06_article_space_g - 22 IEEE Electrical Insulation Magazine...

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Unformatted text preview: 22 IEEE Electrical Insulation Magazine F E A T U R E A R T I C L E F E A T U R E A R T I C L E F E A T U R E A R T I C L E F E A T U R E A R T I C L E F E A T U R E A R T I C L E A Rational Consideration of Space Charge Key Words: Space charge, current density, polymers, water trees, conduction. Steven Boggs Electrical Insulation Research Center, University of Connecticut, Storrs, CT Space charge measurement has become a “growth industry” within the dielectrics community. Space charge has been associated with aging, breakdown, etc. with varying success. Introduction pace charge measurement has become a “growth indus try” within the dielectrics community. Space charge has been associated with aging, breakdown, etc. with varying suc- cess. In some cases, the association is almost certainly not causal, i.e., the same phenomenon that causes the aging causes the space charge, which does not mean that space charge causes aging. In other situations, the relationship is probably causal, as in break- down during the transition from capacitive to resistive grading being a function of the degree of space charge formation. Space Charge Formation Space charge can result from a range of phenomena, but the most fundamental are 1. The combination of a current density and spatially inhomoge- neous resistivity as given in Cartesian coordinates by (1) where ζ is the space charge density, J is the current density, ε is the absolute dielectric constant, and ρ is the electrical resis- tivity. 2. Ionization of species within the dielectric to form heterocharge. 3. “Charge injection” from a stress enhancement; however, such space charge will often be too localized to be detected. 4. Polarization in structures such as water trees. Of these mechanisms, the first two probably account for the majority of space charge detected, and, of these two mechanisms, the first typically dominates. Effect of Sample Specimen Most space charge measurements are carried out on thin, poly- meric samples. Formation of most polymeric films involves sub- stantial heating, which results in surface oxidation. This oxida- tion probably extends to the range of 100 µ m into the sample, with the result that many samples will be entirely in the “surface domain” and may not be typical of a bulk sample. Such surface effects can be found in a number of ways. The late Dr. Das Gupta mentioned in a conversation that he once measured the conduc- tivity of the surface layer of a polyethylene sample and found that the conductivity was 100 to 1000 times greater than the bulk. Similar results have been inferred from space charge measure- ments of thick samples [1]. Interdiffusion between semicons and bulk dielectrics can also change properties in the electrode interface region, as has also been documented in the literature [2]. These electrode interface (surface) effects will certainly re- sult in substantial space charge formation according to (1)....
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06_article_space_g - 22 IEEE Electrical Insulation Magazine...

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