## 2 Materials and Methods  
Materials preparation: ZnS:Cu fluorescent powder was prepared by high-temperature solid-phase method. The specific preparation process is as follows: Firstly, ZnS and copper chloride (CuCl\({}_{2}\)) powder were mixed evenly in methanol at a molar ratio of 99:1 and wet-milled for 120 min until fully dried to obtain the precursor mixture. Secondly, the precursor mixture was placed in an alumina boat and compacted, and the temperature in the tube furnace was set at 1050 \({}^{\circ}\)C and held for 180 min. Then, the alumina boat was taken out of the tube furnace, placed in a muffle furnace, and adjusted to a temperature of 300 \({}^{\circ}\)C for annealing for 30 min. Finally, the obtained sample was re-ground, washed, and dried. A voltage-sensing coating based on ZnS:Cu EL material was prepared. First, E51 epoxy resin and crosslinking agent were mixed in a centrifugal mixer at a ratio of 9:5 and stirred at a stirring speed of 25 krad/min for 30 min for uniform mixing to obtain the mixed material. The 9:5 ratio ensures the curing reaction proceeds efficiently at ambient conditions, forming a densely crosslinked network that achieves full curing within hours at 25 \({}^{\circ}\)C. Then, ZnS:Cu was added to the mixed material with the mass ratio of 1:4. The mixed ZnS:Cu/epoxy mixture was stirred in a centrifugal mixer at a stirring speed of 25 krad/min for 10 min. Finally, the well-mixed coating was sprayed on the surface of the insulator to form a coating. ZnS:Cu EL powder will emit light under the irradiation of ultraviolet lamps. By spraying under ultraviolet lamps, the spraying effect can be observed in real time to ensure uniform spraying. To reduce the distortion of the electric field strength on the insulator surface caused by the coating, the coating should be as thin as possible, with a thickness aimed to approximately 0.1 mm. In controlled deposition processes, thickness is generally maintained as constant unless intentionally varied. Thickness variations across the deposited layer, however, are inevitable due to inherent limitations in manufacturing techniques. Substrate roughness, coating viscosity fluctuations, drying kinetics, or edge effects during deposition can create gradients. Characterization: A 35 kV test transformer was used to apply voltage to both ends of the insulator. The rated capacity of the equipment was 100 kVA, the rated voltage was 35/0.4 kV, and the rated current was 2.86/250 A. One end of the insulator was connected to the high-voltage terminal of the transformer and the other end was connected to ground. The output voltage at the high-voltage terminal was controlled through a computer to change the voltage applied to the insulator, thereby controlling the luminescence and color change effect of the coating. The dielectric spectrum was performed with a concept 80 system (Novocontrol Technologies GmbH, Hundsangen, Germany) with 1 V AC voltage in the temperature range of -100-160 \({}^{\circ}\)C and frequency range of 10\({}^{-1}\)-10\({}^{6}\) Hz. The breakdown test was performed with 25 mm standard sphere-plate copper electrode in insulating oil with a voltage ramping rate of 1 kV/s. The surface flashover experiment of the insulator was carried out in an air environment; the interval between two con secutive surface discharges was 2 min. In each case, three samples were tested, and 11 flashover experiments were conducted on each insulator. With the use of a calibrated high-precision spectrometer (Ocean Insight HDX-VIS-NIR spectrometer), under the illumination of the standard light source D65, the sample to be tested was fixed on the test platform to ensure a flat surface and no interference from environmental stray light. The spectral reflectance/transmittance data of the sample within the wavelength range of 380-780 nm were collected by the spectrometer, and then the chromaticity coordinates (x, y) were derived and calibrated on the CIE 1964 chromaticity diagram.