## 1 Introduction  
Power transformers are at the heart of modern electrical networks, enabling efficient voltage regulation to ensure electricity can be delivered safely and reliably [1]. The durability and performance of these crucial assets rely heavily on the condition of their insulation systems, which traditionally consist of mineral oil and kraft paper [2]. Over time, these materials are subjected to thermal, electrical, and mechanical stress, all of which contribute to gradual aging and eventual degradation of both their insulating and mechanical properties [3]. Among these materials, kraft paper is particularly sensitive to thermal aging. As it degrades, the cellulose structure weakens, leading to reduced mechanical integrity and insulation performance [4, 5]. Mineral oil, used both as a dielectric fluid and an impregnation medium, also suffers from aging; it oxidizes over time and produces acidic compounds and sludge, which further accelerate paper degradation and reduce overall system efficiency [6, 7, 8]. In response to these challenges, growing attention has been directed toward natural esters (NEs) as an alternative insulating liquid. These biodegradable oils provide several advantages over traditional mineral oils, including better thermal stability, higher fire safety due to higher flash point temperature, and improved protection of the paper insulation [9; 10; 11; 12]. Studies show that Kraft paper immersed in natural esters retains a higher degree of polymerization after aging, which suggests better long-term stability [13; 14]. Another key benefit of natural esters is their ability to slow down the formation of harmful aging by-products like furans from cellulose insulation, helping to limit acidification and preserve the dielectric properties of the overall insulation system [14; 15]. However, despite their promising profile, the behavior of these fluids under electrical stress during thermal aging, particularly breakdown conditions, is still not fully understood. Some reports have noted increases in moisture content and acidity over time, which could affect long-term performance [16; 17]. Furthermore, the connection between aging and electrical performance under combined thermal and electrical stress has not been fully explored. Electrical breakdowns, in particular, can drastically change the material properties, often leading to irreversible damage. These events involve localized heating and the formation of discharge paths in the paper, with severe consequences for both dielectric and mechanical performance, especially in mineral oil-impregnated systems [15; 16]. Natural esters, with their better thermal behavior, might offer a more robust alternative [17; 18; 19; 20]. Moreover, while FTIR and FDS are often used separately to assess the condition of insulating materials, their combined use remains rare. A multimodal FTIR-FDS approach could enable earlier and more reliable detection of aging phenomena. It is important to note that natural esters are not a homogeneous group, their performance depends on their chemical composition, source (e.g., rapeseed, soybean), and degree of saturation [21; 22; 23]. Few studies have systematically compared multiple esters under the same thermal aging and electrical stress conditions. Therefore, this study investigates the stability of oil-impregnated paper using both mineral oil and natural esters under simultaneous thermal and electrical stress. Specifically, it presents a detailed comparative analysis of three insulation systems: one based on mineral oil and two based on natural esters (NE1204 and NE1215).