Ageing of Crepe Paper in Mineral Oil and Natural Ester

Several billion liters of transformer oil are used in oil filled transformers worldwide. These machines are voltage transformation devices in which during their operation the heat of windings and iron core firstly is transferred to the insulation oil and then to the cooling medium. Currently, this type of transformers mainly utilizes mineral oils due to their proven good service performance. However, mineral oil has certain limitations such as their low flashpoint and biodegradability which conditions its use in indoor environments, and it may cause a high environmental impact during its use. The development of biodegradable liquids (synthetic and natural esters) has provided an alternative. Nevertheless, a question arises when natural esters are used in power transformers, are they compatible with the rest of materials used in their design. Nowadays, although several studies of accelerated thermal ageing have been undertaken there are few works that study the impact of insulation liquid and temperature on the degradation rate of other cellulosic materials such as crepe paper. This paper can take different shapes of the surface being insulated and has greater surface area which allows to retain more oil reducing the working temperature of the transformer. For this reason, this work has compared the impact of the type of insulation oil (a mineral oil and a natural ester) and the temperature (150°C, 130°C and 110°C) on lifetime of this insulation paper. Degree of polymerization has been measured to quantify the effects. Additionally, the degradation suffered by insulation oils has been evaluated through the measurement of acidity, dissipation factor and DC resistivity. It has been found that the degradation suffered by natural ester is higher than mineral oil, however, the biodegradable liquid extends the life of crepe paper.