Behaviour of free conducting particles and their role on breakdown in oils under AC and DC voltages

The experimental measurements of lift-off fields, transient currents, charge and discharge of spherical and cylindrical conducting particles are in good agreement with the theoretical predictions. The motions of these particles and their effects on prebreakdown and breakdown phenomena have been described. Cylindrical particles can reduce the AC breakdown voltage of uniform gaps to values as low as 20 kV_peak/cm. The breakdown voltage is significantly lower when the particle is free than when the particle is fixed. This is explained by an impulse-type breakdown from the particle tip generated by the particle/electrode micro-discharge rather than by a trigatron-type breakdown. The kind of experiment using a free particle is representative of critical situations that may appear in HV liquid insulated devices at large gaps