HVDC electrical insulation performance based on charge activity in oil-pressboard composite insulation structures

For more reliable HVDC power transmission systems, it is necessary to enhance HVDC electrical insulation performance in power equipment such as an oil-immersed AC/DC converter transformer. In this paper, the electric field distributions under AC, DC steady-state (DC-SS) and DC polarity reversal (DC-PR) conditions are discussed in oil-pressboard (PB) composite insulation structures. Firstly, based on discussion about the charge accumulation and charge dynamics at oil/PB interfaces, the contribution ratio of deposited charges under DC-SS to the following emerging electric stresses at DC-PR conditions are quantitatively clarified. Secondly, we found that the DC electric field distributions could significantly be controlled by PB arrangement in oil. In particular, both PB configurations and the conductivity ratios of oil and PB would play critical roles to determine electric field distribution, under DC-SS conditions. Furthermore, using an actual AC/DC converter transformer structure, electrical insulation performances under various testing conditions such as AC, DC-SS and DC-PR conditions were comparatively investigated. It was concluded that the most critical testing condition against breakdown strength could be the DC-PR conditions under the higher conductivity ratio of oil and PB.