Surface charge accumulation on the spacers is one of the key issues restraining the development of HVDC GIS/GIL. The precise measurement of surface charge properties provides the basis for further study of the surface charge transport mechanism as well as the charge-induced flashover mechanism under DC voltage. In this study, the authors discuss their perspective on the current status, development needs and potential developing orientation of surface charge characterisation techniques. Different surface potential measurement methods and charge inversion algorithms are reviewed regarding the previous studies and future research needs. Drawbacks and outlooks of surface charge measurement techniques are also discussed with the background of laboratory experiment results and on-site measurements. It is hopefully that this study can serve as a useful guide reference for researchers within the same research field. More importantly, it is authors' hope that this study can inspire some novel ideas for readers into developing of more accurate and scientific interface charge characterisation techniques.
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.
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).
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