FLOW ELECTRIFICATION MEASUREMENTS OF TRANSFORMER INSULATION USING A COUETTE-FLOW FACILITY

A rotating cylindrical electrode apparatus, which provided cylindrical Couette flow, was used to simulate flow electrification in an electri c power transformer. The apparatus had Shell Diala A transformer oil f illing the annulus between coaxial cylindrical stainless steel electro des that were either bare metal, or covered by a thin copper sheet and /or EHV-Weidmann HiVal pressboard insulation. Extensive experiments ch aracterized the time transient and steady state behavior of the electr ification through measurements of the volume charge density the termin al voltage, and the terminal current as the system was driven out of e quilibrium by changes in the flow rate (inner cylinder rotation rates of 100 to 1400 rpm, Reynolds numbers of 5 x 10(3) to 5 x 10(5)), tempe rature (15 to 70 degrees C), insulation moisture content (0.5 to 20 pp m in the oil), applied voltage (0 to 2 kV dc), and concentration of th e non-ionizable antistatic additive 1,2,3 benzotriazole (BTA, 0 to 60 ppm). Generally, the electrification increased with flow rate and temp erature but the BTA appeared to cause competing effects: it decreased the volume charge density on the liquid side of the interface by a fac tor of 4, which reduces the electrification, but also decreased the oi l conductivity by a factor of 10, which enhances the electrification. A critical oil BTA concentration of 5 to 8 ppm minimized the electrifi cation. The volume charge density was sensitive to the current flowing through the interface and surface charge accumulation. With pressboar d covering the cylinders, the electrification effects of the temperatu re and moisture were decoupled during the transient following step red uctions in the temperature. The oil moisture content did not affect th e oil conductivity significantly, but the volume charge density varied inversely with the moisture content during an initial equilibration p eriod just after the oil had been added to the system. The highest ele ctrification was observed during this equilibration period.