Sammendrag
This report presents results from an experimental investigation using Rogowski shaped test objects with needle implants, either fully insulated or in contact with one of the electrodes. In both cases abrupt grounding caused electric breakdown to be initiated at a voltage much lower than the short term breakdown voltage. Reduced breakdown strength occurred due to increased DC pre-stress level, number of rapid voltage groundings and rate of voltage reduction. The observations are explained by the concept of a field limited space charge injection (FLSC). When slowly increasing the applied voltage a cloud of homo charge is likely to be injected into the high field region around the needle tip resulting in reduced local electric stress preventing electrical tree formation. In case of rapid voltage grounding the injected charge will move in the opposite direction, towards the tip electrode. Due to the high local field enhancement, increased electric conductivity, high dissipated power and electromechanical forces may cause electrical treeing and breakdown.