Effects of temperature and moisture on flow electrification in transformer pressboard/oil insulation

A novel Couette charger facility has been built to simulate flow electrification processes in transformers, where transformer oil fills the annulus between coaxial cylindrical electrodes that can be bare metal or transformer pressboard covered. Flow electrification charge density measurements as a function of inner cylinder speed have been performed using a newly developed absolute charge sensor (ACS) which brings a sample of fluid from the turbulent core into a Faraday cage and measures the total charge with no extraneous contributions from charge separated within the ACS itself. Transient measurements with a step change in temperature have shown the charge density to change from an initial to a new steady-state value. These charge density values depend on equilibrium moisture levels which in turn depend on the temperature. For all oils tested thus far with bare metal and T4 pressboard, the oil charge density is positive, but with HiVal pressboard there are some moisture and temperature regimes where the charge polarity reverses. The effects of transformer energization were also simulated by applying low-frequency (1 to 4 Hz) high voltages (field strength up to 1.8 kV/cm peak) across the cylinder of the Couette charger.<>