## 2 Construction of Experimental Platform and Experimental Methods  
The experimental environment is characterized by a temperature of approximately 15 \({}^{\circ}\)C, a relative humidity of around 75%, and an atmospheric pressure of 101.5 kPa. In this study, mini corona cages were selected to simulate transmission lines. The corona cage structure employed a commonly used steel framework and metal mesh configuration. To ensure a uniform distribution of the electric field intensity on the conductor surface during measurements and to avoid end effects, the corona cage was divided into three sections. The total length of the corona cage was 1 m, with the middle section measuring 50 cm and both sides measuring 25 cm each. The diameter of the cage was 30 cm. As shown in Figure 1, the middle section served as the measurement segment, which uniformly surrounded the conductor. The two ends were directly grounded as protective segments to weaken the end effects and were not used for measurements. The measurement segment in the middle and the protective sections on both sides were insulated through an air gap. In the experiment, the conductors were placed in the center of the corona cage using insulating supports. Following the methods recommended by the International Council on Large Electric Systems (CIGRE) and the International Special Committee on Radio Interference (CISPR), the actual operating split conductor is first equivalent to a single conductor, and then the conductor radius is reduced proportionally to allow it to corona at a lower voltage. In the experiment, smooth, stainless-steel rods with diameters of 2 mm, 3 mm, and 5 mm were used to simulate actual transmission lines. The main equipment included in the experimental setup consisted of a DC high-voltage generator, which was used to generate the required high voltage for the experiment. In this experiment, the model of the high-voltage generator used was JYDHV-100 kV/3 mA. It has an adjustable output DC voltage range of 0 to 100 kV, with a maximum protection current of 3 mA. One ammeter was used to measure the current on the conductor, which has a measurement range of 0 to 2 mA. During the experiment, the STUV-11 ultraviolet imaging camera, which is a day-blind type, was positioned directly facing the discharge location. Only the instrument gain and observation distance were varied, while the other parameters were kept at their default settings. During the experiment, an ultraviolet imaging camera and ammeters were used to measure the UV image and corona current, respectively, to reflect the changes in the severity of the corona discharge depending on the voltage applied to the conductor and the observation distance and gain of the UV imaging device. STUV-11 UV imagers were used in the experiments, as shown in Figure 2. The basic parameters of the UV imagers are shown in Table 1.  
Under the influence of DC voltage, the dispersion of the DC breakdown voltage in air gaps is generally minimal. The equation for estimating the corona inception field strength for positive polarity conductors can be approximated using the empirical formula proposed by [PERSON], as shown in Equation (1) [21]. \[E_{on+}=33.7m\delta(1+\frac{0.24}{\sqrt{r_{0}\delta}}) \tag{1}\]  
In the equation, \(E_{on+}\) represents the corona inception field strength for positive polarity, and \(\delta\) is the relative density of air, \(\delta=0.00289p/(273+\theta)\). \(p\) is the atmospheric pressure in pascals (Pa), \(\theta\) is the temperature in degrees Celsius (\({}^{\circ}\)C), and \(m\) is the roughness factor for smooth conductors; as used in this study, the roughness factor can be taken as 1. For the coaxial cylindrical electrode structure used in this study, the relationship between the corona inception field strength and the corona inception voltage can be expressed