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As the transmission capacity of gas insulated transmission line (GIL) increases, the synergistic improvement in dielectric and mechanical strengths of GIL insulators is urgently needed for the development of advanced power systems. Mechanical defects in power equipment are prone to induce discharges during high voltage operations, which need to be mitigated. In this paper, the strains on the GIL insulator surface are measured by fiber Bragg grating (FBG) strain sensors during hydrostatic tests. An abnormal change of strain is detected near the interface between center conductor and the spacer. The numerical results of stress distributions of GIL insulator during manufacturing and high-voltage operation are calculated by the finite element method. The results indicate that the mechanical stresses are concentrated at the interface, which lead to the development of conductor-spacer interface separation. In addition, based on the hydrostatic test and the FEM calculations, the dielectric interfacial strength, including the electric field distribution, and flashover are investigated by a combination of numerical and experimental studies. The results indicate that the air gaps generated by the separated interface distort the electric field at the tip of the gap, which is likely to cause discharges and reduce the gas-solid interfacial flashover strength of the insulator, placing concerns of insulation failure and even operating failures of GIL. This work sheds light on the importance of interfacial structure of power equipment and will guide the design and the manufacture of GIL insulator.

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Gas‐filled internal crack might appear in thermoset materials like epoxy resin during the equipment manufacturing, which would become a vulnerable local region to initiate the electrical tree, thus prone to cause insulation failure. The withstand voltage test was carried on epoxy samples with artificial cracks based on a rod‐plane electrode arrangement. Simultaneously, surface state variation and tree evolution with crack were observed by an optical microscope in conjunction with a charge‐coupled device camera. The changes in morphology and chemical status of the crack surface were characterized by scanning electron microscopy, laser Raman spectrometer and energy dispersive spectrometer, respectively. It was found that the erosion and tree started from the borderline of crack under a relatively low electric field strength; however, the area near the electrode had relatively little damage. The breaking of epoxy molecular chains coarsens the crack surface and further forms deep channels on a micro‐level, which is the forerunner of the electrical tree inception. Based on these, the initiation mechanisms of the crack‐induced electrical tree and the reasons for the erosion near the borderline have been revealed. This study provides a train thought for the polymer degradation opening up into the initial tree channel during tree evolution processes.