## 5 Compatibility of WCO with Solid Insulators (Papers)  
In transformer insulation systems, both liquid and solid insulators are critical to the overall reliability and longevity of the equipment. While the liquid provides insulation and cooling, the paper insulators offer thermal, electrical, and mechanical protection for transformer windings. Numerous studies have shown that the life expectancy of a transformer largely depends on the condition of the solid insulators, primarily cellulose-based papers; unlike insulating liquids, damaged solid insulation cannot be replaced [175; 176]. These solid insulators are directly impregnated by the insulating liquid, making compatibility between the two materials essential [177]. Therefore, any alternative insulating liquid, such as transesterified waste cooking oil, must demonstrate strong compatibility with solid insulation to ensure long-term transformer performance and safety. Although there are limited reports on the compatibility of WCO-methyl esters in the literature, the report made by Ref. [100] gives substantial information concerning the long-term compatibility of papers and used cooking oil methyl ester (UCO/ME). The accelerated thermal aging of impregnated thermally enhanced Kraft paper (TEKI) and enhanced Nomex 910 (EN910I) insulation was investigated at 0 h, 48 h, 96 h, and 144 h in two different oils: transformer mineral oil (TO) and used cooking oil methyl ester. The visual aging behavior of the insulating papers immersed in different oil samples over various durations can be found in Ref. [100]. A progressive darkening of the paper samples was observed with increased aging time, indicating thermal and oxidative degradation. In the case of TEKI-TO and TEKI-UCO/ME, the darkening was more pronounced, especially after 144 h, suggesting a higher generation of acidic and oxidative byproducts that accelerate the degradation of cellulose in the paper. Interestingly, for thermally upgraded Kraft paper immersed in TEKI-TO and TEKI-UCO/ME, the papers exhibited similar visual appearances throughout the aging period, with only a slight difference at 96 h. This similarity implies that both mineral oil and methyl ester provided comparable protection to the paper during thermal aging, indicating a similar interaction or stabilization effect from the oils. In contrast, the EN910I papers aged in TO and UCO/ME maintained a more stable coloration even after 144 h, suggesting better oxidative stability and compatibility of these oils with solid insulation. Furthermore, the subsequent micrograph image of these samples presented in Ref. [100] indicates that EN910I paper aged in transformer oil demonstrates the best compatibility, maintaining relatively well-preserved fiber structures even after prolonged thermal exposure. This suggests that transformer oil provides a more stable aging environment, effectively limiting the extent of fiber breakdown and surface deterioration. In contrast, EN910I aged in used cooking oil methyl ester exhibits significant fiber damage, including pronounced distortion, surface cracking, and delamination, resulting in a highly disordered structure with broken and entangled fibrils. These severe structural changes highlight the compromised mechanical integrity of the paper, showcasing the aggressive oxidative environment and acidic by-products associated with the ester, which can accelerate cellulose degradation. This indicates that, despite the potential cooling and dielectric advantages of used cooking oil methyl ester, its long-term compatibility with paper insulation is significantly lower than that of conventional transformer oil. Due to the limited availability of data in the existing literature on the compatibility between waste cooking oil methyl ester and insulation paper, further research is essential to gain a comprehensive understanding of the degradation behavior of paper in the presence of methyl esters. This will help establish the long-term viability and reliability of such alternative insulating fluids in transformer applications.