With the increase of working time

a fluorescence optical fiber temperature sensor sheath will inevitably degrade in power transformer. According to electrostatic simulation results

the sensor and its sheath normally endures a quiet small electric force. Thus

thermal aging is the main factor influencing the insulation performance of the sheath during long working time. In this study

the fluorescence optical fiber temperature sensor sheaths made of three kinds of materials

such as cross-linked polyolefin

polyurethane and polyvinyl chloride

were subjected in hot transformer oil to degrade. The Fourier transform infrared spectroscopy（FTIR） experiments were used to explore the chemical structure of the sheaths. Moreover

the sheath insulation conditions were estimated based on the mechanical tensile test and the isothermal relaxation current test using a setup of surface electrodes. The FTIR results show that the cross-linked polyolefin and polyurethane sheaths may decompose after aging in transformer oil

whereas no obvious change of polyvinyl chloride sheath is observed. The tensile tests results manifest that the tensile strength and the elongation of cross-linked polyolefin sheath decrease dramatically. The elongation of polyurethane sheath decreases whereas the tensile strength increases with the increase of aging time. However

the elongation and the tensile strength of polyvinyl chloride sheath do not change much after aging. The isothermal relaxation current results demonstrate that the aging factor of aged sheath is larger than that of non-aged sheath. To sum up

the performances in hot transformer oil of different sheaths can be evaluated. The cross-linked polyolefin sheath is the worst

and the anti-aging characteristic of polyurethane sheath is better than cross-linked polyolefin sheath. The polyvinyl chloride sheath is the best.