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DC wall bushing is one of the key equipment in high-voltage direct current (HVDC) transmission project. At present, post insulators based on epoxy composites are widely used for supporting insulation in SF6-insulated apparatus. However, the flashover faults caused by surface charge accumulation can be a severe threat to the long-term operation of the post insulators under dc field. Therefore, the issue of charge accumulation on epoxy post insulators in HVDC wall bushing and its effect on flashover has been widely concerned. Aiming at this problem, the flashover characteristics of epoxy samples in SF6 gas under polarity reversal conditions with dc voltage prestressed for 10–600 min were measured in this article. The flashover process is analyzed through surface charge distribution measurement and electric field computation. The results show that with the increase of prestressed time, the accumulated negative charge density could reach $-14.8\,\,\mu \text{C}/\text{m}^{{2}}$ , which led to a 12% increase in maximum field strength. As a result, the polarity reversal flashover voltage could decrease by 12% and the flashover time delay could decrease by 84.6%. This article can offer as a reference for the effect of surface charge accumulation on flashover in SF6 gas-insulated equipment. [ABSTRACT FROM AUTHOR]

The design of the oil–pressboard/paper insulation structure is the key to maintain the safe and reliable operation of power transformers, which mainly relies on simulation and verification test undertaken with simple or equivalent models, falling short of the physical entity verification undertaken in the large-scale structure. Based on the Kerr electro-optic effect and the optical fiber technology, a novel electric field measuring sensor was developed in this paper for the application test in the large-scale oil–pressboard structure. The sensor is contained of collimator, total reflection prism, coupler, and customized optical elements, whose appearance is a miniature capsule-shaped configuration. The key technical performance parameters of sensor were obtained, for instance, and the effect of imported sensor on the electric field under measurement is an average 1.17% reduction in intensity; the sensitivity of designed system is 162.84 V/mm under ac voltage and 40.43 V/mm under dc voltage. Under dc voltage, the measuring accuracy of the designed system is greater than 97.42%, while in the context of ac voltage, it could reach 97.14%. The application test was conducted to capture the dynamic process of the electric field in oil in the large-scale outlet device model, which has three uniformly spaced oil spacings divided by pressboards, under ac voltage, dc voltage, and polarity reversal voltage. The experimental results verify that the developed electric field sensor could be effectively and reliably applied in the large-scale oil–pressboard insulation, and it could be confidently inferred that the larger the scale of insulation structure, the better is the performance of sensor. [ABSTRACT FROM AUTHOR]

The Resistive-Capacitive (RC) model has been widely adopted for analysis of the electric field in oil-pressboard insulation under DC voltage. However, studies in this regard have indicated that the charge polarity effect in oil-pressboard insulation, namely the stronger adsorption of negative charges by pressboard, could not be adequately captured by the RC model. In order to analyze the electric field with due consideration of charge polarity effect, the present paper adopted the Kerr electro-optic techniques to have measured the electric field in the oil spacing under DC voltage for two experiments models with Model I having two oil spacing divided by a pressboard at the spacing ratio of 1:1, and the Model II at the spacing ratio of 1:2. As Model I is concerned, the strength of electric field in the upper oil spacing reached 1.61 times of the initial one under positive voltage and 1.60 times under negative voltage, while the field in the lower oil spacing registered as one eighth of the initial one under both positive and negative voltages. For Model II, the electric field strength in the upper oil spacing reached 2.3 times of the initial field under positive voltage and one fourth of that under negative voltage, showing much stronger charge polarity effect compared to that in Model I. In order to analyze the effect of interface charge on electric field, an electric field analysis model based on interface charge, named Interface Charge Polarity Effect (ICPE) Model, is proposed in this paper. Referring to the different adhesive abilities of positive and negative charges on pressboard, the influence of interface charge on electric field can be analyzed by the proposed model, which could make the contribution to the insulating design and proofread for the converter transformer. [ABSTRACT FROM PUBLISHER]