## 1 Introduction  
The power transformer is one of the most important electrical pieces of equipment in power systems. In recent years, due to the unique advantages of optical fiber sensors such as high immunity to electromagnetic interference (EMI), strong corrosion resistance and low unit cost [1; 2; 3], optical fiber sensors are widely used in online monitoring to improve the reliability of power transformers. However, when the optical fiber laid along transformer windings is running at high temperatures for a long time, the oil insulation system of the transformer is affected by the degraded optical fiber sheath [4]. The development of better fiber sheaths and assessment of thermal aging characteristics of the oil containing fiber have become the focus of worldwide research. Oil-paper insulation is an important part of internal insulation in oil-immersed power transformers [5; 6; 7; 8; 9; 10]. As the dielectric constant of the insulating oil is much lower than that of the paper, the field strength of the oil is higher than that of the paper in this insulating scenario [11]. Thus, partial discharge in the oil can occur easily [12; 13]. The influence of thermal aging on the electrical properties of the oil and the corresponding mechanisms is a concern which can impact the operational stability of transformers. In the process of thermal aging, the main reason for the deterioration of insulating oil is the pyrolysis reaction. Thermal aging experiments have been performed by many researchers over the past decades to study the aging mechanisms of oil and variations of aging parameters [14; 15; 16; 17; 18]. However, most studies have focused on the effects of insulating paper, copper, water, and oxygen on the oil. Few results are available for fiber-containing oil. Optical fiber sensors in transformers, based on the fiber Bragg grating (FBG), fluorescence, the Brillouin scattering principle, and Raman scattering principle, are used to measure the partial discharge, temperature, and winding deformation [19; 20; 21; 22; 23]. Ordinary optical fiber sheaths, such as polyethylene, low-smoke zero-halogen, and polyvinyl chloride have successively been used in the field of communications [24]. However, they cannot operate at high temperatures in transformers. Special optical fibers which have polyimide coating and heat resistant outer sheaths are the most ideal built-in optical fiber sensors used in transformers. Two typical commercial sheaths are thermoplastic polyester elastomer (TPEE) and poly tetra fluoroethylene (PTFE) [25]. These two special optical fiber variants are being used in the insulation of wires and cables that are exposed to highly corrosive environments across the word [26]. Some previous studies have been carried out to test the thermal aging properties of TPEE and PTFE in air. Only a few reports have focused on changes in the properties of oil-polymeric materials during the thermal aging process. In 2017, [PERSON] [27] analyzed the thermal aging stability of fluorescent fiber sheaths of cross-linked polyolefins, polyurethanes, and polyvinyl chlorides at different temperatures in oil. The experimental results show that the cross-linked polyolefin and polyurethane sheaths are decomposed, and the performance of the polyvinyl chloride (PVC) sheath had little change. However, the influence of TPEE and PTFE sheaths on aging characteristics of oil and related mechanisms have not been fully addressed. In this paper, transformer oil was aged with TPEE and PTFE in parallel to investigate the influence of the sheaths of optical fibers on the thermal aging characteristics of oil. Firstly, accelerated thermal aging tests were performed at 130 \({}^{\circ}\)C. At the same time, pure oil without any optical fibers was set as a reference sample. Secondly, the volume resistivity, dielectric dissipation factor (DDF), and breakdown voltage of oil were measured to indicate insulation strength.