The Influence Of The Operating Enviroment To The Surface Discharge On The Oil-Impregnated Pressboard Under Combined AC-DC Electric Field

Converter transformer is very important in HVDC system, which is wildly used in the power grid nowadays. Converter transformer’s operation condition is complex, and it withstands AC/DC combined electric field. A large amount of interface charge on the oil impregnated pressboard can be accumulated owing to the DC voltage. And the charge will be trapped in or off the interface of insulation material due to the AC electric field. Thus the insulation aging and deterioration will be accelerated, and may cause surface flashover. Owing to the complexity of the operation condition, the DC component of of the AC-DC composite field, temperature, and the moisture of the oil-impregnated pressboard will be alternated with the operation condition. And the oil-impregnated pressboard which the converter transformer used is different with normal AC transformer. Thus it’s necessary to study the influence of the operating enviromnet to the surface creepage discharge under the AC-DC combined electric field, and emphasize on the DC component, temperature, moisture and type of the oil-impregnated pressboard. Meanwhile, the effect of the interface charge will be considered.With the help of oil-paper insulation test platform and the interface charge measurement platform, we use the needle-plate electrode model to study the surface creepage discharge and the interface charge accumulation under different DC component of the AC-DC composite filed, which the ration between the AC voltage and DC voltage is 1:1,1:3, and 1:5. And the DC composite is positive. What’more, we carried out the temperature characteristics and the influence of the moisture of the oil-impregnated pressboard to the surface creepage discharge and the interface charge accumulation under the AC-DC combined electric field. The analysis of the type of oil-impregnated pressboard between the converter transformer and the normal AC transformer is also carried out.The result showed that the accumulation of interface charge will fluctuate with the AC phase under the AC-DC combined electric field, and will increase with the increase of the DC component. When the DC component increases, the surface creepage discharge will be difficult to occur, and the PD1V will increase, the number of the discharge and the discharge quantity is less. When the ration of the AAC voltage and DC voltage reach 1:3 and 1:5, the discharge energy is less enough so the the carbon branch doesn’t appear. The more DC component, the less surface creepage discharge, and the PDIV will increase. This is because the interface charge will weaken the electric field with the DC component increase and inhibit the surface creepage discharge.The temperature and the moisture of the pressboard also have an effect on the surface creepage discharge under the AC-DC combined electric field. When the temperature rises, the PDIV will decrease, and the number of discharge and discharge quantity will increase obviously. And the interface charge accumulation will decrease when the temperature rises, so the inhibitory effect to the surface creepage discharge is weakened.When the moisture of the oil-impregnated pressboard raised from 0.61% to 4.68%, the interface will decrease obviously. Meanwhile, the PDIV decreased and the number of discharge and discharge quantity increased. This also reflected the inhibitory effect to the surface discharge is weakened.What’s more, the different between the oil-impregnated pressboard used in converter transformer or in the normal AC transformer is also studies. The type of converter transformer has better insulation performance, the voltage of surface flashover is higher and the partial discharge is not obvious before flashover.