This streamer onset voltage agrees well with measured values. The conditions for streamer onset are found to exist whenever the average gradient of the field is higher than a certain value specified by the initial avalanche length.

Digital Computer Calculation of the Potential and Its Gradient of a Twin Cylindrical Conductor

A knowledge of the electric field is required to estimate the onset voltages of corona breakdown for extra-high voltage lines with bundle conductors. A method is described for computing the potential and electric field in the gap between unipolar twin- bundle conductors and ground based on a charge simulation technique and the principle of images. In this method the charge on the conductors is represented by several lumped charges. Boundary conditions on the conductors surface allow formulation of simultaneous equations, whose digital solution yields the assumed lumped charges, from which the potential and electric field are directly computed. The digital computer program provided values of the potential and electric field, magnitude, and direction, anywhere in the gap with an accuracy of 0.1 percent. The results of a sample calculation for an actual 345-kV conductor are given for varying parameters.

Some Physical Properties of Electrical Discharges on Contaminated-Surfaces

The various discharge phenomena caused by contamination of insulator surfaces were investigated by means of slow and fast oscillographic recordings of the discharge current. It was found that the surface discharges have distinct stages of development. The visual manifestations were recorded simultaneously with the current pulses, providing a good correlation and an exact physical interpretation. It was found that contaminated surface discharges can be classified in three main types: the arc, streamer, and glow. The arc discharge has a highly ionized plasma channel with a small voltage drop while the other two are of low ionization density and carry a relatively high voltage. The ignition mechanism was also investigated and explained. Both streamer and glow discharges are accompanied by steep current pulses and are believed to be responsible for the high-intensity radio noise associated with contaminated surface currents. The use of such pulses or bursts of current to evaluate insulator performance is critically discussed.

Field Criterion for Sustained Streamer Propagation

Experiments have shown that streamers are astonishingly capable of penetrating low‐field regions before finally coming to a stop. This lead to a proposed theory of low‐field requirements for continued propagation of the ionizing potential space wave constituting the streamer. The long range of impulse streamers in atmospheric air from a positive point was investigated in order to establish their spatial properties and a criterion that allows the prediction of such ranges. This was accomplished by axial and lateral measurements of the ranges of streamers for various gap spacings. A parallel study based on charge simulation and using a digital computer delivered the field intensity at the various points of the gap. The correlation of the results of both studies showed that streamers tend to stop whenever the applied field drops below 500 to 600 V/cm regardless of gap spacing or streamer location. Thus, streamers are not capable of propagating under an external zero field and require this minimum field for sustained advance.

Role of the Cathode Field Emission in the Streamer‐Spark Transition

In a point‐to‐plane gap in atmospheric air streamers propagate under a voltage pulse from the point anode into the low‐field region at high velocities and under continuous branching. It was found that electron emission from the copper cathode occurs under the effect of the high field between the streamer tips and the cathode just before impact. Photographic evidence by means of paper films placed on the cathode with sensitive emulsion facing it showed that at the points where strong streamer tips approached the back side of the film great concentrations of minute dots appeared. These dots are attributed to electron avalanches produced by those electrons liberated from the cathode and accelerated towards the streamer tip. The liberation of electrons was ascribed to the high field intensities at the tips since radiation cannot penetrate the paper. To confirm this, the back side of the paper was blackened at spots with no change in pictures. To further substantiate this, a thin metallic layer was placed at t...

Spark breakdown in air at a positive point

When a voltage across an insulating gas is increased beyond a certain limit, the gas breaks down, becoming a conductor of electricity. A photographic method, the Lichtenberg figure technique, is used to study the process. Above a certain voltage an ionization wave, called a streamer, proceeds from the highly stressed electrode, branching out along the way and extending far into low-field regions. At elevated voltages the vigorous streamers reach the cathode with a high-potential front. Owing to the front, the short-lived local field at the cathode triggers electron emission, also producing negative streamers. The negative streamers greatly increase the density of the ionized particles in the channel, yielding what is known as a backstroke. For long air gaps, streamers are incapable of reaching the cathode; intensive secondary channels therefore develop at the anode and proceed toward the cathode. Either the backstroke or the secondary channel paves the way for the full ionization of the channel and spark breakdown.

Calculation of the Potential Gradient of Twin-Cylindrical Bipolar Conductors with Various Geometrical Parameters

All theoretical methods for calculating corona thresholds in a nonuniform electric field depend on precise knowledge of the field distribution near the stressed electrode. The charge simulation technique and the method of images were applied to two twin parallel cylindrical conductors, all located in the same horizontal plane above ground. The charge on the positive and negative subconductors is represented by several lumped charges, and a set of equations are developed whose simultaneous solutions yield equipotential surfaces on the conductors circumference. These equations were solved for the magnitudes of the lumped charges, whose optimum locations were determined by considering the results over a wide range of line dimensions. A digital computer program was developed to provide values for the potential and electric field, magnitude, and direction anywhere in the gap. The error in computing the electrode surface potential is about 0.02%. When applied to bundle-conductor lines, this method has no restrictions with regard to spacing between the bundles or height above ground. A study on the effect of varying the line geometrical parameters on the potential and electric field has revealed interesting relations.