The measuring system of charges accumulated on an insulating plate is developed with an electrostatic probe whose sensing electrode is of 0.5 mm diameter. In the inverse calculation from the probe outputs to the charge distribution, Tikhonov's regularization technique is effectively used to suppress the excessive amplification of the noise. In the case of measuring a 2 mm thick PMMA plate, the spatial resolution of the measuring system reaches 1.0 mm. With this system, the residual surface charge distribution on an insulating plate just after the occurrence of a positive surface discharge at 25 kPa (190 Torr) air is measured. On the head of a positive surface streamer, there remains 10−11 C surface charge, which satisfies the criterion of an electron avalanche-to-streamer transformation. The charge density across a streamer takes a minimum on its centre and a maximum on its sheath. This fact suggests that a quasineutral channel exists in the centre of a streamer, and positive ions remain on the sheath of it.

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The phenomenon of accumulated charges on solid insulator surfaces is one of the critical parameters to consider at the insulation design stage, for ac electric power equipment as well as for dc equipment, so it is important to investigate the characteristics and predominant factors underlying various charging mechanisms. Several researches related to this theme have been reported, but independently, and cross-sectional comparison and evaluation from a unified viewpoint are meaningful. In this paper, the resistance of solid insulator is first discussed, showing that the resistances found by diverse measurements are in a fairly good agreement under similar conditions of the temperature and electric field. Next, three kinds of electric charging mechanisms, i.e. volume conduction, surface conduction and electric field emission are characterized in terms of the time constant, applied voltage and charge distribution. Then, eight cases of recent measurements on the charging time are investigated and their charging mechanisms are classified. Electric field emissions are likely to occur with model spacers made in routine GIS manufacturing process at the electric field level used. Further, three examples in cases with metallic particles are introduced, of simulating charge from edge on the tank, spacer surface charging phenomena, and influence of charge on spacer surface flashover. It is expected that this paper will be helpful for understanding charging phenomena e.g. on insulation spacers in gas insulated switchgears.

This paper describes discharge properties of N 2 and CO 2 -based gas mixtures including a perfluorocarbon (PFC) gas such as CF 4 , C 3 F 8 and c-C 4 F 8 under non-uniform field. The mixture ratio between a base gas of N 2 or CO 2 and the additive PFC gas was fixed as 9:1; namely, 90%N 2 /10%PFC or 90%CO 2 /10%PFC gas mixture. The PFC gases have even smaller global warming potential (GWP) than SF 6 gas and have good insulation properties as SF 6 gas. Thus, PFC gas mixture is expected to be a SF 6 substitute without highly pressurizing the gas over the conventional pressure of 0.5 to 0.6 MPa. In this study, in order to compare the partial discharge (PD) inception voltage V PD and breakdown voltage V B properties between N 2 and CO 2 -based gas mixtures, as well as between the additive gas of PFC and SF 6 gas, we investigated these properties of the gas mixtures with a needle to plane electrode under ac high voltage application. The gas pressure was changed from 0.1 to 0.6 MPa. As a result, it was found that V PD and V B characteristics of N 2 and CO 2 -based gas mixtures differed considerably, especially the gas pressure dependence of V B (so-called the N shape characteristics). V B characteristics of N 2 -based gas mixture including c-C 4 F 8 proved to be excellent within the test conditions over the wide gas pressure region, showing the maximum breakdown voltage. In terms of V PD properties, CO 2 -based gas mixture had an advantage over N 2 -based gas mixture due to higher V PD . Furthermore, we discussed the synergy effects of V PD and V B for N 2 and CO 2 -based gas mixtures using the index R n which was defined to quantify the degree of the effect. R n for CO 2 -based gas mixture was higher than that of N 2 -based gas mixture.Index Terms -SF 6 gas, perfluorocarbon (PFC) gas, partial discharge, breakdown voltage, non-uniform field.

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Partial discharge (PD) detection using a UHF (ultra high frequency) band signal is a well known advanced insulation diagnosis method in gas insulated switchgear (GIS), and has been actively studied. Detailed investigation of electromagnetic (EM) wave propagation inside the GIS tank is required for significant improvement of detecting PD signal by UHF method. When practically applying the UHF method to GIS insulation diagnostics, it is necessary to examine the effects of GIS components such as circuit breakers, isolators and disconnectors on EM wave propagation properties. In this paper, attention is paid to the effects of a disconnecting part of a high voltage (HV) conductor like a circuit breaker or a disconnector in GIS.To examine the effects of disconnecting part, the gap length of the disconnecting part was set as parameter, and waveforms and frequency spectra of the propagation PD-induced EM wave were measured with UHF sensors. For the purpose of discussing the effects of the disconnecting part theoretically, a finite difference time domain (FD-TD) simulation was also carried out. The experimental results show that the PD-induced wave could propagate through the disconnecting part with higher frequency components over the cutoff frequency components of TE11 mode for disconnecting part, i.e. cylindrical shape formed by GIS tank without HV conductor. The propagation of the lower frequency components below the TE11 mode depended on the gap length of the disconnecting part.Index Terms -Gas insulated switchgear (GIS), partial discharge (PD), electromagnetic (EM) wave, UHF (ultra high frequency) method, TE11 mode, cutoff frequency, insulation diagnosis, disconnecting part, finite difference time domain (FD-TD).