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 now electrification in an electric power transformer. The apparatus had Shell Diala A transformer oil filling the annulus between coaxial cylindrical stainless steel electrodes that were either bare metal, or covered by a thin copper sheet and/or EHV-Weidmann HiVal pressboard insulation. Extensive experiments characterized the time transient and steady state behaviour of the electrification through measurements of the volume charge density, the terminal voltage, and the terminal current as the system was driven out of equilibrium by changes in the flow rate (inner cylinder rotation rates of 100 to 1400 rpm, Reynolds numbers of 5×10³ to 5×10⁵), temperature (15 to 70°C), insulation moisture content (0.5 to 20 ppm in the oil), applied voltage (0 to 2 kV dc), and concentration of the non-ionizable antistatic additive 1,2,3 benzotriazole (BTA, 0 to 60 ppm). Generally, the electrification increased with flow rate and temperature but the BTA appeared to cause competing effects: it decreased the volume charge density on the liquid side of the interface by a factor of 4, which reduces the electrification, but also decreased the oil conductivity by a factor of 10, which enhances the electrification. A critical oil BTA concentration of 5 to 8 ppm minimized the electrification. The volume charge density was sensitive to the current flowing through the interface and surface charge accumulation. With pressboard covering the cylinders, the electrification effects of the temperature and moisture were decoupled during the transient following step reductions in the temperature. The oil moisture content did not affect the oil conductivity significantly, but the volume charge density varied inversely with the moisture content during an initial equilibration period just after the oil had been added to the system. The highest electrification was observed during this equilibration period