Electrical Currents in Liquids: Case of Vegetable Olive Oils

In the mind of alternative to the conventional mineral oils used as insulating material in power transformers, this work aims to study the electrical behavior of one of the biodegradable vegetable oils so called olive oil. The tests consist to submitting samples of the olive oil to various DC electric fields and the electrical response is obtained, under DC stress and in rest state, by measuring respectively the electrical charging and discharging currents. Obviously, on basis of the different features of the current vs. time curves, a confrontation is made between the electrical behavior of mineral oil and that of the tested vegetable oil. Comparison between the two cases is also undertaken on the basis of the components of the charging/discharging currents, which are the transient and the conduction currents. The results indicate that the olive oil presents several similarities compared to the conventional transformer oil. Indeed, under electrical DC stress, the measured current seems to evolves, in both cases, as
$I_{c}(t)=I_{0}.t^{b}$
. The transient current feature in olive oil reaches that of the mineral oil as soon as the applied
$DC$
electric field becomes sufficient to initiate the supposed SCLC process. The conduction components, expressed through the I(V) characteristics, meet closely in qualitative point of view in both cases. When the DC voltage is suppressed, the discharging current
$I_{d}(t)$
evolves as
$exp(-t/\tau)$
in the mineral oil, contrary to the vegetable oil for which it does follows any specific feature. This behavior is interpreted as space charge limited conduction in the mineral oil, and, in the case of the vegetable oil, it seems that the electrical response is masked by the impurities (water's molecules, oxidation, solid particles…)