Roy E. Wootton

AC Corona and Breakdown Characteristics for Rod Gaps in Compressed Hydrogen, SF6 AND Hydrogen-SF6 Mixtures

The ac corona onset and breakdown voltages have been measured for rod-plane and rod-rod gaps in hydrogen, SF6, and hydrogen-SF6 mixtures for pressures between 1 and 5 bar, with gaps up to 14 cm, and voltages up to 250 kV rms. The addition to hydrogen of quantities of SF6 as low as 0.002% by volume resulted in a substantial increase in breakdown voltage of typically 70%. The corona onset voltage of the hydrogen was only increased by large additions of SF6 above 1%. With the mixtures for rod-rod gaps, breakdown values were obtained up to 70% higher than for SF6 alone, and there was no maximum in the breakdown voltage-pressure characteristic in contrast to the case for SF6. Very intense, stable glow currents of up to 20 mA were recorded. The characteristics and mechanisms of the various corona modes are discussed.

A Test for the Effect of High Energy Arcs on the Flash-over Strength of Insulators in Compressed SF6

A test method for evaluation of solid insulating material for insulators in Compressed Gas Insulated Transmission (CGIT) systems has been developed. The test measures the ability of an insulating surface to withstand voltage after being subjected to a high energy power arc across its surface. The test has been used to evaluate several different solid insulators in SF6 at 0.4 MPa. The test circuit is designed to effect an efficient transfer of energy from a capacitor bank to an arc on the insulator surface. Voltage reversals in the capacitor bank are minimized. Flash-over of the insulator is initiated using an impulse generator. After exposing the insulator to the arc, the power frequency flash-over voltage is used as a relative measure of the ability of the material to withstand power arcs. The test circuit, sample geometry and representative measurements are described. Epoxy systems with the best performance in these tests showed little or no decrease in power-frequency flash-over voltage after arcing, while large reducitons were observed in other epoxies after only a few power arc-overs. This arc damage test was developed as part of an ERDA Contract to develop a prototype 1200 kV CGIT system.

Some Aspects of Breakdown in Gases

This paper discusses the occurrence of corona in small, highly-stressed, regions in large apparatus.

Particle-initiated ac and dc breakdown in compressed nitrogen, SF 6 , and nitrogen-SF 6 mixtures

The preliminary results presented here are an extension of our previous studies into the effect of metallic particle contaminants on the high voltage electrical breakdown in compressed gases.1,2 The earlier work dealt mainly with ac breakdown in SF6 gas in coaxial electrode geometries, under conditions where it was not possible to directly view the gap under voltage. The present study used a new test chamber in which direct observations of particle behavior between plane parallel electrodes at elevated gas pressures were possible.

Electric charge accumulation on HVDC insulators in compressed-SF 6 -insulated transmission lines

This paper describes measurements of the magnitude and distribution of residual fields around a tri-post insulator in compressed SF6, after being subjected to sustained dc voltages of several hundred kV, of negative and positive polarity. The effects of geometry associated with insulator leg fittings, particle-trap slots and residual dust were studied. Comparative measurements were also made on an annular polyethylene insulator in the same coaxial system, and on uniform thickness disks of polyethylene and filled-epoxy resin in both uniform and non-uniform fields in air. The decay of the residual field involved (in some cases) field reversal leading to the conclusion that some part of field measured initially was due to a slow polarization or bulk charging of the dielectric rather than conduction through and/or over the surface of the insulator and drift of ions through the gas. This work was carried out under Department of Energy Contract DE-AC01-79ET-29059 for the design of a ±600kVdc gas insulated transmission line.

Mapping of Electrostatic Charge Using a Robot Controlled by a Computer

This paper is intended to fulfill two purposes. The first objective is to describe a particular laboratory system which maps the distribution and decay of residual electric fields around flat or curved insulating surfaces. Such a system may have applications in the noudestructive testing of insulation in the form of boards, cylinders, cones, and other shapes. The second objective is to point out the general usefulness of a system consisting of one or more sensors positioned by a robot controlled by a computer for reliably mapping in detail many kinds of fields in three dimensions. The particular implementation described uses a modified Microbot Teachmover robot to move the measuring head of a Monroe Type 230A field meter through a two-dimensional rectangular scanning grid over a flat sheet specimen. A DEC 11/23 MINC laboratory computer is used to both control the robots movements, and to digitize and store in memory the readings of the field meter. The computer also plots a field map on the video screen during the scan, and stores field data on disk for later retrieval and processing. Results of scans of residual field are given for a typical plastic (polypropylene). Plots of the rate of change of residual field (obtained by taking the difference between scans) are also given. The significance of these results for nondestructive testing is discussed. The combination of one or more sensors, robot, and a computer, forms a flexible and convenient bench top system for the measurement, processing, real-time display, and storage of large amounts of data on two- and three-dimensional specimens.

Triggered breakdown time-lags of transformer oil subjected to uniform power frequency fields

In this paper we report some of the results obtained in a study of triggered breakdown in technical grade transformer oil. The gap studied consisted of two brass hemispheres of 12 cm diameter with the gap axis mounted horizontally in an oil tank. One of these electrodes was connected to a 60 Hz high voltage transformer, the other was permanently grounded and contained within it a 1.5 mm diameter tungsten rod set in a 3.2 mm diameter hole to form a trigger gap of approximately 0.85 mm. The tip of the trigger pin was mounted flush with the hemispherical surface. Figure 1 shows a simplified diagram of the trigger circuit. Additional circuitry permitted firing the trigger circuit at any phase position of the 60 Hz voltage applied to the main.

A test for the effect of high energy arcs on the flash-over strength of insulators in compressed SF 6

A test method for evaluation of solid insulating material for insulators in Compressed Gas Insulated Transmission (CGIT) systems has been developed. The test measures the ability of an insulating surface to withstand voltage after being subjected to a high energy power arc across its surface. The test has been used to evaluate several different solid insulators in SF6 at 45 psig. The test circuit is designed to effect an efficient transfer of energy from a capacitor bank to an arc on the insulator surface. Voltage reversals in the capacitor bank are minimized. Flash-over of the insulator is initiated using an impulse generator. After exposing the insulator to the arc, the power frequency flash-over voltage is used as a relative measure of the ability of the material to withstand power arcs. The test circuit, sample geometry and representative measurements are described. Epoxy systems with the best performance in these tests showed little or no decrease in power-frequency flash-over voltage after arcing, while large reductions were observed in other epoxies after only a few power arc-overs. This arc damage test was developed as part of an ERDA Contract to develop a prototype 1200 kV CGIT system.

Time-lags of triggered breakdown in transformer oil subjected to power frequency stress

Previous work1 by the authors has shown that the breakdown time-lags for oil-filled trigatrons subjected to power frequency voltages can range from a small fraction of a cycle to several cycles of 60 Hz voltage. In general it was shown that long breakdown delays of one quarter to several cycles of power frequency voltage occurred with large amounts of available trigger energy (∼1 J) and low stresses across the main gap (−50 kV/cm rms), while short delays (<1 ms) resulted from the use of higher main-gap stresses and lower available trigger energies (tens of mJ). The present study has sought to examine in greater detail the time-to-breakdown characteristics in the submillisecond delay region and to discuss various breakdown mechanisms in terms of the dependence of the breakdown probability on time.

Electrical discharges and breakdown in gases

The scope and format of the chapter are similar to those of previous years, the literature being reviewed under the following major headings: Monoenergetic Beam Experiments; Swarm Data; Uniform Field Breakdown; Nonuniform Field Breakdown; Breakdown with High Frequency and Magnetic Fields; Laser-Induced Breakdown; Gas Lasers; and Applications. The material was gathered directly from the literature and with the use of Physics Abstracts and Electrical and Electronic Abstracts.