## 2 Ageing of Samples and Measurements of Electrical Properties
### Experimental Setups  
The Nicolet Fourier-transform infrared spectrometer (FTIR) was used to measure and analyze the molecular groups on the surface of silicone rubber insulating materials peeled from cable terminals aged for different times. The test method was total reflection infrared spectroscopy (ATR method) with the measured wave number ranging from 4000 to 600 cm\({}^{-1}\). A Novocontrol Concept 80 dielectric spectrometer was used to test the dielectric spectra of silicone rubber samples. Before the measurement, silicon rubber samples were cut into a circle with a diameter of 40 mm, and an ion sputtering device (Q150T, Quorum Technologies Ltd, East Sussex, United Kingdom) was utilized to perform gold spraying on the upper and lower surfaces of the sample in vacuum. The samples were put into the test chamber of the dielectric spectrometer. The test temperature of dielectric spectroscopy was at 20 \({}^{\circ}\)C, the frequency range was in the range of 10\({}^{3}\)-10\({}^{-1}\) Hz, and the effective value of the ac voltage was 1 V. The dielectric constant and dielectric loss of the silicone rubber samples were obtained. Figure 1a shows a schematic of the surface potential decay measurement system, which consists of a needle-grid electrode and the surface potential measurement apparatus. The needle-grid electrode includes three electrodes: high-voltage electrode, metal grid electrode, and grounded electrode. During the experiment, the needle electrode and the grid electrode were connected to voltage sources at the same time. The voltage of the needle electrode was higher than that of the grid electrode. In this way, due to the potential difference between the two electrodes, the needle appeared as corona discharge andquantities of charges were generated. Then, the charges were transferred directionally onto the silicone rubber sample under the effect of the electric field. In the surface potential decay test, the sample was firstly charged by the applied voltage source and then moved quickly below the probe for the detection of the potential on the sample surface, and the corresponding data was uploaded to the computer, which was running the collecting data program complied by the Labview (National Instruments, Austin, U.S.). Two groups of experiments (negative corona charging and positive corona charging) including four aged silicone rubber samples with diverse ageing times and one pristine sample were conducted in this paper. All samples were paved and fixed on the copper plate with conductive tapes. The voltages were set as \(-10\) and \(-5\) kV for the needle and grid electrodes respectively, when silicone rubber samples were charged negatively. The voltages were set as \(10\) and \(5\) kV for needle and grid electrodes, respectively, when silicone rubber samples were charged positively. All the samples were charged for \(2\) min, and the detection periods for surface potentials were \(4\) h. The testing was performed at room temperature and the relative humidity was limited to around \(25\%\). Figure 1b demonstrates the volume conductivity measurement system. The test used a three-electrode system, which was placed in the shielding box and included high-voltage, measuring, and grounding electrodes. The diameter of the measuring and high-voltage electrodes were \(25\) and \(50\) mm, respectively. The guarding gap between the measuring electrode and grounding electrode was \(2\) mm. The leakage current of the testing sample was measured by a high-accuracy pico-ammeter. The volume conductivity of all aged silicone rubber samples and the pristine one was tested through the measurement system. The applied voltage was set as \(5\) kV and the current of the sample at steady status was obtained when the voltage source was applied for \(1\) min. The testing temperature was \(20\)\({}^{\circ}\)C and the relative humidity was around \(30\%\), which was controlled by the desiccant.