## 4 Properties of Treated Liquids from Waste Cooking Oil as an Alternative Insulating Liquid
### Influence of WCO Treatment on Thermal Properties
#### 4.1.5 Breakdown Voltage (BDV) of WCO  
The breakdown voltage of an insulating liquid is a critical parameter that defines its ability to withstand electrical stress without failure. It reflects the dielectric strength of the fluid and serves as a key indicator of its suitability for transformer insulation. Among the standard diagnostic tests conducted on insulating liquids, the BDV test is one of the most fundamental and widely used due to its simplicity, rapidity, and ability to provide insightful information about the quality and purity of the liquid. A high BDV generally indicates low moisture and contaminant content, while a low BDV suggests the presence of degradation byproducts, water, or particulates that compromise the insulating performance. For modified waste cooking oil, ensuring a high BDV is essential to validate its potential as an alternative dielectric fluid in power transformers.  
Table 12 presents the breakdown voltage values of waste cooking oils from different authors. A marked enhancement in BDV is evident across all studies, showing the efficacy of the transesterification process in improving the dielectric strength of WCOs for transformer insulation applications. The oils treated with NaOH exhibited the most significant increases, with BDV rising from 7 kV to 33.4 kV, a 377.14% enhancement [42], while another sample increased from 2.5 kV to 6 kV [161], further affirming NaOH's effectiveness. Similarly, KOH-catalyzed samples recorded a BDV improvement of 328.57% [46], indicating the superior performance of homogeneous alkali catalysts (NaOH and KOH) in enhancing the electrical insulating properties. In contrast, heterogeneous catalysts such as CaO and coal ash showed more modest improvements, with BDV increases of 10.55% [140] and 79.13% [100], respectively. This disparity emphasizes the influence of catalyst type on BDV enhancement, with homogeneous catalysts generally delivering higher dielectric performance improvements.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Reference**} & \multicolumn{2}{c}{**Acidity of**} & \multicolumn{2}{c}{**Acidity After**} & \multirow{2}{*}{**Catalyst**} & \multirow{2}{*}{**Percentage**} \\  & **Waste WCO** & & & **Modification** & **Decrease** \\ \hline [82] & 0.13 & 0.06 & NaOH & 53.84 \\ [76] & 2.5 & 0.4 & CaO & 84 \\ [42] & 2.7972 & 0.2578 & NaOH & 90.78 \\ [46] & 2.7972 & 0.2561 & KOH & 90.84 \\ [140] & 2.793 & 0.144 & CaO & 94.84 \\ [100] & 2.82 & 0.03 & Coal ash & 98.94 \\ [161] & 1.82 & 0.13 & NaOH & 92.86 \\ [141] & 0.1 & 0.06 & - & \\ [142] & 1.122 & 0.16 & KOH & 85.74 \\ \hline \hline \end{tabular}
\end{table}
Table 11: Acid value of transesterified waste cooking oil.  
From a practical standpoint, industry standards typically require a minimum BDV of 35 kV for transformer insulating oils according to ASTM 1816 and IEC 60156 [162; 163]. Several modified WCO samples in Table 12 meet or exceed this benchmark, confirming their potential for use as viable alternatives to conventional transformer oils. However, some modified oils fall below this threshold, suggesting the need for further process optimization, including post-treatment and purification, to fully comply with application-specific requirements.