Synthetic esters are miscible with mineral oil as well as ester oils in any proportion. Synthetic esters can not be used to retro-fill silicone oil-filled units. Silicone oil mixed with esters can cause foaming during the filling of a transformer under a vacuum. The mixture of mineral oil and synthetic esters differ from both base insulating oils. This needs to be taken into consideration when operating and diagnostics of transformers [1]


Retro-filling is the process where one insulating liquid in the transformer tank is replaced with another liquid. Previously this process was applied for environmental reasons when oil with high PCB levels was replaced with a liquid with an acceptable PCB concentration.
Nowadays, mineral oil is replaced with either synthetic or natural ester oils. The reason for this retro-fill can be one of the following or both. Esters have a better fire property compared to mineral oils, the other reason would be for the environmentally friendly properties, esters are readily biodegradable. [1,2,3]
There are also other technical and non-technical benefits related to the replacement of mineral oil with synthetic esters.
* It will increase the transformer life due to its ability to tolerate high concentrations of water, this leads to an increased solid insulation lifetime
* A lower cost of insurance because the fire hazard is very low
Fire safety is greatly dependant on the quality of the liquid exchange treatment done. It should be ensured that the largest possible part of the mineral oil is removed from the system as the mineral oil will adversely affect the properties of the synthetic ester the customer is looking to incorporate into his transformer by performing the retro-fill. The mineral oil that is left in the core will slowly diffuse into the synthetic ester, it has been calculated that up to about 10% of the mineral can be left in the cellulose of the core after retro-fill, sometimes also from the bottom of the tank as well as the walls of the tank.
The percentage of the cellulose paper to insulating fluid might also affect the amount of mineral oil left after retro-fill. A rinsing procedure of the core and windings with heated ester oils should be used to have a more successful end-result.
Figure 1: The flash and fire points depending on the content of mineral oil in synthetic ester, based on data from [4]

For distribution transformers up to 33kV, no problems relating to retro-filling are anticipated. For larger units additional steps are required, first to prepare the unit, and then the steps are set out in [5] We will discuss this in a later document. 


1. L Leland, “Laboratory evaluation of several synthetic and agricultural-based dielectric liquids” Doble International Client Conference,2001
2. H Borsi, “DIelectric behavior of Silicone and ester fluids for use in distribution transformers” Transactions on Electrical insulation, VOl 26 no 4 pp 755-762 Ieee 1991
3. Dombek, G.; Nadolny, Z.; Marcinkowska, A. Effects of Nanoparticles Materials on Heat Transfer in
Electro-Insulating Liquids. Appl. Sci. 2018, 8, 2538. [CrossRef]
4. Dombek, G.; Gielniak, J. Fire safety and electrical properties of mixtures of synthetic ester/mineral oil and
synthetic ester/natural ester. IEEE Trans. Dielectr. Electr. Insul. 2018, 25, 1846–1852. [CrossRef]
5. Insulect-Energy Blog. Available online: https://insulect.com/energy-blog/can-i-use-midel-fluids-to-retrofilltransformers- at-high-voltages (accessed on 25 August 2020).
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