Abnormal changes in the state of insulating gas within gas-insulated switchgear (GIS) can significantly influence the equipment’s insulation performance. Existing gas state detection devices are difficult to accurately reflect the actual internal gas states due to environmental interference. To address this issue

this paper designs a gas state sensing unit centered on a micro-electro-mechanical systems (MEMS) sensor

integrated with a built-in ultra-high frequency (UHF) sensor

enabling internal monitoring of the gas states. To tackle the harsh electromagnetic environment within high-voltage equipment

a full-scale simulation model combining the composite sensor and GIS was developed to analyze the electric field distribution around the sensing unit and electromagnetic interference coupling paths under typical disturbances. Moreover

a multi-layer shielded enclosure design for the sensing unit was proposed. The results indicate that the superposition of capacitive and inductive coupling between the interference source and the gas state sensing unit presents the characteristics of multi-modal and wide frequency domain. The designed grounded shielding enclosure

with a silicon steel main body

effectively shields power-frequency voltages and currents while absorbing high-frequency electromagnetic waves. The top aluminum alloy layer reflects most high-frequency electromagnetic waves

and the shielding mesh at the airflow hole further enhances shielding performance. After shielding

the induced voltage on the sensing unit remains within ±0.2 V

meeting safety requirements for unit operation. This research provides valuable insights for built-in monitoring of high-voltage equipment and electromagnetic compatibility design for low-voltage units integrated into high-voltage systems.