Electrical Reliability of DC Line Insulators

Recent investigations have shown that the environment of dc lines is more damaging to insulation than that of comparable ac lines. Various mechanisms which can lead to failure of ceramic or composite insulators have been investigated. Results are reported for ceramic insulators on thermal breakdown and ionic migration. For composite insulators, we have studied ionic migration through the fiber glass rod and through the housing. The harmful effect of humidity in the fiberglass material is again evidenced. We also studied erosion due to dry band arcing. The erosion rate on the housing materials commonly used is three to five times larger than in a comparable ac test. This shows the importance of the design of the insulator on its reliability. In some cases, materials that do not track in ac, track under a dc stress.