## 1 Introduction
These include poor oxidation stability, which limits their use in free-breathing transformers; inadequate fluidity in subpolar regions; high viscosity; and lower breakdown voltage under lightning impulse at long oil gaps compared to mineral oil [18]. Additionally, the high cost of natural esters, from cultivation to the final product, poses a significant barrier to their commercial viability. Over the years, various enhancement techniques have been explored to improve the performance of natural esters, including chemical modifications [19; 20] and the use of additives such as pour point depressants [21; 22; 23], antioxidants [24; 25; 26; 27], viscosity modifiers [9; 28; 29], and nanoparticles [30; 31; 32; 33; 34; 35; 36; 37]. However, despite these advancements, the global application of natural esters in transformers remains limited. A key concern is that natural esters are primarily derived from cultivated plants, and agricultural practices are known to significantly contribute to anthropogenic greenhouse gas emissions. Reports indicate that agriculture accounts for approximately 84% of global nitrous oxide emissions and 52% of methane emissions. Moreover, to meet the growing demand for food and livestock feed, global crop production is projected to increase by 100% by 2050. To mitigate these environmental impacts, there is a growing interest in utilizing waste-derived vegetable oils, such as waste cooking oil (WCO), as a sustainable alternative to fresh vegetable oils. This approach not only reduces food competition but also minimizes the environmental footprint of agricultural production. Recent advancements have demonstrated the successful conversion of waste cooking oil into biodiesel and diesel blends for fueling diesel engines [38; 39; 40; 41]. Extending this concept to transformer insulation, researchers have begun investigating the potential of waste cooking oils as alternative insulating liquids. Studies have shown that properly treated waste cooking oil can meet standard requirements for breakdown voltage and moisture content, although challenges remain in terms of acidity control [42]. The study presented in Ref. [43] demonstrates that appropriate modification and treatment of waste cooking oil (WCO) can effectively reduce its acidity to below 0.06 mgKOH/g, meeting the standard requirement set by ASTM [44]. Furthermore, research findings in Refs. [45; 46] indicate that treated waste cooking oil exhibits a breakdown voltage and thermal conductivity that are 48% and 33% higher, respectively, compared to mineral oil, highlighting its potential as a viable alternative insulating liquid. These findings indicate that waste cooking oil has the potential to become a viable alternative insulating liquid in the near future. Given the limited knowledge available on the potential use of waste cooking oil (WCO) as a transformer insulating liquid, this review explores its feasibility as a sustainable alternative. It examines the necessary purification processes, including physical and chemical treatments, to enhance its suitability for insulation applications. Additionally, the study evaluates the cooling and dielectric properties of purified WCO based on existing literature. A life cycle assessment is also discussed to assess its environmental impact and sustainability. While WCO has not yet been adopted for transformer applications, this review aims to provide a comprehensive assessment of its potential, highlight key challenges associated with its implementation, and outline future research directions to optimize its properties for high-voltage transformer insulation.