Pollution flashover behavior of hydrophobic polymeric insulating interfaces at insulators under DC voltage stress and combined DC and AC voltage stress

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The demand of electrical energy is increasing to achieve the goals to limit the global warming process and to replace natural gas and oil due to the actual war, because the electrical energy is a key resource in all sectors. To transfer the electrical energy from the generation plants to the users an increasing capability of transmission lines are necessary. Additionally, the time period (gradient) of the significant increase in the electrical energy demand is short, on the other side the erection of new transmission power lines will take certain (usually a longer) time period from the planning till the commissioning due to different reasons. Back to the early 1970s, the transmission of electrical energy by using a mixed AC and DC voltage with the motivation of increasing the maximum transferable electrical power have been discussed. Based on the maximum transferable load can be upgraded by approx. 85 % at existing transmission lines by using a mixed of AC and DC transfer. The insulators need to provide the necessary insulation for the mixed AC and DC voltage in combination with the polluted surrounding. For the immediate usage of the mixed AC and DC voltage no change at the transmission line insulators and towers needs to be done. The design of porcelain, glass and composite insulates is understood and standardized by long-term practical experience for AC and DC. For the behavior and design of mixed insulators for combined AC and DC limited experience and investigations are available. Especially, the behavior of dry band discharges but also the pollution flashover characteristic may be different related to the missing zero crossing of the current (the pre-arcs will not distinguish). This different behavior will have effects to the behavior of polluted insulators with hydrophilic behavior (glass and porcelain insulators) as well also the hydrophobic behavior (composite insulators). The main goal of this research project is to investigate the differences between pure AC, DC and mixed AC and DC stress in the pollution flash over characteristic and the tracking erosion behavior, extract the processes and based on the result to adapt the available models. With these investigations, the fundamental data for the design and usage of transmission lines with a mixed AC and DC voltage are provided.