Converter transformer is the core equipment in HVDC transmission system

which plays a key role in connecting AC and DC system. Oil-paper insulation is the main insulation of converter transformer

whose insulation performance is of great significance to the transmission system. Moisture will inevitably exist in the oil-paper insulation system due to insulation aging and external moisture intrusion. Meanwhile

moisture not only significantly reduces the dielectric strength of the insulation

but also is adsorbed onto the surface of the insulation paper

increases the number of interface defects and distorts the local electric field

and causes partial discharge or even insulation failure

which pose a great threat to the reliability of the oil-paper insulation system and the stable operation of the converter transformer. We review its research status in the world

analyze the source

existing state and distribution characteristics of moisture in converter transformers

explore the internal mechanism and dynamic law of moisture balance and migration in oil-paper insulation

and summarize the law and mechanism of moisture effect on the characteristics of charge and partial discharge of oil-paper insulation under operating conditions. The dynamic behavior of moisture at the interface of oil-paper insulation is emphasized especially. It is pointed out that the influence of transient process of water migration on the charge and partial discharge of oil-paper insulation under operating conditions can not be neglected.