رویکرد فازی برای ارزیابی باقی عمر عایق کاغذی Fuzzy approach for residual life assessment of paper insulation

I. INTRODUCTION In today’s life, the uninterrupted supply of electricity to consumers is in close relation to the consistent function of the transformers. Thus, the major components of the transformers should be maintained to achieve its designed service life span [1]. The life of transformer relies on the life of their insulation system that undergoes a number of stresses, such as electrical, thermal, mechanical and environmental. The transformers are unable to operate for the designed service period [2], due to active participation of such stresses. It is therefore necessary for the utility to have continuous monitoring different insulations used in transformer. The transformer insulating system, primarily consisting of two special dielectric media; the insulating mineral oil whose ionic conduction varies and the oil impregnated paper (or pressboard), whose life is limited, even under ideal circumstances. The existence of transformer typically depends upon the health condition of its CIP. The fundamental element of pressboard and paper is cellulose, which is polymer of glucose wherein glucose units link to each other in a chain like structure. Chemically cellulose is represented as [C6H10O5]n, here ‘n’ refers to number of glucose units for each polymeric chain, more specifically Degree of Polymerization (DP); a significant factor of paper degradation. Thus the value of DP specifies the number of monomer unit in the polymer as cellulose is the linear polymer constituent of individual anhydrous glucose units associated with glucosidic bonds. It have been verified that there is direct relationship between mechanical tensile strength of the paper cellulose and the corresponding value of DP. Generally, a fresh cellulose paper has a DP value of 1200 which reduces slightly when subjected to stresses. The cellulose paper life comes to an end when the value of DP approaching to 200. The Kraft/Cellulose paper losses its tensile strength as it is subjected to ageing and, at some point, does not survive on the short circuit stresses. The mechanism involve in paper deterioration is quite difficult and sturdily influenced by the working environment. Degradation of solid insulation occurs due to schism of the 1, 4-glycoside bonds among two consecutive glucose monomers present in it. As a result of these chain scissions, the DP gets reduced that ultimately lowers the mechanical tensile strength of the paper. During ageing, cellulose is decomposed into various substances including furanic compounds, acids, CO2 and CO. These byproducts are partially soluble in oil and are generally characterized as indicators for cellulose ageing and hence show the end of life of power transformers. Several authors contributed their research towards deterioration of cellulose under moisture and thermal stress to enunciate the end of life of power transformers using Degree of Polymerization (DP) as a failure index. A mutual stress failure model was developed to perform a chain of thermal aging experiment on kraft paper of power transformer on a laboratory scale [3].Diagnostic and health monitoring approaches based on the value of the ratio of carbon dioxide and carbon Monoxide (i.e. CO2/CO) gases were also investigated in [4]. Soft computing techniques like Neural Network (NN), Fuzzy Logic (FL), Neural Fuzzy System, Support Vector Machine (SVM) and Wavelet Network [5][6][7]were applied to assess the health condition of CIP. These models used carbon oxide gases to evaluate the degradation of paper cellulose. The main objective of this work is to monitor the condition of CIP using the fuzzy model that correlate the concentrations of CO, CO2 and 2-FAL with the value of Degree of Polymerization (DP). Input to the developed FIS is 2-FAL, CO2 and CO and output is the value of DP which ultimately decides the residual life of transformers.