No abstract

Ultraviolet (UV) radiation decreases the hydrophobicity of silicone rubber (SiR) insulators. Lack of hydrophobicity on the surface of such insulators lowers the flashover voltage and increases the Leakage Current (LC) of insulators in the moist environment. This paper presents the artificial UV radiation effects on three types of 20 kV SiR insulators in a chamber with nine UV-C lamps (50 W/m 2 ) for up to 5000 hours.To simulate the moist environment, solid layer method is applied according to IEC 60507 that includes artificial pollution deposition on the insulator surface. Fast Fourier transform of the LC waveforms indicates that the third and the fifth harmonic components are quite sensitive to any incremental discharge on the insulator surface. This sensitivity makes the frequency spectrum of the LC a good criterion to distinguish three distinct SiR insulator situations under the electric stress: 1) no discharge, 2) dry band arcing/corona discharge and 3) continuous arc. Thermogravimetric analysis (TGA) and scanning electron microscope images depict that the polluted insulators include much larger degraded parts under electric stress after aging. Measurements also show that the flashover voltages and the hydrophobicity of aged insulators decrease as the UV exposure time increases for different levels of moisture and pollution.

Streamer branching in liquid dielectrics is driven by stochastic and deterministic factors. The presence of stochastic causes of streamer branching such as inhomogeneities inherited from noisy initial states, impurities, or charge carrier density fluctuations is inevitable in any dielectric. A fully three-dimensional streamer model presented in this paper indicates that deterministic origins of branching are intrinsic attributes of streamers, which in some cases make the branching inevitable depending on shape and velocity of the volume charge at the streamer frontier. Specifically, any given inhomogeneous perturbation can result in streamer branching if the volume charge layer at the original streamer head is relatively thin and slow enough. Furthermore, discrete nature of electrons at the leading edge of an ionization front always guarantees the existence of a non-zero inhomogeneous perturbation ahead of the streamer head propagating even in perfectly homogeneous dielectric. Based on the modeling results for streamers propagating in a liquid dielectric, a gauge on the streamer head geometry is introduced that determines whether the branching occurs under particular inhomogeneous circumstances. Estimated number, diameter, and velocity of the born branches agree qualitatively with experimental images of the streamer branching. V

No abstract

No abstract