## 3 Experimental Results and Analysis  
### Effect of Temperature on the Breakdown Voltage of Nanomodified Transformer Oil  
Firstly, several samples of nanofluids with a water content of 26 ppm and a nanofluid concentration of 0.01 g/L were prepared. For comparison, several samples of pure oil with a water content of 26 ppm were also prepared. Breakdown tests were conducted at six different temperatures ranging from \(-30\)\({}^{\circ}\)C to 20 \({}^{\circ}\)C. Additionally, tests were performed on nanofluid samples with a water content of 85 ppm. The results at low water content are shown in Figure 5, where the absolute values of the breakdown voltages for pure oil and nanofluid varied with temperature. The minimum value was observed at \(0\)\({}^{\circ}\)C, where the breakdown voltages of both types of oil were the closest. Within the temperature range of \(-30\) to \(30\)\({}^{\circ}\)C, the breakdown voltage of nanofluid was significantly higher than that of pure oil. Figure 5: Experimental results of oil samples at low water contents. Results of the breakdown tests on high-water-content pure oil and nanofluid are illustrated in Figure 6. Compared with Figure 5, it can be observed that the breakdown voltage of the oil samples was lower, reaching its minimum value at 0 \({}^{\circ}\)C. Within the temperature range of \(-\)10 to 0 \({}^{\circ}\)C, for the high-water-content oil samples, the rate of increase in breakdown voltage relative to decreasing temperatures was significantly lower compared to the low-water-content oil samples. The breakdown voltages of the two oil samples under AC voltage varied with temperature. When the temperature exceeded 0 \({}^{\circ}\)C, the breakdown voltage increased with temperature, with a relatively slow rate of increase between 0 \({}^{\circ}\)C and 10 \({}^{\circ}\)C, and the fastest rate of increase was observed beyond 10 \({}^{\circ}\)C. On the other hand, when the temperature was below 0 \({}^{\circ}\)C, the breakdown voltage increased with decreasing temperature, with the slowest rate of increase between \(-\)10 and 0 \({}^{\circ}\)C and a slightly faster rate of increase below \(-\)10 \({}^{\circ}\)C. Nevertheless, the rate of increase remained lower than that above 10 \({}^{\circ}\)C. The overall trend of this curve resembled a \"V\" or \"U\" shape, with the minimum value still occurring at 0 \({}^{\circ}\)C.  
This phenomenon occurs because, at higher temperatures, moisture exists in oil primarily in the form of dissolved water, which has no significant impact on the breakdown voltage. As the solubility decreases with decreasing temperatures, the moisture gradually precipitates and exists in the form of suspended water in the oil. Around 0 \({}^{\circ}\)C, the suspension of water in the oil reaches its maximum. This suspended water causes the distortion of the electric field, and, under the influence of the electric field, the suspended water droplets elongate and deform, forming conductive bridges in an orderly arrangement. Consequently, the breakdown voltage of the oil gap is significantly reduced. As the temperature continues to decrease, the suspended water particles undergo solidification and transform into ice crystals. At this stage, moisture in the form of impurities affects the breakdown voltage of the oil gap. The ice particles cannot be elongated by the electric field, and their dielectric constant is similar to that of insulating oil. This results in the weakened distortion of the electric field. Additionally, as the temperature decreases, the viscosity of the oil increases, causing hindrance to the formation of conductive bridges. Therefore, the breakdown voltage increases below 0 \({}^{\circ}\)C as the temperature decreases. This phenomenon can be explained by the \"bridge theory\".