DC fog is the suspension insulator specially developed for DC transmission system.

In a DC transmission system, anti-contamination performance is one of the most decisive factors for the insulation design.

As the behavior of DC local discharge on insulators is quite different from that of AC local discharge, we cannot simply use conventional AC Fog type suspension insulators for DC lines. A rather unusual DC Fog, which has shown excellent performance as expected, has been determined by our long-term research and experiences on the behavior of DC local discharge under contaminated condition.

Because of no-polarity alternation in ,DC, the DC local discharge on the surface of the disc is apt to continue for a long time. This is the very reason why partial arcs easily bridge over the tips of adjacent ribs in the case of the conventional AC fog disc, nullifying the leakage distance of the ribs.

AC withstand voltage is almost in proportion to the leakage distance of insulator. On the other hand Dc withstand voltage is determined not only by leakage distance but also by the shed profile. DC Fog has the optimal design in this respect.

DC Fog suspension insulator designed in accordance with the above criteria shows excellent performance compared with AC Fog insulator even though it has the same leakage distance.

 

(From internet)