Electric field homogenization for converter transformer with nano modified pressboard

The oil/pressboard insulation system in converter transformer undertakes AC superimposed DC and polarity reversal voltages in operation. In order to match the conductivity of oil and pressboard to reduce the differences of field intensity between in oil and in pressboard under direct current and polarity reversal voltages, and also to improve the space charge characteristics at oil/pressboard interface, the modification method with nonlinear nano SiC (the silicon carbide with the diameter of 60 nanometers) for pressboard was put forward. The samples doped with different mass fractions of nanoparticles were manufactured, and their dielectric properties were studied in laboratory. At the same time, the simplest model of oil/pressboard structure was built and the simulations of electric field distribution and space charge characteristics at oil/pressboard interface were carried out to inspect the influence of pressboard modification on them. The experiment results show that the mass fraction of nanoparticle has little influence on the permittivity of samples, and the permittivity almost keeps constant with the applied electric field intensity increasing. The conductivity of the samples rises rapidly with the nanoparticle mass fraction increasing, and it shows obvious nonlinearity with the electric field intensity when the nanoparticles mass fraction exceeds a certain ratio. The simulation results show that with nano modified pressboard, the field distribution in oil-pressboard system is more uniform under direct current or polarity reversal voltages, and the space charge characteristics at the oil/pressboard interface in converter transformer can be well improved under polarity reversal voltage.