IEC 60076-7:2018 pdf download – Power transformers – Part 7: Loading guide for mineral-oil-immersed power transformers.
2 Normative references The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC 60076-2, Power transformers – Part 2: Temperature rise for liquid-immersed transformers IEC 60076-1 4, Power transformers – Part 14: Liquid-immersed power transformers using high-temperature insulation materials 3 Terms and definitions For the purposes of this document, the following terms and definitions apply. 3.1 small power transformer power transformer without attached radiators, coolers or tubes including corrugated tank irrespective of rating 3.2 medium power transformer power transformer with a maximum rating of 1 00 MVA three-phase or 33,3 MVA single-phase 3.3 large power transformer power transformer with a maximum rating of greater than 1 00 MVA three-phase or greater than 33,3 MVA single-phase 3.4 cyclic loading loading with cyclic variations (the duration of the cycle usually being 24 h) which is regarded in terms of the accumulated amount of ageing that occurs during the cycle Note 1 to entry: The cyclic loading may either be a normal loading or a long-time emergency loading.
6.2 Insulation life In recent years, extensive work on paper degradation has been carried out and published in references [9] to [1 5], indicating that cellulose ageing may be described by combination of the three processes, i.e. oxidation, hydrolysis and pyrolysis. The oxidation is a process possibly dominant at lower temperature. The oxidizing agent in this environment is oxygen from air ingress, and as the ultimate end product of the process appears water. The hydrolysis of cellulose is a catalytically governed process where the rate of chain scissions depends on carboxylic acids dissociated in water. As both water and carboxylic acids are produced during ageing of cellulose this process is auto accelerating. The pyrolysis is a process that can take place without access to water and/or oxygen, or any other agent to initiate the decomposition. At normal operating or overload temperatures, (i.e. < 1 40 °C), such processes are considered to be of little relevance. In a real transformer all these processes – hydrolysis, oxidation and pyrolysis – act simultaneously. This hampers the application of one model describing the full complexity of the degradation processes. Which process will dominate depends on the temperature and the condition (i.e. oxygen, water and acid content). Different parameters might be used to characterize cellulose degradation process during ageing. In reality it is the mechanical strength that is important for the winding paper to resist the shear stresses occurring during short circuits. However, due to the folded geometry of paper in a transformer, it is not possible to analyse tensile strength of paper sampled from used transformers. Hence, it is more convenient to characterize the degree of polymerization (DP) in order to describe the state of an insulation paper. Figure 2 shows a typical correlation between tensile strength and DP value (see [1 1 ]), the same correlation is valid for the thermally upgraded and non-thermally upgraded paper.
The degree of polymerization (DP) is the average number (n) of glycosidic rings in a cellulose macromolecule, which ranges between 1  1 00 and 1  400 for unbleached soft wood kraft before processing. Depending on the transformer drying process, the DP value may be reduced further to a lesser or higher degree. During ageing, the lengths of these polymeric cellulose molecules are reduced due to breakage of the covalent bonds between the anhydrous-β- glucose monomers. The change of DP over time of non-thermally and thermally upgraded paper exposed to a temperature of 1 40 °C, oxygen of < 6 000 ppm and water of 0,5 % is shown in Figure 3 (see [1 5]). The nitrogen content of the thermally upgraded paper used in this experiment was 1 ,8 %. When the DP is reduced to 200 % or 35 % retained tensile strength, the quality of the paper (i.e. the mechanical strength) is normally considered so poor that this defines the “end of life” for such insulating material (see [1 1 ]), although the insulating material dielectric strength may be still at an acceptable level. Annex A gives further elaboration of the paper ageing theory providing a mathematical methodology for estimation of the expected insulation life considering different ageing factors such as moisture, oxygen and temperature. The corresponding results for the non-thermally and thermally upgraded paper are presented in Figure 4 and Figure 5, respectively. The illustrated difference in thermal ageing behaviour has been taken into account in industrial standards as follows: • The relative ageing rate V = 1 ,0 corresponds to a temperature of 98 °C for non-thermally upgraded paper and 1 1 0 °C for thermally upgraded paper.