In electrical transformers, insulation is mainly ensured by the sum of solid materials, such as kraft paper and insulating fluids, mainly mineral oils. This important innovation was invented and claimed by the well-known scientist Nikola Tesla of patent no. 655,838 “Electrical conductor insulation method” of August 14, 1900.
The solid insulator, also called insulating or cartographic paper, is mainly derived from the production processes of kraft paper. The result is a product that offers surprising properties, both electrical and mechanical. In this area, kraft paper has found one of its most important applications, especially in the insulation of electrical equipment up to high voltages.
Over time, thanks to the use of specific additives, kraft paper has been improved mainly in its temperature-related behavior, giving rise to thermal update paper (TUP). Synthetic polymers, such as Dupont’s Nomex material, a meta-aramid compound, are also available on the market.
As for liquid insulator, in addition to mineral oils, there are also natural esters, synthetic esters, silicone fluids and, in the past, also Askarel based on PCBs.
Kraft paper
Insulating papers are impregnated with oil or other insulating liquids. At the end of the impregnation cycle (typically under vacuum, 60-80 ° C and at least 72 hours), kraft paper soaks the oil up to 150-180% of its initial mass.
Kraft paper covers copper or aluminum conductors to insulate them electrically and, therefore, is exposed to thermal, electrical and mechanical stress.
The main property of the work is DP (IEC 450: 1974), degree of polymerization. This parameter characterizes the material’s properties, which are mainly: tensile, elongation, flexural strength, modulus of elasticity, loss factor, specific resistivity. A typical kraft paper has a DP between 1000 and 1500.
During the actual life cycle of the transformer, the DP gradually decreases to a value of about 200 (about 80% reduction over the new one) to which the conventional term corresponds to the term “thermal”. In this condition, the paper loses its mechanical properties, but not the electrical properties, which, however, remain able to guarantee the required insulation.
Electrical insulation can be considered the heart of the transformer, if the direct consequence is electrical failure (metaphorically, like a heart attack). In the presence of strong electric power strings, the failure can trigger insulating oil, which is combustible, causing bursts of transformers and fires and possible serious accidents.
The degradation of paper
The degradation processes of kraft paper are the result of the interaction of 3 mechanisms: hydrolysis, oxidation, pyrolysis.
The processes of paper degradation are extremely complex in themselves and, in addition to the effects of oil degradation, the mechanisms influenced by many critical factors of the difficult quantitative formalization result. The critical factors that determine the aging of insulating papers are temperature, water, oxygen, whether the system is closed or open, thermal cycles and the relationship with the load profile of the transformer.
A series of experiments were carried out to determine the relationship between paper degradation and oil degradation. One of these tests, according to the IEC 62535 standard (in a 20 ml bottle with 10 ml of oil, inserts a copper test with about 3 g of weight, wrapped with approximately 23 g of paper, at 150 ° C for 72 hours), showed a progressive loss of paper weight (up to 25%) and a reduction in DP (up to 80% compared to the new value and 60% less compared to an initially non-acidic oil) increasing the acidity of the analyzed oil (TAN).
The main degradation products of insulating papers are: water, acids, CO2, CO, furan compounds, methanol, ethanol, particles.
Geartech BR currently occupies a prominent position in the country importing films, polyester compounds, papers and insulating tapes. Currently, our products are used by manufacturers of electric motors, transformers, cables, compressors, fans, etc. Contact us to find out more! It will be a pleasure for us!
