Power transformers utilize oil as a heat transfer medium and a dielectric material, together with cellulose. Breakdown voltage (dielectric strength) is one of the most important parameters of transformer oil. It is measured when the transformer is taken into use and typically monitored by sampling during its operational lifetime.
It has been reported that breakdown voltage is affected by several factors, such as moisture, particles, acidity, and pressure [3,4,5,6]. The objective of this study is to investigate the dependency of the breakdown voltage on moisture using a representative set of oil samples and to point out the advantages of online moisture measurement.

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