The more demanding application the cleaner the paper needs to be. Paper machines are run with deionised or even distilled process water when producing higher grades of electrical insulation paper. Electrical insulation papers are made from well delignified unbleached kraft pulp.
Electrical cables are categorized by the voltage and current used. Telephone cables have moderate voltage and current associated with cables leading moderate electric current or transmitting electrical signals. The telephone cables have a large number of conductors that are individually insulated. In order to not become excessively thick the paper needs to be thin (30-40 g/m2). A normal power cable needs more insulation and therefore paper with higher paper density is used, normally 60-190 g/m2. The paper needs to be strong, elastic, uniform and free of holes or debris. These applications are being replaced by plastic insulation.
Submarine power cables at very high voltages (> 400 kV) are a very demanding application. The paper is normally 65-155 g/m2 and mostly produced on two ply paper machines. An advantage of using paper in sea cables is that in case of leakage, the paper will swell and prevent water from flowing along the cable.
This paper is used in capacitors and is an extremely clean and thin tissue paper (normally 6-12 g/m2) that is super calendered. The pulp is clean unbleached kraft pulp that is extremely refined. The paper is made on small paper machines with slow speeds because the stock has to be drained very slowly.
Transformer board is used mainly in oil-filled transformers where a solid insulating structure is needed. This is a pressboard up to 8 mm in thickness. The board is built up wet on forming cylinders and cut off when at the desired thickness. This makes a sheet with the size of the width and circumference of the drum. The wet sheets are hot- or cold-press dried and finished on separate machines.