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Mineral oils are commonly used for cooling and electrical insulation purposes in electrical power transformers. Nevertheless, these oils are especially problematic for the environment in the case of burning and leakage. Thanks to their higher fire point and excellent biodegradability, ester oils could be a good candidate to replace mineral oils. Thus, some transformer manufacturers have launched investigations by filling medium power transformers with ester oils and especially natural ester oils. In large power transformer where forced convection of the insulating liquid is the predominant heat transfer mean, the electrification risk can appear. This phenomenon has been suspected to be responsible for several failures. Studies have shown, in addition to the transformer design (shell or core type), that the type of liquid used is quite important in limiting these hazards. The goal of this paper is to present an analysis of ester oil behavior versus electrostatic hazard due to flow electrification. Four ester oils were tested in the laboratory of the institute Pprime in Poitiers University in collaboration with Alstom Grid Power Transformers. Two experimental studies were conducted: the first one is for measuring the Electrostatic Charging Tendency (ECT) by using the Westinghouse protocol (ministatic charge tester), and the second one is made by mean of a test loop simulating the oil flow as in a real transformer and equipped with a capacitive sensor. This sensor is able to measure both the charge accumulation at the pressboard surface and the generating current. All these results are compared with those of mineral oils which are actually used for large power transformers. It was shown that even if ester oils increase the charge generation in comparison with mineral oil, the charge accumulation on the solid surface is not excessive thus limiting the potential at the solid/liquid interface and associated electrification hazards. The higher conductivity of esters enable- - s better evacuation of generated charges.