Moisture dynamics in natural-ester filled transformers

Abstract

Due to the global trend in the industry to work towards sustainability and clean energy, utilities and transformer manufacturers focus on the design of environmentally friendly devices. In this context, the use of natural esters as transformer’s liquid insulation is becoming an habitual practice in distribution transformers. These fluids make possible the operation of transformers with lower fire risk, what is important for certain applications. Additionally natural esters are biodegradable and their application minimizes the risk of soil contamination in case of fluid’s leak. One of the differential characteristics of ester fluids is that they have higher affinity for moisture than mineral oils, absorbing masses of water up to ten times greater than conventional insulating fluids under similar working conditions. This fact changes the water distribution between the solid and liquid insulation that is typically found in mineral-oil filled transformers. Moisture content in insulating materials remains as one of the most important parameters determining the life cycle of transformers, hence the importance of fully understanding its behaviour. Although many actions are being taken to spread the use of ester fluids, not many works about the performance of water in ester-cellulose systems haven been published to date. In this work, an analysis on the moisture dynamics in transformers insulated with natural esters is presented. In the first part of the paper the main parameters that influence the moisture distribution between oil and paper are obtained either for the case of an ester or a mineral oil being used as insulating fluids. Then, simulations are carried out in which different temperature profiles are assumed and the evolution of the moisture distribution between the solid and liquid insulation is analysed. The reported parameters together with the theoretical model proposed in this work can be used to evaluate the maximum admissible load of transformers insulated with natural ester taking into account not only the thermal aspects but also the moisture dynamics of the oil-paper insulation. The application of the presented approach can lead to a more reliable diagnosis of the transformer as moisture-related critical points could be identified and subsequent actions be taken to correct the risk. Identification of critical events, such as the presence of excessive moisture in the liquid insulation with the subsequent risk of partial discharges, breakdown or formation of water in liquid state, will help the transformer operator to take decisions that make possible a safer transformer exploitation.

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