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Abstract A 3D particle‐in‐cell/Monte Carlo collision model is used to investigate the streamer discharge inception in CO2 at elevated pressures including gaseous, liquid and supercritical phases. Generation of free electrons is a prerequisite for initiation and development of positive streamers. Field ionization from impurity molecules of low density and low ionization energy is assumed as the source of primary ionization in the model. The field dependent generation rate of electrons is calculated by Zener's model. Tree‐like streamer with filamentary shape presents at the needle tip and propagates towards the plane electrode in all three phases of CO2, with differing in length and propagation speed. Furthermore, the influence of different parameters such as applied voltage, electrode tip size, and ionizable density and ionization potential of impurity molecules on the evolution of streamer discharge in supercritical CO2 are investigated. Simulation results obtained from the 3D model agree well with experimental observations in the literature, which show that field ionization might be one of possible ionization sources in pressurized CO2 in the presence of high electric field.

Ultra-high-frequency (UHF) method is regarded as an effective approach for partial discharge (PD) detection in gas insulated substation (GIS). The feature parameters representing UHF signal characteristics can be extracted from its waveform, and applied to PD source classification and mixed signals separation. In this study, an UHF signal feature extraction algorithm based on cumulative energy (CE) technique is proposed. The CE functions in time domain (TCE) and frequency domain (FCE) are calculated from UHF waveforms and their fast Fourier transform spectrums, respectively. The mathematical morphology gradient (MMG) operation is applied to TCE and FCE to characterise their rising behavior. The feature parameters including width, area and sharpness are extracted from CEs and MMGs in both time and frequency domain. Moreover, the extracted features are applied to UHF signals classification and mixed signals separation. First, experiments on four kinds of typical defects in a 252 kV GIS are performed, and the classification performance of the extracted features is examined with the acquired signals. Second, by clustering the extracted features with fuzzy maximum-likelihood algorithm, mixed UHF signals originated from multiple PD sources are successfully separated. The results indicate that the proposed features are effective for representing UHF signal characteristics. [ABSTRACT FROM AUTHOR]

Surface charge accumulation on insulators becomes an important bottleneck to restrain the development of a DC energy transmission system. This study investigates the effects of SiC/epoxy coating on surface charge behaviors and flashover performance of alumina/epoxy spacers. A nonlinear conductive coating consisting of different contents of SiC fillers and epoxy matrix was fabricated and sprayed on only half surface of the spacers. The dependence of surface charges on the contents of SiC fillers is characterized by three stages. Surface charge accumulation is aggravated at deterioration stage, then almost unaltered at balance stage and inhibited at suppression stage. These three stages are determined by the competition between the increased surface trap density from interface region and the improved surface conductivity from SiC fillers. The variation trend of flashover in air and SF6/N2 mixtures is distinguished by three stages as well. Flashover voltage decreases at descent stage where deficient contents of SiC fillers are appended and then goes up at improved stage with moderate supply of SiC fillers. Finally, it ends up with drastically collapse at degenerated stage when affluent SiC fillers are brought in. The theoretical analysis is proposed based on surface trap distribution and conductivity measurement results to reveal the regulation mechanism of SiC/epoxy coating on surface charge behavior and flashover performance. [ABSTRACT FROM AUTHOR]

An investigation is conducted regarding the influence of surface roughness on the flashover strength of an insulator in vacuum. A series of experiments is first presented, showing the relationship between surface roughness and flashover voltage, combined with measurement of surface potential distribution. It is found that surface charging on a roughened dielectric is suppressed remarkably; also the flashover voltage threshold depicts a rise-and-fall trend with roughness increasing, exhibiting a voltage summit at a certain roughness value. In addition, optical observation is implemented to draw a parallel with an electron trajectory in vacuum. A proposal can therefore be made that multipactor expansion tends to be mitigated as electron bombardment on a dielectric occurs with a lower secondary electron yield. Then a theoretical model considering the microscopic physical process is established to explain the above phenomena, consisting of an internal charge migration layer, an interfacial electron absorption layer and an external electron obstruction layer. The validity of this theoretical model can be confirmed by obtaining the surface trap distribution, microscopic morphology and net secondary electron yield. [ABSTRACT FROM AUTHOR]