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Composite insulators are widely used in power grids at home and abroad. The anti-pollution flashover performance, hydrophobicity and hydrophobic migration, and pollution flashover mechanism of composite insulators have been the focus of international attention. Temperature is one of the important parameters to describe the flashover of composite insulators. In this paper, the internal temperatures of a composite insulation sample were detected based on FBGs (fiber Bragg gratings). The sample was layered and made of one layer of silicone rubber 3 mm in thickness and one layer of epoxy-GFRP (Glass Fiber Reinforced Plastics) similar to 220 kV composite insulators. Three gratings were embedded in the interface of the two layers between long and short electrodes on the surface of the sample. Temperature calibration tests were carried out on the gratings in the sample. Then, discharges tests were done on the sample with embedded FBGs. Internal temperatures as well as leakage currents were measured, and the arcs process of the discharges on the sample was photographed. The test conditions are as follows: the output voltage was increased by approximately 0.5 kV/s; the equivalent salt deposit density (ESDD) was about 0.5 mg/cm2 and the non-soluble deposit density (NSDD) was about 1.2 mg/cm2.The results show that the internal FBGs could sense the flashover on the surface of the sample. An interface temperature rise caused by the discharges is within 11 °Cin this paper. There is a time delay of 1.4 s from the leakage current arise to the temperature rise during the discharges. The arcs always develops from the small electrode to the large electrode on the surface of the sample, the rise process of the internal temperature at the interface under the middle surface position of the two electrodes is significantly different from that near the small electrode, the arcs starting electrode, while similar to that near the large electrode, the flashover electrode.

The discharges of water columns and droplets between the sheds make the leakage distance not effectively used, which is one of the main reasons for flashover of composite post insulators under heavy rainfall. To study the influence of shed parameters on surface rainwater characteristics, artificial rain tests were carried out on the large-diameter composite post insulators under the rainfall intensity of 2–15 mm/min. Lwc (the length of water columns at the edge of large sheds), Nwc (the number of water columns at the edge of large sheds), Nwde (the number of water droplets at the edge of large sheds) and Nwds (the number of water droplets in the space between two adjacent large sheds) were proposed as the parameters of surface rainwater characteristics. The influences of large shed spacing, large shed overhang and rod diameter on the parameters of surface rainwater characteristics under different rainfall intensities were analyzed. The experimental results show that, under the same rainfall intensity, with the rise in large shed spacing, large shed overhang or rod diameter, Lwc, Nwc, Nwde and Nwds all increase. Under different rainfall intensities, the trends of the parameters with the change in shed parameters are basically invariant; however, the change ranges of the parameters are different. The increases in the parameters with the rises in shed parameters and rainfall intensity are mainly attributed to the change in the rainfall on the insulator surface. The experimental results can provide references for the quantitative description of surface rainwater characteristics and the design of large-diameter composite post insulators for DC transmission systems.