III. FORMULATION OF A LOW-VISCOSITY
ESTER COMPOSITION
In collaboration with transformer manufacturers and an
oleo-chemistry research centre, EDF has developed a new
dielectric fluid based on vegetable oil (TGs) and derivative
(natural-like esters) which can be used as an insulating and
cooling liquid in distribution transformers. The challenge was
to obtain the simplest composition (i.e. the cheapest one) that
meets the technical requirements for current low/medium
voltage transformers with ONAN cooling system [1]. So the
formulation has been optimized in order to improve the
physical, chemical and electrical properties of natural esters,
and then to achieve characteristics close to those of insulating
mineral oils.
The final composition is a mixture of oleic rapeseed oil and
ethyl-hexanolic ester (1:1) plus 0.3% inhibitor di-tertio-butylpara-
cresol (DBPC). The fatty esters are produced by the transesterification
of the triglycerides of the rapeseed oil with 2-
ethyl-1-hexanol. That means that the fatty acid profile of the
resulting monoesters is the same as that of the TGs of the
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