I. INTRODUCTION
Environmental considerations demand alternatives for the
current mineral oils used in many electrical equipment, such as
transformers. Because of their excellent biodegradability and
non-toxicity relative to petroleum products, the use of
vegetable oils is on the rise as insulating and heat transfer
liquid in electrical devices. In addition these natural esters have
a favorable lower flammability, but their behavior at low
temperature is not well suited to applications in cold climate
and/or with natural cooling system. In this context, and
considering that current distribution transformers (MV/LV and
usually with ONAN [Oil Natural - Air Natural] cooling mode)
deserves a specific approach, the development of a lowviscosity
ester fluid was deemed profitable by some end-users.
Indeed, the cost of ownership of a single unit is quite
moderate with respect to other assets but, as the fleet is quite
significant (from 10,000 units for small utilities, to more than
700,000 MV/LV transformers for a utility such as the French
DSO) and is scattered throughout the land, these transformers
may be dicey for the environment. Furthermore, the costs of
implementation of leakage protective system, and/or those
involved in developing new environmental friendly