Dielectric integrity associated with circulating insulating fluids

Two important electrokinetic effects in transformer oil subjected to forced circulation are experimentally investigated using a laboratory model duct. Careful breakdown measurements with sustained voltage on circulating fluids have been extended to pulse voltages with a view to establishing the nature of time dependencies. The results, together with previous optical evidence and simulation, indicate that electrohydrodynamic (EHD) effects are intimately involved with the breakdown of flowing oil. The interaction of EHD motion and forced flow results in a local turbulence, and thus an inhomogeneous gap state, which is thought to initiate the breakdown of the gap. The relationship between this process and that of flow-electrification is also covered by highlighting the effects of entrained charges on dielectric integrity downstream. The result demonstrates that not only is charge in the bulk able to distort electric fields in the stressed gap, but charge deposited on a dielectric surface can affect the withstand voltage of an oil/cellulose structure.