The gas state of the insulating gas significantly impacts the development and extent of the partial discharge (PD) in gas-insulated switchgear (GIS). Correlating these two factors allows for a more comprehensive diagnosis of insulation defects. At present, the ultrahigh-frequency (UHF) method has been widely used for PD detection in GIS. However, the measurements of the insulating gas's state in the field are primarily conducted offline or at gas valves, providing ineffective data for reference. To address this challenge, this article introduces a novel composite sensor capable of simultaneous UHF and gas state measurements. Building upon the disk-type UHF sensor structure, the composite sensor integrates a gas state sensing unit with the micro-electromechanical systems (MEMS) sensor as the core. A robust shielding shell is designed through finite element simulation to ensure the safety of the sensor, with the maximum induced voltage and electric field strength on the sensing unit measuring 0.28 V and 414 V/m, respectively. The average UHF measurement sensitivity of the composite sensor reaches 13.3 mm. In addition, the composite sensor exhibits low errors (±0.2 °C, ±3 kPa, and ± 10 μ L/L), high consistency and repeatability in temperature, pressure, and moisture measurement. Finally, the composite sensor has demonstrated its capability to simultaneously and safely detect PD signals and the gas state within GIS.

During the long-term operation of converter transformers, the aging and decomposition of oil-paper insulation lead to the presence of cellulose particles in transformer oil, which affects the safe operation of the equipment. The motion characteristics of cellulose particles in short oil gaps and long oil gaps under a DC electric field are studied. The forces acting on a single particle, the aggregation characteristics of multiple particles, and the conditions for particle chain breakage are calculated and analyzed. An experimental platform is constructed to validate the conclusions. The results show that cellulose particles in transformer oil are subjected to the combined action of various forces under a DC electric field. Under the influence of the dipole force, multiple cellulose particles attract each other and form particle chains along the direction of the electric field. Changes in internal forces within the particle chain can cause the chain to break. The movement of particles in oil is closely related to the adsorption time of particle chains on the electrode surface and their movement time within the oil gap, which is affected by the resistivity, particle size, charge quantity, and external electric field intensity of the particles. In the experiments, particles form cellulose bridges in short oil gaps, while in long oil gaps, they move freely in transformer oil in the form of particle chains.