Fundamental study on the application of natural esters on intrument transformers

Esters are nowadays very popular among high voltage power transformers manufacturers, because of their outstanding advantages in terms of thermal performance of extended overload possibilities. Instruments transformers manufacturers, on the contrary, have not started investigating their possible use yet, because of the lack of immediate performance increase, being low power machines. A starting testing campaign is described in this paper and aimed at verifying the feasibility of the ester liquids replacement in instrument transformers without affecting the current design where the dominant insulating fluid is mineral oil. High voltage current and voltage transformer prototypes were designed, dried and impregnated with Envirotemp FR3 and their performance was assessed by dielectric routine and type tests. An interesting polarity effect was noticed during the lightning impulse tests and currently under investigation, together with the endurance tests aimed at assessing the long term performance of the units.