Chathan M. Cooke

Measurement of spatial charge distribution in thick dielectrics using the pulsed electroacoustic method

Measurement techniques of volume charge distribution within insulating materials are developed using a pulsed electroacoustic method. The fundamental equation of the relation between the charge distribution in the insulating material and the signal voltage from the electroacoustic transducer is presented along with the typical measured charge profiles in polymethylmethacrylate (PMMA) during and after electron-beam irradiation. >

Influence of particles on AC and DC electrical performance of gas insulated systems at extra-high-voltage

High-voltage breakdown measurements were made in two similar particle contaminated coaxial test systems, one with AC and the other with DC voltages. Information is presented on the effects of particle size, shape, and material for both SF6and N2gases at pressures up to 15 atm in a plain coaxial gap and a coaxial gap including a post-type support spacer. Particle motion and location were found to strongly influence insulation performance. Measured values of electric fields which lifted and drove the particles, so that they bounced vertically and laterally, compare favorably with calculated levels. Movement into the the higher stress region at the center conductor was correlated with the initiation of sparkover. These breakdowns could be at levels more than a factor of five lower than those obtained when contamination was not introduced. Large variations in breakdown voltage of as much as 3 to 1 encountered under DC correspond to conditions where particle motion could be restricted, presumably by corona discharge, to motion near the outer electrode. AC sparkover levels were typically at the lower limits of the DC range. Both free and attached particles on the dielectric spacer surface would trigger flashover at the same low levels as were measured in the gas gap.

Charging of Insulator Surfaces by Ionization and Transport in Gases

Insulator surfaces in gases collect charge when the rate of charge arrival exceeds the rate of conduction by the insulator from the surface region. The source of the collected charge may be in close proximity to, and hence greatly influenced by, the surface. Alternately, the source may be remote so that it releases charge independent of the surface accumulation. For the latter arrangement, charge transport through the gas greatly influences where it is collected. Surface charging is desired in some situations, such as for producing images or for processing. Charging is undesirable and hazardous in other situations; for example, where electrical failure may be triggered. A third possibility makes use of surface charge collection as a diagnostic procedure for materials and transport studies. This paper is concerned with the basic production and accumulation of surface charges from the adjacent gas and presents results on the processes involved. Transport parameters of drift, diffusion, and spacecharge effects are considered. Examples of charging and measured distribution under different conditions, including saturation effects, are analyzed.

Pulsed electro-acoustic method for the measurement of volume charges in E-beam irradiated PMMA

The measurement of volume charges within insulators has been attempted by various methods, including thermal, optical, mechanical, and electrical stimulation of the sample. A new method incorporating pulse electrical stimulation and pulsed acoustic detection is reported here. This technique is an extension of a previously reported [1] high frequency AC steady state electro-acoustic measurements in which electric fields at an electrode surface are determined by measuring the AC high frequency vibration of the electrode in response to a DC plus high frequency AC excitation. The use of pulse operation allows the measurements of time domain delay information which yields the spatial distribution of the volume charge.

Kerr Electro-Optic Field Mapping Measurements in Electron Beam Irradiated Polymethylmethacrylate

Kerr electro-optic field mapping measurements are presented in electron beam irradiated polymethylmethacrylate (PMMA) where the accumulated trapped charge results in large self-electric fields of the order of 1 to 2.5 MV/ cm. The resulting numerous light maximum and minimum recorded on photographic film and videotape allow accurate measurement of the time dependence of the electric field and space charge distributions.

Bulk charging of epoxy insulation under DC stress

Stress enhancement due to the migration of charges can be a critical factor in the design of solid insulation for high voltage DC systems. The effect of bulk currents on surface and internal charge accumulation is discussed; and equations are formulated which predict steady-state conditions in arbitrary configurations o Experimental evidence is given for the validity of this analysis in epoxy insulators.

The Nature and Practice of Gases as Electrical Insulators

The gaseous state of matter is an excellent high voltage insulator which nonetheless is subject to overstresses and electrical breakdown. The main parameters which govern insulation strength in gases are identified as gas density and time duration of applied voltage. Several gases show similar effects of density and time changes so that generalized breakdown curves are presented. Practical considerations for the design of high voltage equipment are also developed through examples of insulation performance in compressed SF6. Features emphasized include the influence of electrode surfaces, particle contamination and solid insulating supports. Significant achievements in the application of gases to high voltage equipment and expected areas for future fundamental and applied developments are discussed.

Post-Type Support Spacers for Compressed Gas-Insulated Cables

The flashover performance of post-type support spacers for concentric compressed gas-insulated transmission lines was investigated. The experiments were made with SF6 gas at 4.4 atm. abs. and low frequency AC voltage in the EHV range. Nine cast epoxy spacers with metal inserts at each end shaped to produce different electric field distributions along the solid-gas interface were tested. The results show that flashover voltage is controlled by the electric field distribution and is inversely proportional to the stress enhancement in the gas. This conclusion is shown to be consistent with ionization-initiated breakdown at a certain stress limit about 300 kV/cm in these studies. The better designed spacers were limited by sparkover of the radial gas-gap at 1300 kV peak and not by surface flashover at the insulator.

Ionization, electrode surfaces and discharges in SF 6 at extra-high-voltages

The influence on DC electrical performance of SF6gas from small protrusions on electrode surfaces has been investigated experimentally in a system of moderate size over the pressure range from 1/2 to 15 atm. abs. The protrusions used were steel or aluminum spheres with 0.079cm radius or a rod 10 times higher than its tip radius of 0.039cm. Over the complete voltage range, 100 kV to 1500 kV, good agreement was found between measured values and those calculated using a simple ionization development model for discharge initiation. This simple model was applied to calculate the effect of other protrusion shapes. In each case when the product of gas pressure times protrusion height above a flat electrode exceeded 80 atm-microns their presence decreased the breakdown stress. Scanning microscope views of the sphere tips after sparking showed microscopic protrusions which account for the deterioration found after the first spark.

Electrical Breakdown and the Similarity Law in SF6 at Extra-High-Voltages

The DC electrical breakdown of SF6 gas at extra-high- voltages was studied experimentally in coaxial systems of moderate size. The results were compared to a similarity relationship of electric stress for breakdown based on ionization of the gas. For laboratory-clean systems, theory and experiment agree when the macroscopic gradient does not exceed 150 to 200 kV/cm. If the active area of the stressed electrode is small the law remains accurate for higher gradients. Departures from the law are qualitatively explained by including the electrode surface microstructure in its application.