Eugene J. McMahon

The Chemistry of Corona Degradation of Organic Insulating Materials in High-Voltage Fields and under Mechanical Strain

Organic materials used as high-voltage electrical insulation are altered chemically and physically by corona discharges. The basic behavior of polyolefin and fluorocarbon resins were investigated under conditions of high electrical stress. The presence of corona subjects a material to ozone, acid, ultraviolet light, and bombardment by electrons and ions. Degradation under these conditions is usually confined to the surface rather than the bulk of the material. The surface may be the outer surface of the material or the inside face of voids within the material. The present study includes the identification of by-products such as, for example, oxalic acid, which is found on the surface of polyethylene subjected to corona in the presence of humidity. The kinds of failure, cracking or erosion, and the effects of a number of variables including mechanical strain and humidity are presented. Also included is a discussion of electrical failures between the points of sharp needles imbedded opposite each other in a solid. Dendritic channels initiated by corona bombardment at the interface between the tip of one of the needles and the solid polymer slowly progress across the gap. The rate of dendrite growth is related to the gas pressure generated within the dendrite.

A Tree Growth Inhibiting Insulation for Power Cable

This paper describes a power cable insulation with a tree growth inhibiting additive that is nowommer commercially available for use by the electric utilities. The new insulating resin inhibits the growth of both electrical and water trees. A major cause of insulation failure is the progressive ive degradation process called treeing. There is general agreement that electrical and water trees cause insulation degradation leading to cable failure. This paper deals with a thermoplastic polyethylene insulating resin for use in power cable systems. Dodecanol is added to polyethylene to inhibit the growth of both electrical and water trees. The differences between electrical and water trees are reviewed. Test methods for evaluating insulating compositions for resistance to electrical and water tree growth are presented using small laboratory samples in accelerated tests. Data obtained in special model cables show the extent of migration of the dodecanol in accelerated tests. Test results on full sized commercial cables are discussed.

Techniques for studying volume discharges and treeing in organic insulating compounds

In this study of volume discharges and treeing we have abandoned uniform field electrode systems and are using sharp-pointed electrodes which permit us to observe the progressive development of channels referred to as “trees” or “dendrites.”

Dissipation Factor of Composite Polymer-and Oil-Insulating Structures on Extended Exposure to Simultaneous Thermal and Voltage Stress

The combination of a low-loss polymer film with a low-loss impregnating oil does not always give a low-loss composite insulation, particularly when the system is aged under simultaneous thermal and voltage stress. This anomalous behavior appears to be related to a loss phenomenon at the oil-to-polymer interface. For the combination of fluorinated ethylene propylene (FEP) film and polybutene oil a remarkable reduction in dissipation factor was achieved by increasing the degree of wetting at the interface. This was brought about by modifications tending to increase the surface energy of the solid (polymer) and to decrease the surface tension of the liquid (oil). The resulting system gave low and stable dissipation factors on extended aging and, in addition, did not show loss peaks as a function of voltage as observed on the original insulation.

Volume discharges and treeing — A primer

Since this paper is directed to the practical electrical designer who uses electrical insulation as only one of several elements in a complete design, it is assumed that he is not familiar with the more theoretical investigations of how and why insulations fail under high voltage stresses. For this reason, in attempting to discuss the subject of Volume Discharges and Treeing — which is a highly theoretical subject — we are approaching it in a very elementary fashion. If this simple approach helps some of you to a better understanding of electrical breakdown, we need not apologize to those who are expert in the area.

Electrical properties of plastic and oil composite insulating structures

The object of this program is to measure the electrical properties of solid polymeric films in composite structures with oil. There are many routes one can take to evaluate electrical insulating materials for use, for example, in high-voltage-cable applications. You can build a cable and test it — a very expensive procedure — or you can wrap models using small-diameter rods and a limited number of wraps of tapes. Another approach is to use a uniform-field flat-film system using guarded electrodes. It is possible that accurate data can be obtained easily and inexpensively from measurements made in a uniform field.

High voltage dielectric behavior of fluorocarbon resins

Since 1945 the Du Pont Company has been carrying out a systematic study of the behavior of electrical insulating materials under high voltage gradients. The effects have been studied both in the presence of and in the absence of corona or ionizing discharges. This paper presents some of the conclusions based on our studies of Teflon ® tetrafluoroethylene resins.

A study of the effects of corona on polyethylene

The deleterious effect of corona on the insulation of wires and cables subjected to high voltages has long been recognized. This basic work was undertaken to investigate the effects of corona on commercial Fawcett-type polyethylenes (branched high-pressure free-radical polymers) used as insulation in high-voltage (HV) applications. Two questions were to be answered. What are the factors involved in the design of an HV cable? What are the criteria for selecting the best polyethylene resin for this use?