## 4 Properties of Treated Liquids from Waste Cooking Oil as an Alternative Insulating Liquid
### Influence of WCO Treatment on Thermal Properties
#### 4.1.6 Oxidation Stability of WCO  
In transformer insulation systems, the oxidation stability of insulating liquids is a critical parameter that governs the long-term performance and reliability of the equipment. Liquids with high oxidation stability help maintain the operational integrity of transformers by preserving both cooling and insulating functions over extended service durations. Conversely, insulating liquids that are prone to oxidation degrade over time, producing by-products that negatively affect their dielectric strength and cooling capacity, as well as the mechanical and dielectric properties of the associated solid insulation (typically cellulose-based paper). Among the common oxidation by-products are aldehydes, ketones, carboxylic acids, and moisture, which accelerate the aging of both the liquid and paper insulation [164]. It is noteworthy that plant-based insulating liquids, such as waste cooking oil (WCO), are inherently more susceptible to oxidation. This susceptibility stems from their molecular composition, typically consisting of fatty acid chains with carbon lengths ranging from 8 to 24 atoms and containing varying degrees of unsaturation (from saturated to mono- and polyunsaturated structures) [62]. The oxidation stability of these oils is directly linked to the structure and quantity of double bonds in the fatty acid chains. A carbon-carbon double bond consists of a sigma (\(\sigma\)) bond and a pi (\(\pi\)) bond, which arise from the sp\({}^{2}\) hybridization of the carbon atoms [165]. The overlapping of sp\({}^{2}\) orbitals between two carbon atoms forms a sigma bond (\(\sigma\)), which houses two electrons. The remaining two electrons form the pi bond (\(\pi\)), which is typically more reactive and thus determines the degree of unsaturation and oxidation susceptibility of the oil. Oils with higher levels of unsaturation (i.e., more \(\pi\) bonds) tend to oxidize more rapidly, making them less stable under prolonged thermal and oxidative stress [101]. From Table 2, waste cooking oil exhibits a high proportion of unsaturated fatty acids, which contributes to its increased susceptibility to oxidation. The oxidation stability of methyl esters derived from WCO has been comparatively evaluated against those produced from castor oil, as reported in Ref. [134], using TGA analysis under an oxygen atmosphere. The results demonstrate that castor oil methyl ester possesses significantly higher oxidative onset temperatures (218-230 \({}^{\circ}\)C) compared to waste cooking oil methyl ester (187-200 \({}^{\circ}\)C), indicating greater resistance to initial thermal oxidation. Furthermore, \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Reference**} & **BDV of WCO** & **BDV After** & \multirow{2}{*}{**Catalyst**} & **Percentage** \\  & **(kV)** & **Modification (kV)** & & **Increase** \\ \hline
[82] & 27 & 52 & NaOH & 92.59 \\
[76] & 35 & 40 & CaO & 14.29 \\
[42] & 7 & 33.4 & NaOH & 377.14 \\
[46] & 7 & 30 & KOH & 328.57 \\
[140] & 18 & 37 & Cao & 10.55 \\
[100] & 25.4 & 45.5 & Coal ash & 79.13 \\
[161] & 2.5 & 6 & NaOH & 140 \\
[141] & 10.5 & 38.3 & - & 264.76 \\ \hline \end{tabular}
\end{table} 
Table 12: Breakdown voltage of treated waste cooking oils. castor oil methyl ester required higher temperatures to reach 10%, 50%, and 90% mass loss than waste cooking oil methyl ester, confirming its superior thermal stability under oxidative conditions. These differences in thermal and oxidative behavior are closely related to the fatty acid profiles of the two esters [134]. Due to the high percentage of unsaturated fatty acids in the methyl ester from WCO, the oxidation stability seems challenging and may need the inclusion of antioxidants and chemical modification, as reported by Refs. [140; 166], respectively. When antioxidants are added to the base samples, they impede the degree of degradation in the oil by preventing the radicals from attacking the oil molecules.