Space charge behaviour in thick oil-impregnated pressboard under HVDC stresses

As the main insulation applied in HVDC convertor transformer, the dielectric properties of oil-pressboard insulation system play an important role in the reliability of power system. One of the key problems related to any HVDC insulation systems is the easy formation of space charge within the bulk and at the interfaces. In this paper, space charge behaviour in the impregnated pressboard with 1mm in thickness is investigated using the pulsed electroacoustic (PEA) technique under different DC stresses at the room temperature. To study the influence of transformer oil status, the 1mm thick dried pressboards are separately impregnated with fresh oil and serviced aged oil. The PEA results show that homo-charge injection occurs for all the measurements. Both applied stress and oil status can affect the space charge behaviour, however, it has been found the oil status is the dominant factor in the present study. The electric field enhanced by the charges is about 45% in the aged oil samples and 15% in the fresh oil samples.