DC Flashover Voltage Characeteristics and Their Calculation Method for Oil-Immersed Insulation Systems in HVDC Transformers

The effects of the laminated paper layer direction on the conductivity and the dc breakdown strength in oil-impregnated paper were experimentally clarified. It was confirmed that not only oil and paper discharges through the laminated paper layers, but also paper discharges along them triggered total flashover in oil and oil-impregnated paper composite insulation. Analyses verified the usefulness of the dc flashover voltage calculation method, based on a non-linear directional field calculations and the lowest flashover voltage estimations between paper discharges along the laminated paper layers and oil and paper discharges through them.