Distribution of water and degradation processes of insulation system of power transformers filled with mineral oil and natural ester oil

Predmet istraţivanja ove doktorske disertacije je raspodela vlage i degradacija izolacionog sistema energetskih transformatora ĉiji su bakarni provodnici izolovani Kraft papirom i izolacionim uljem mineralnog, odnosno biljnog porekla. Izolacioni sistem uljem punjenih energetskih transformatora saĉinjen je od izolacionih ulja i ĉvrste izolacije na bazi celuloze. Najvaţniji deo izolacije predstavlja izolacioni papir oko bakarnih provodnika i smatra se da je transformator dostigao kraj svoj ţivotnog veka onda kada je papir degradirao u tolikoj meri da je izgubio mehaniĉku ĉvrstinu i sposobnost da podnese dalja elektriĉna, a posebno mehaniĉka naprezanja Kao jedna od mera degradacije papira, odnosno mera mehaniĉke ĉvrstoće, uzima se vrednost stepena polimerizacije papira (DP). Ovaj parametar se moţe odrediti tek nakon što transformator završi svoj radni vek, tako da je tokom pogona transformatora moguće vršiti samo indirektne procene stepena degradacije papirne izolacije. Jedan od naĉina za procenu stanja papirne izolacije jeste odreĊivanjem sadrţaja derivata furana u ulju koji nastaju prilikom degradacije papirne izolacije uz korišćenje odgovarajućih korelacionih jednaĉina. Korelacione jednaĉine koje su trenutno dostupne u literaturi ne uvaţavaju parametre koji utiĉu na rastvorljivost derivata furana iz papira u ulje kao što je prisustvo vlage i kiselina u izolacionom sistemu. Istraţivaĉki rad bio je organizovan u dva dela. Prvi deo istraţivanja obuhvatao je prouĉavanje fenomena raspodele vlage izmeĊu papirne izolacije i izolacionog ulja mineralnog i biljnog porekla. Formirani su ravnoteţni dijagrami raspodele vlage izmeĊu izolacionog Kraft papira, presborda i mineralnog, odnosno biljnog izolacionog ulja, za opseg temperatura 40 °C - 120 °C i sadrţaja vlage u izolaciji 0,5% – 5%. Potvrda mogućnosti primene formiranih dijagrama u odrţavanju realnih energetskih transformatora ostvarena je poreĊenjem rezultata procene sadrţaja vlage u celuloznoj izolaciji kod 23 realna energetska transformatora dobijenih na dva naĉina: primenom elektriĉne metode FDS i primenom predmetnih dijagrama uz korišćenje izmerenih sadrţaja vode u ulju i radne temperature ulja. U okviru istraţivanja fenomena raspodele vlage utvrĊeno je da u sluĉaju biljnih izolacionih ulja postoji nejednaka raspodela vlage izmeĊu papira i presborda, naroĉito na umerenim temperaturama i za niţe sadrţaje vlage, što moţe biti veoma korisno prilikom procene ovlaţenosti izolacionog sistema realnih energetskih transformatora punjenih uljem biljnog porekla...Scope of this phd research was investigation of moisture distribution and ageing phenomena of the insulation system of power transformers with kraft paper insulated copper conductors and filled with mineral or natural ester oil. The insulation system of oil-filled power transformers is composed of insulating oils and cellulose-based solid insulation. The most important part of the insulation is the insulating paper wrapped around copper conductors. Transformer is considered to have reached the end of its life when the paper have lost it‘s mechanical strength and ability to withstand further electrical and mechanical stresses due to degradation. A common way to express the condition of paper insulation in transformers, ie mechanical strength of paper, is the degree of polymerization (DP). This parameter can be determined only after the transformer has completed its service life. During the operation of the transformer it is only possible to make estimation of DP based on furan derivatives content in the oil, compounds produced during paper ageing, using appropriate correlation equations. The correlation equations currently available in the literature do not take into account parameters that affect the solubility of furan derivatives in oil, such as the presence of moisture and acids. Research was orriented in two areas. First area had covered investigation of moisture distribution phenomena between paper and oil, mineral and natural ester. Based on experimentally data moisture equilibrium charts for kraft paper and pressboard insulation and mineral and natural ester oil were developed, for temperature range 40 ° C – 120 ° C and moisture content in insulation 0.5% - 5%. A comparative analysis of estimated moisture contents in the cellulose insulation using measurements of water content in the oil and equilibrium curves, and moisture contents obtained by applying electrical frequency dependent spectroscopic method (FDS) are provided for 23 transformers in service. A good agreement of these results confirmed applicability of these charts for assesing the water content in solid insulation of power transformers in service. Due to the higher solubility of moisture in natural ester compared to in mineral oil, water equilibrium in a system of natural ester oil, paper and pressboard was moved to the oil. In the system of naural ester oil uneven distribution of water between paper and pressboard was also observed, especially for moderate initial water contents and moderate temperatures. This fact could be very useful in assessing the moisture content in paper in power transformers filled with natural ester oil..