normal ageing. The heat removal, from copper coils towards
the outside of transformer, by the cooling fluid is driven by the
following thermo-physical properties: kinematic viscosity,
specific heat (or thermal capacity), coefficient of expansion and
thermal conductivity. Table 1 shows that these properties differ
from one fluid to another. Simulations carried out using a
network thermo-hydraulic model on a typical disk winding
indicate that, in forced oil convection mode, lower temperature
rise could be expected by using ester fluids and, in natural oil
convection, the low viscosity ester composition (and synthetic
esters) behaves better than vegetable oil. But deeper thermal
studies should be undertaken to confirm these conclusions. [4]
F. Conclusion of laboratory investigations
Compared to other insulating natural esters (i.e. vegetable
oils or triglycerides), the low viscosity ester composition is
very similar to standard mineral oils regarding their physical
and thermal properties, and it appears to behave the same way
in the transformer surroundings. So, this insulating fluid seems
well suited to replace mineral oil in medium voltage ONAN
transformers without requiring any modification of their design
and manufacturing.