Efficiency of ageing markers for different transformer insulation systems

The lifetime of a transformer is driven by the lifetime of its solid insulation, namely cellulosic materials (impregnated with liquid). Indeed, this later cannot be easily replaced as the insulating liquid. The most important parameter to assess cellulose aging status in transformer is to measure its degree of polymerization (DPv), giving data on its mechanical behavior. Nevertheless, the drawback of this method is the non-accessibility of cellulosic samples in service. This is the reason why the monitoring of cellulose insulation is generally made through oil sampling followed by laboratory analysis. Some of main markers, dissolved in the oil, and giving such evaluation are: water content, 2-FAL furanic compound and more recently Methanol. These chemical tracers are currently used in mineral oil and still under study for natural ester (vegetable oil). This paper presents humidity equilibrium charts developed for natural ester/cellulose systems, including Kraft paper and pressboard. Another part is dedicated to ageing studies realized on different transformer insulation systems (mineral oil / vegetable oil / Kraft paper / TUP) and discusses the efficiency of Methanol and 2-FAL ageing markers.