The effect of hydrostatic pressure on electrical treeing in silicone cable joints

The purpose of this work was to study the effect of hydrostatic pressure on the initiation and growth of electrical trees in a silicone insulation material for electrical apparatus. In this study a commercial available light transparent silicone material was subjected to a hydrostatic pressure in the range 1 to 100 bar at 30°C. The test objects were concentric cylinders with a tinned copper conductor as a high voltage electrode. The insulation thickness was 4 mm and stainless steel needles with tip radius of 4 μm were inserted in the conductor with a distance of 2 and 3 mm from the insulation surface. The joints were submerged in water which also acted as a ground electrode. During the high pressure tests the samples were placed in a stainless steel vessel with optical windows. High voltage was fed into the vessel by using specially designed miniature cable penetrators. In order to avoid partial discharges at the cable ends outside the vessel, silicone oil filled cable terminations were installed. The tree growth was monitored by measuring partial discharges (pd), together with recording of light emission from the discharge channel. The results show that tree growth in silicone rubber rapidly leads to breakdown. Pd inception is suppressed with increasing hydrostatic pressure, while the tree growth rate is slightly increased.