*2.2. Typical Particle Size Distribution Characteristics*

The Hami South-Zhengzhou (Ha-Zheng Line) ±800 kV ultra-high voltage (UHV)-DC transmission line and the Southeastern Shanxi-Nanyang-Jingmen (Chang-Nan Line) 1000 kV UHV-AC transmission line pass through Henan, China. Additionally, these two lines have a short distance to transmit power in parallel. Therefore, this was a good experimental environment to compare the difference of contamination accumulation characteristics of UHV-DC and UHV-AC transmission lines, under the same natural conditions. During the outage of these two UHV transmission lines in December 2014, 16 samples of natural contamination on the surface of the XP-160 insulator were collected from 4 towers of No. 249 and No. 251 (±800 kV, Ha-Zheng Line), and No. 3937 and No. 3938 (1000 kV Chang-Nan Line). In addition, the particle size was measured using a laser particle size analyzer and scanning electron microscope. The scanning electron microscope graph and particle size distribution graph of the contaminated samples, which were under the conditions of 1000 kV (AC), +800 kV and −800 kV (DC), are shown in Figures 1 and 2, respectively. The accumulation period of these 16 samples was from 21 March 2014 to 21 December 2014. Experiment towers were in the same area (distance less than 2 km), and the main pollution sources were farmland and chemical plants.

It can be seen from Figures 1 and 2, that the size of contaminated particles on the insulator surface is mainly concentrated in a specific range, which is 5–50 μm. Specifically, the average size was 19.76 μm, and the size of 90% of the particles was larger than 2.89 μm and less than 46.37 μm. Based on the above measurement results, this paper proposed a new conclusion, that there are obvious statistical characteristics. In other words, the size distribution of contaminated particles on the porcelain insulator surface was basically in logarithmic normal distribution, and the median size of adhered particles was about 20 μm, and the distribution of particles with sizes larger than 50 μm and less than 5 μm was rare.

**Figure 1.** Scanning electron microscopy diagram of typical contaminated particle samples: (**a**) +800 kV (DC); (**b**) −800 kV (DC); and (**c**) 1000 kV (AC).

**Figure 2.** Size distribution diagram of typical contaminated particle samples.

#### *2.3. Particle Size Distribution Characteristics*

To verify whether the size distribution characteristics of contaminated particles, proposed in the previous section is generally applicable, 34 samples of contamination on the porcelain insulator surface were collected from 27 provinces and cities in China, and their particle size was measured using a laser particle size analyzer. The measurement results and essential information are shown in Table 1. In the samples, the voltage levels included AC (110, 220, 500, 750, and 1000 kV) and DC (±500, ±660, and ±800 kV). The results showed that the average particle size was mainly distributed in the range of 5–25 μm. It can be considered that these measurement results supported the particle size distribution characteristics proposed in this paper.


**Table 1.** Measurement results of particle size and essential information of contamination samples.

**Table 1.** *Cont.*


\* SPS (site pollution severity) level is used to evaluate contamination accumulation degree in a region defined by IEC 60815 [27].

### *2.4. Particle Size Measurment Results of References*

The measurement results of other researchers also support the conclusion that the size distribution of contaminated particles on the insulator surface is mainly concentrated in a specific range. Xu et al. [28] measured the particle size of contamination collected from a porcelain insulator surface, under charged and non-charged conditions. The measurement results showed that the average particle size was mainly concentrated in the range of 17.97–24.64 μm. At the same time, 90% of the particle size was greater than 6.55 μm, and 90% of the particle size was less than 41.9 μm. Su et al. [29] measured the contamination collected on the porcelain insulator, which worked in a natural contamination test station and converter station. The results showed that 50% of the particle size was less than 15 μm, and 90% of the particle size was less than 50 μm. Tu et al. [30] measured the particle size of the contamination on the porcelain insulator surface under the condition of haze and fog, and found that particle size was in logarithmic normal distribution, 90% of the particle size was less than 14.6 μm.

#### **3. Motion Characteristics of Particle**

There are two processes in which particles move from the air to the insulator surface. The first one is that the particles move toward the surface with the effect of fluid drag force, electric force, gravity, and other forces. The second one is that particles with a certain initial velocity, collide with the surface and adhere to the surface. To research the motion characteristics of particles more clearly, it is necessary to carry out detailed research on these two physical processes [31].
