DC prestressing effect on breakdown characteristic of insulating paper in liquid nitrogen and insulating oil

Currently, high temperature superconducting (HTS) cables are developed mainly as AC cables with PPLP^®-liquid nitrogen composite insulating system. On the other hand, solar and wind power are expected as alternative energy of fossil fuels in the future. DC transmission using superconducting technology having low transmission loss and environmental burden is most suitable for such new energies. However, DC HTS cable may raise the problem that charge injection may degrade the insulating performance, which is the same as the problem in existing DC oil immersion cables. Moreover, the influence of charge injection on breakdown characteristics of PPLP^® in liquid nitrogen (LN₂) is not clarified yet. In this paper, DC prestressing effects on impulse breakdown strength (Fb) of PPLP^® was investigated, and the charge injection model in LN₂ was discussed from the conventional knowledge to charge injection in DDB oil. As a result, the opposite polarity relations in impulse Fb and the prestressing effects in LN₂ appeared compared with the results in DDB oil. The DC prestress effect on impulse Fb in DDB oil can be explained based on the charge injection model in LN₂ with the opposite polarity charge injection. Therefore, the difference of breakdown characteristics of PPLP^® in DDB oil and in LN₂ are influenced by the polarity of the charge that is easy to inject into KP layer and can explain using the charge injection model developed for the breakdown characteristics in LN₂.