Effect Of Adhesive On Partial Discharge Characteristics Of Pressboard

The converter transformer is one of the important equipment in the HVDC transmission system,which can makes it possible to convert between alternating current and direct current together with the converter valve,and insulate the AC and DC system.Therefore,the oil-paper insulation structure inside the converter transformer will be sorely tested.The pressboard is the main component in the converter transformer,and its reliability should be highly valued.However,the effect of adhesive on partial discharge characteristics of the pressboard is still unclear.In this paper,a partial discharge test platform that can superimpose AC on any ratio of DC,according to the actual situation of the transformer.By using the conventional pulse current method to detect the partial discharge signal,the effects of AC,positive DC voltage,negative DC voltage and different ratio of AC to DC voltage on partial discharge of pressboard with different adhesives are studied.Moreover,the partial discharge inception voltage and breakdown voltage,as well as the characteristic parameters such as the number of discharges n,the apparent discharge q and the q-(p scatter diagram,and the n-φ histogram are analyzed,which can characterize the discharge severity during the development of the discharge.This research found that the pressboard with or without adhesive have little effects on the partial discharge process and the trend of the characteristic parameters under column plate electrode,when the applied voltage is AC,DC or compound voltage.The deviation is around 6%,when the three types of research objects are at the same electrode.The main reason is that the presence of the adhesive is only equivalent to the addition of an interface,whereas the oil-paper interface under the column plate electrode is more prone to occur charge accumulation.By analyzing the discharge phenomenon of different directions of adhesive pressboard under AC,AC and DC compound voltage.It was found that the inception and flashover voltages of different adhesive pressboards under the same layer direction were not much different whether the adhesive direction perpendicular or parallel to the electric field.By contrast,the average deviation is about 3%,and the maximum deviation is less than 8.6%,which indicates that the type of adhesive has little effect on the partial discharge under the same electric fieldHowever,the insulation strength when the adhesive direction parallel to the electric field is lower than that perpendicular to the electric field.The maximum reduction of the inception and flashover voltages is 18.43%when the applied voltage is AC voltage.It can be seen that the presence of the adhesive is a weak link of the entire insulation structure.Therefore,when designing the insulation structure of the transformer,it is necessary to avoid the situation that the direction of the glue layer is parallel to the electric field.