# Wet Snow Flashover Characteristics of 500-kV AC Insulator Strings with Different Arrangements
## 2 Experimental Setup, Specimens, and Test Procedure
### Test Procedure  
The pollution deposited on the insulator surface has two main components, namely soluble pollution that forms a conductive layer when wetted, and non-soluble pollution that forms a binding layer for the soluble pollution [20; 21]. The flashover performance of the insulator is closely related to the soluble pollution component and its electrical conductivity is normally expressed as equivalent \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Shed Type**} & **Arcing** & **Leakage** & **Shed Diameters** & **Shed Dodec** & **Rod** & **Shed Spacing** \\  & **Distance** & **Distance** & **D1/D2/D3 (mm)** & **Numbers** & **Diameter** & **d1/d2/d3/d4** \\  & **(mm)** & **(mm)** & **N1/N2/N3** & **D (mm)** & **(mm)** \\ \hline The anti-icing type & 4195 & 14,000 & 300/170/135 & 6/35/36 & 34 & 40/45/70/80 \\ The common type & 4154 & 14,000 & 170/135 & 46/47 & 34 & 41/47 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters of the common and anti-icing insulators. Figure 4: Various arrangements examples of snow-covered insulator strings: (**a**) Single string, (**b**) 450 mm II-string, (**c**) 650 mm II-string, (**d**) 2\({}^{\circ}\)\(\Lambda\)-string (450 mm-600 mm string), (**e**) 30\({}^{\circ}\)\(\Lambda\)-string (450 mm string), (**f**) 60\({}^{\circ}\)\(\Lambda\)-string (450 mm string), (**g**) 90\({}^{\circ}\)\(\Lambda\)-string (450 mm string), (**h**) 60\({}^{\circ}\) V-string (450 mm string)salt deposit density (ESDD). The insoluble fraction of the pollution layer is commonly reported as non-soluble deposit density (NSDD). To simulate the actual pollution level in Tianjin during the flashover test, detailed measurements of ESDD and NSDD were carried out on exposed field insulators. The measured average ESDD and NSDD were 0.1 mg/cm\({}^{2}\) and 0.6 mg/cm\({}^{2}\), respectively. This result confirmed that Tianjin, a coastal city, is exposed to a severe combination of salt and dust, and that its pollution level could be classified as Heavy according to the standardization adopted in IEC TS 60815 [20]. The test procedure adopted was in accordance with GB T 4585, IEC 60507, and IEEE Std 1783-2009 [22; 23; 24], and can be described as follows:  
1) Pre-contamination  
First, the insulator strings are carefully cleaned and dried. Before the first contamination, insulator strings are washed with a slurry of water and kaolin, and then rinsed with tap water. When dried, the specimens are artificially contaminated using the solid layer method with the measured ESDD and NSDD as mentioned above [20]. Then, the specimens are dried naturally for 24 hours. 2) Wet snow accretion  
The specimens are suspended in the climate chamber in a desired arrangement, as shown in Figure 4. To deposit the wet snow, the climate chamber is first cooled down to a preset temperature of \(-16\)\({}^{\circ}\)C and then kept constant. The water used for spraying has a conductivity of 100 \(\upmu\)s/cm and is precooled to 4-5\({}^{\circ}\)C before the spraying. After the preset temperature is reached, one hour is allowed for temperature stabilization. Then, the snow cannon is turned on to produce the wet snow. The water supply pressure is set at 280 PSI and the snow production is 20 m\({}^{3}\)/h. Under the above-mentioned conditions, the measured liquid water mass fraction of wet snow was 20-25%. As icing degree is an important factor that influences the flashover voltage, each wet snow accretion lasted 20 min for the same snowing degree to be obtained. At this snowing degree, more than 90% of the single string with common sheds was bridged, as shown in Figure 4a. After that, the temperature of the chamber was kept constant at \(-16\)\({}^{\circ}\)C for another 15 min to harden the snow. Finally, the chamber door was opened to increase the ambient temperature to 0 \({}^{\circ}\)C at 3 \({}^{\circ}\)C/h. 3) Wet snow flashover test  
Using the even-rising method, the average flashover voltage was obtained for the various insulator string arrangements in the wet snow condition.