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The brittle fracture (BF) surface of a 500 kV composite insulator was analyzed. It was operated for 6 years. Chemical compositions on the field failed BF surface were examined by X-ray fluorescence spectrum and FTIR spectroscopy. Test results showed that the chemical functional groups N[O.sub.3.sup.-], N[O.sub.2.sup.-], together with S[O.sub.4.sup.2-], were formed both on the BF surface of FRP rod and in the rust on end fitting near BF crack. Laboratory simulated BF by stress corrosion under nitric acid or sulfuric acid were all analyzed and compared with the field failed BF surface. Analysis of the field failed insulator reveals that the BF is influenced not only by nitric acid, which may be formed by electrical discharge, but also by sulfuric acid, which may come from acid rain. Index Terms--Composite insulator, brittle fracture, stress corrosion, FTIR.

Water drops on surface of polymer insulation materials may cause corona because they can enhance the electric field nearby. Water drop corona plays an important negative role in the long-term performance of composite insulators. In this paper, based on a model with two parallel electrodes water drop corona is studied. The relationship between initial corona voltage and different sets of water drops is obtained from experiments. Then several factors, which may affect the electric field analysis of water drop corona, are studied by numerical calculation on computer. Calculation is mainly on a 2-D model. However, a 3-D model is also used to study the effect of water drops distributed in space. Index Terms--Composite insulation, corona, electric fields, numerical analysis.

Analyzing the nonuniformity of electrical parameters of ZnO varistors is useful to select and coordinate the ZnO varistors in parallel operation and improve the protection capability of surge-protection device (SPD). The nonuniformity of 1-mA dc voltage and impulse residual voltage of commercial low-voltage ZnO varistors were estimated by statistic analysis in this paper. The Minimum Gobble Distribution was used to describe the distribution of 1-mA dc voltage and impulse residual voltage, ZnO varistors were verified conforming to the Minimum Gobble Distribution by the Kolmogorov-Smirnov test. The relationship between the impulse residual voltage and 1-mA dc voltage has a chaos phenomenon, and they have a close relative degree by t-test, the impulse residual voltage can be estimated by voltage ratio and 1-mA dc voltage. The nonuniform phenomena of electrical parameters of ZnO varistors were simulated by Voronoi network. The current overload of ZnO varistor with reduced residual voltage is serious, in order to keep safe operation of SPDs, when we designa SPD with ZnO varistors in parallel, a safe margin coefficient of discharge withstand current should be considered according to the actual relative standard deviation of surge residual voltages of used ZnO varistors. Index Terms--Discharge withstand current, electrical parameter, Kolmogorov-Smirnov test, minimum gobble distribution, nonuniformity, statistic analysis, surge-protection device, surge residual voltage, ZnO varistor, 1-mA dc voltage.