The ultra-high frequency(UHF) built-in sensors are arranged inside the gas insulated switchgear

which can monitor the UHF electromagnetic wave signals generated by partial discharge in real time.However

due to factors such as equipment vibration and environmental temperature during long-term operation

the performance of the built-in sensors may deteriorate

resulting in the inability to accurately monitor partial discharge signals.Therefore

it is necessary to study the onsite calibration method of the built-in sensors

so as to regularly evaluate the performance of the built-in sensors during operation.This article analyzed the physical meaning of the return loss of built-in sensors based on the structural principle of GIS built-in sensors and the theory of multi-port networks

and confirmed that the return loss can effectively reflect the operating status of the built-in sensors.Subsequently

based on the structural dimensions of typical GIS chambers

a threedimensional simulation model with an equal proportion of UHF built-in sensors was established

and the correctness of the simulation model was verified by comparing the simulation results of the return loss curve with on-site measured data.Further

the characteristics of return loss curves were researched by simulation for two typical defects of built-in sensor

namely insulation degradation of the dielectric layer and poor contact of the feeder pole

which provided reference for on-site verification and defect diagnosis of the built-in sensor during operation.