Temperature and moisture transient flow electrification measurements of transformer pressboard/oil insulation using a Couette facility

To simulate the flow electrification processes in transformers, a `Couette facility´ has been developed, where transformer oil fills the annulus between coaxial cylindrical electrodes which can be bare metal or covered with transformer pressboard. Rotation of the inner cylinder provides controlled turbulent flow. This turbulent mixing transports electric charge from the electrical double layer at the liquid/solid interfaces to the oil volume. Measurements of the volume charge density are performed using an absolute charge sensor, which brings a sample of fluid from the turbulent core into a Faraday cage. In this facility, the temperature can be controlled from 15°C to 140°C and vacuum drying is used to reduce the moisture level below 1% in the pressboard. The oil conductivity and moisture content can be monitored continuously. Transient measurements with a step change in temperature have shown that the volume charge density follows the temperature