What types of capacitive bushings are there?

For voltage levels of 110kV and above, using oil-immersed bushings would result in excessively large dimensions. Therefore, it is necessary to employ solid insulation manufactured based on the principle of capacitive voltage division as the primary insulation, namely by using a capacitive core. Depending on the material and manufacturing method of the capacitive core, capacitive bushings are divided into two types: resin-impregnated paper (RIP) type and oil-impregnated paper (OIP) type.

Resin-Impregnated Paper (RIP) Capacitive Bushing. Its capacitive core is formed by interleaving single-sided resin-impregnated paper and aluminum foils through a process of heating, pressing, and rolling. After curing through heating, the outer surface undergoes mechanical processing and is impregnated with moisture-resistant varnish. Aluminum foils are placed between resin-impregnated paper layers every 1-2mm, forming numerous concentric cylindrical capacitive screens, achieving a uniform radial and axial potential distribution. The innermost screen serves as the zero screen, connected to the conductor at the same potential; the outermost screen serves as the ground screen, grounded together with a flange through a grounding bushing. The grounding bushing can be used to measure the tanδ of the bushing.

The mechanical clamping method of RIP capacitive bushing usually involves inserting a conical ring into the precision inner part of the capacitive core at the center, and then using a flange to press the conical ring tightly. The head is clamped using tightening screws, adopting a segmented clamping approach.

Oil-Impregnated Paper (OIP) Capacitive Bushing. The radial dimensions of OIP capacitive bushing are smaller compared to the RIP type, as the voltage causing corona discharge in oil is higher, allowing for a thinner insulation thickness between the capacitive screens. However, OIP capacitive bushing requires excellent sealing performance and must have a porcelain insulator at its lower part, resulting in a longer lower section. The capacitive core of OIP is formed by interleaving cable paper and aluminum foils, pressed and rolled, mechanically processed on the surface, and then subjected to vacuum drying and oil impregnation.

OIP capacitive bushing adopts a complete series-connected method, with a powerful spring at the head applying pressure through the conductor by borrowing the base. To ensure unimpeded oil flow between the upper and lower parts of the bushing, there needs to be a certain gap between the inner wall of the flange and the capacitive core. Consequently, its mechanical strength is not as good as RIP capacitive bushing, and the installation angle cannot be too large.

OIP capacitive bushing exhibits lower levels of partial discharge, making it a common choice for transformers.

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