Breakdown initiation fields for surface flashover of dielectric materials immersed in transformer oil

The applied electrical fields required to initiate surface flashover of different types of polymeric insulating material immersed in transformer oil have been investigated. Cylindrical samples of polypropylene, ultra-high molecular weight polyethylene, low-density polyethylene, and Rexolite were tested. Average electrical fields up to ~700 kV/cm were applied to samples via a 10-stage, inverting Marx generator. The samples were held between two electrodes immersed in 20 litres of EOS Ltd. (UK) L10B transformer oil. The stage capacitance of the Marx is 80 nF, and the rise time is around 100 ns with the test cell load. The applied electrical field required to initiate surface flashover was found by firing the system with an increasing charging voltage until a breakdown occurred. Experiments were performed with plane-plane electrodes and two types of point-plane electrode gap. The first point-plane gap was achieved by deploying a high-voltage electrode with a 1-mmdiameter pin protruding perpendicular to the surface, and the second by a high-voltage electrode with a 25-mm-diameter collar centred on the surface. Of the materials tested, polypropylene showed the highest average peak applied fields to initiate the first surface flashover, varying from 325 kV/cm in non-uniform fields to 637 kV/cm in uniform fields.