Study on Insulation Parameters of Epoxy Resin Impregnated Paper Bushings with Typical Defects

Epoxy Resin Impregnated Paper (RIP) bushings are widely used as transformer bushings and wall bushings. This type of bushing has stable electrical properties, explosion-proof, small size, light weight and other advantages. But its manufacturing process is complicated and difficult, improper process control will result in defects, especially for large size bushings. In order to find out the characteristic parameter of defects, some samples were manufactured to simulate defects and thier insulation parameters were investigated in this paper. For example, the dry time was shorten and the moisture is not dried completely during the heating & drying process. Crack was resulted by uneven release of thermal stress during solidification process. Incomplete degassing of material and residual gas creates bubbles in the condenser core. Insulation parameters such as insulation resistance, dielectric loss factor, capacitance, Frequency-domain Dielectric Spectroscopy(FDS), Polarization and Depolarization Current(PDC) are tested and compared with these parameters of normal bushing samples which have a same design. Test results indicated that all type of defects may lead to a larger tanδ than normal bushing, the incompletely dried core has a smallest insulation resistance but largest tanδ when the test frequency is less than 0.01Hz. The microcrack defect can be tested out by tanδ-f curve where it has a larger tanδ when the frequency is more than 100Hz. In the low frequency band(<; 1Hz), the tanδ-f curve of the core with microcrack is similar to that curve of the core with microbubbles. Different defect bushings exhibit different rates of decrease and stability value at the polarization current. This study can provide reference for factory test and on-site test to diagnose the insulation state of the bushing.