Couette charger for measurement of equilibrium and energization flow electrification parameters: application to transformer insulation

The fluid under test (e.g. transformer oil) fills the annulus between coaxial circular cylinders that are covered by the solid of interest (e.g., metal or oil-impregnated paper backed by metal). The inner cylinder can rotate at speeds giving controlled laminar and turbulent flows (for fluids as viscous as transformer oil). This relatively compact apparatus allows for flexibility in testing liquids, trace impurities, and solid insulation without involving large amounts of material. By virtue of the reentrant flow, it acts like a pipe of infinite length and thus provides for the measurement of basic electrification parameters even in highly insulating systems, where the electrical development length for a pipe flow system is likely to be longer than the practical length of a test section. By sampling the charge entrained in the azimuthal flow at a sufficiently slow rate, equilibrium parameters can be deduced.