Electric field distribution with dynamic charge activity in composite insulation systems under HVDC and DC-polarity reversal conditions

As for the development of AC/DC converter power transformers, it is necessary to enhance their electrical insulation performance while taking specific DC behaviors, such as DC electric field distribution, dynamic charge accumulation and DC polarity reversal characteristics, into consideration. In this paper, the electric field distributions in an oil-impregnated pressboard composite structure were calculated under AC, DC steady-state and DC polarity reversal conditions, by an analysis with the Finite Element Method (FEM), and their electrical insulation performances were quantitatively investigated and discussed. The results clarified that it was very important to well arrange the pressboard barriers in oil to reduce the emerging electric stresses, based on the beforehand accumulated charge amount under DC steady-state conditions, especially at the instant of DC polarity reversal, which will be the most critical conditions.