We also refer to a resin-bonded paper bushing or a synthetic resin-bonded paper bushing, respectively, as SRBP or RBP bushings, in short form. Before entering the actual discussion of resin-bonded paper bushings, let us understand the meaning of resin-bonded paper.
What is the meaning of resin-bonded?
We all know that paper is a cellulose material, hence it has good insulating properties. Obviously, the thickness of a single paper sheet is not enough to withstand the required voltage stress. So we have to make it thicker. Therefore, we make multiple layers of paper to achieve the required dielectric strength for withstanding the operating voltage. However, while doing this, there should not be any voids or air gaps between the layers of paper. To avoid this, we bond the sheets together with resin, one upon another. In this way, we create a solid cylinder of thick paper.
Process of Manufacturing
We use high-grade electrical insulating kraft paper for this purpose because it has a very uniform thickness and high dielectric strength. Then we coat the paper with phenolic resin or synthetic resin and stick one sheet upon another. The resin penetrates the fibers of the paper and bonds the paper sheets together.
After that, we apply heat treatment under pressure. The layered assembly is compressed under controlled high temperature. During curing, the resin polymerizes and forms a rigid laminated structure.
Then we insert this cylinder into the hollow bushing with the help of the required machinery. In this way, we make a homogeneous, dense, and rigid cylindrical insulation of paper. Normally, the dielectric strength of this cylindrical paper structure is around 6 to 10 kV per millimeter. Obviously, this is a Class-A type insulation.
Due to the presence of resin, this type of structure normally does not absorb moisture.
Advantages of SRBP or RBP bushing
Its construction is simple. It is also mechanically robust, as the resin becomes hard like rock after compression and heat treatment. It is cost-effective and quite easy to manufacture.
Disadvantages of SRBP or RBP bushing
As this type of bushing does not have any capacitive grading, it is not suitable for voltage levels above 72.5 kV. The voltage distribution is not uniform between the live and earthed parts of any electrical equipment. The electric field near the live conductor is much higher than that near the earthed metallic body.
Since the voltage distribution is naturally non-uniform, for very high voltage applications, the size of this SRBP bushing becomes quite large, which is not ideal for manufacturing, handling, or installation.
The aging effect of resin-bonded paper bushings is also poor. After long-term use, the bonding between the layers may become loose due to aging. Moisture absorption at these weak points can reduce the dielectric strength of the bushing. Thermal degradation is also a problem with this type of bushing.
We normally use this type of bushing in distribution transformers of 11 kV to 33 kV class. It is also used in current transformers and voltage transformers. As it is affected by atmospheric conditions, it is always recommended for indoor switchgear applications.