Investigation of electric stresses caused by applying DC and AC Voltages on the insulation of converter transformers

The insulation system of converter transformers is more stressed electrically than of AC transformers while both types use the same insulation materials. However, the structure of insulation around valve winding in converter transformers is designed for withstanding AC and DC electric fields. In the DC field, electrical conductivity is the most important factor influencing the electric field distribution. Moreover, the electric conductivity is strongly dependent on the temperature and electric field. In this article, the dielectric parameters (DC conductivity and relative permittivity) were used in the COMSOL model to investigate the variation of electric field distribution at different temperatures and AC, DC and hybrid fields. The electrical conductivity of oil and pressboard was defined constant for simplicity. The results showed the great effect of temperature and DC voltage percentage on the electric field strength of pressboards used in the converter transformers.