5 CONCLUSIONPartial discharges in a

5 CONCLUSION
Partial discharges in a liquid nitrogen/solid dielectric insulation system have been investigated experimentally using a point-to-plane configuration. The inception voltage for PD in LN2 can be estimated under the hypothesis that there is a threshold electric field above which PD streamers are initiated. In this study, this value was found in the order of 1.55 MVmm-1.
Phase resolved analysis using φ-q-n plotting shows that PD events occur primarily in the first and third quadrants of the voltage waveform. The inception phase shifts towards the zero crossing as the applied voltage increases and eventually back discharge occurs. The foregoing phenomenon indicates the importance of charge storage on the PD patterns in a composite insulation system. It is difficult to conclude on how the charge is stored. However, the difference in the PD patterns between PTFE and G10 barriers together with streamer attachments observed by the shadowgraphs would suggest that charge carriers were stored on the surface of the solid dielectric barrier.
One concern is the degradation of the solid materials due the streamers discharge in LN2. The study has proposed certain voltage and charge levels in which streamer attachment happens. Below these values, the discharges only happen in the vicinity of the needle tips and do not affect the solid materials. Previous work at Southampton [26] has shown the presence of fields in excess of 2 kV/mm-1 can cause a reduction in the hysteresis between convection in the liquid and nucleate boiling. When combining this result with the findings here, it is possible to hypothesis that the presence, under fault condition, of a very high electric field could have two significant effects: first the formation of filamentary