Pearl-chain-type Trees in Silicone Gel Under AC Voltage

The tree growth has been usually classified to three stages; inception, propagation and runaway stage under AV voltage. When the two-dimensional (2-D) specimen of silicone rubber was used, the propagation stage was divided to two stages: straight and isotropic growth stages. 2-D specimen is useful to observe AC trees with many branches since the number of the branches of AC tree is usually larger than DC and impulse tree. When the silicone rubber is fabricated, two types of silicone gel is mixed up; one includes catalytic agent. If the mixed ratio is changed from 1:1, silicone rubber has fluidity or viscosity. In theses cases, breakdown or partial discharge phenomena also change. Under certain conditions of gel, tree has been developing with bubble-like and string-like pattern alternatively. This tree is called pearl-chain-like tree. At first the tree developed like bubble and then a projection like cone was generated on the surface of the bubble. The tip of the cone expanded and formed a new bubble. This process was repeated. If two tips of the cone are formed on the surface of the bubble, the branching occurs. The shape of tree is like pearl-chain. The pattern of the tree is usually string (or straight) channel and branching. However In the pearl-chain like tree, string channel has consisted of two channels; bubble and string. Branching has occurred at bubbles. These data is important to investigate the mechanism of tree.