RIP bushing is a kind of condenser bushing or capacitor-graded bushing. The full form of RIP bushing is Resin Impregnated Paper Bushing. The working principle of an RIP bushing is similar to that of an OIP bushing, and construction-wise, they are almost similar. The only difference is that in an OIP bushing, we use oil to impregnate the kraft paper, whereas in an RIP bushing, we use resin to impregnate the kraft paper insulation.
Difference between OIP Bushing and RIP Bushing
In OIP bushings, we have to fill oil in the bushing chamber. So, we need to provide an expansion chamber at the top of the bushing housing. Because of the liquid oil inside the bushing housing, there is always a chance of oil leakage.
Another drawback of the OIP bushing is that it must always be stored in the prescribed alignment, so the entire oil-impregnated paper condenser with aluminum foil remains under oil. Not a single part of the condenser core should remain out of the oil during storage. In RIP bushings, we use epoxy resin to impregnate the paper layers.
After curing, the insulation becomes solid. So the RIP bushing is free from oil leakage problems. It also eliminates the storage and handling limitations of OIP bushings.
This solid graded capacitor insulation makes the bushing almost maintenance-free. Since there is no oil leakage, the chance of fire hazard is very minimal in an RIP bushing.
Applications of RIP Bushings
OIP bushings are ideal for oil-filled transformers and oil-filled bead tank circuit breakers. However, in Gas Insulated Switchgear (GIS) installations, the inner side of the bushing is in an SF6 gas environment, while the outer side remains in air. If we use oil-filled bushings here, there may be oil leakage inside the SF6 chamber. Oil leakage is absolutely undesirable because it may react with or contaminate the gas. In such cases, the only alternative is an RIP bushing. Because of its solid insulation structure, there is no chance of oil contamination.
In underground substations, tunnels, or indoor power stations, oil-filled bushings always pose a fire hazard risk due to possible oil leakage. Here again, the best alternative is the RIP bushing.
Offshore platforms and marine substations also demand maintenance-free and vibration-resistant insulation systems. The solid-filled design of RIP bushings serves this purpose well.
In renewable installations, such as offshore wind transformer stations and solar inverter stations, oil leakage is unacceptable. RIP bushings perform very well here because they are dry-type and eco-friendly.
Construction of an RIP Bushing
A central rod or hollow pipe serves as the main conducting part of the bushing. Here also, we wind kraft paper with embedded aluminum foil around the conductor in the required number of layers. This arrangement creates a series of concentric hollow cylindrical capacitors.
Then, we place the entire assembly with its central conductor inside a high-temperature vacuum chamber for resin impregnation.
After it becomes dry, we inject resin to impregnate the kraft paper with epoxy. After curing, it becomes a hard, solid capacitor-graded insulation system. This type of insulation can resist moisture ingress strongly.
Then, we insert the entire assembly into a silicone composite or porcelain housing (casing). This housing provides mechanical support and the required creepage distance.
Here also, the lower portion of the housing does not require any sheds to increase creepage distance, as it remains inside the pressurized SF6 chamber of the GIS. Manufacturers also provide a tan-delta terminal at the flange of the bushing.
This RIP bushing (Resin Impregnated Paper bushing) is hermetically sealed from the top and bottom to prevent moisture ingress.
Manufacturing Process of RIP Bushing
The process starts by winding dried kraft paper on the central conducting core. In this paper, we wrap aluminum foil. We use pre-calculated length and width of the kraft paper and foil to ensure proper voltage stress distribution.
We use a pre-determined number of layers of kraft paper embedded with aluminum foil of descending axial lengths, wound around the conducting part. This arrangement ensures uniform voltage grading across the insulation.
Then, the assembly moves to a vacuum chamber. Here, it becomes dried under controlled high temperature and vacuum conditions. This process enhances the dielectric strength and eliminates any voids in the paper insulation.
After drying, we inject liquid epoxy resin under vacuum. The vacuum facilitates easy impregnation of the epoxy into the papers. In this way, the resin fills the microscopic voids in the paper and forms a dense, homogeneous dielectric medium.
The resin-impregnated core is then cured in a controlled oven at a specific temperature and time. After curing, the core becomes solid, hard, and mechanically strong.
Next, the solid core is machined to precise dimensions and a smooth surface finish. Then we insert the entire assembly into a porcelain or composite insulator housing. Finally, we seal the top and bottom along with tan-delta tap fittings with gaskets.