# Non-Uniform Distribution of Contamination on Composite Insulators in HVDC Transmission Lines
## 2 Natural Pollution Test in the Operating HVDC Transmission Lines
### Contamination Non-Uniformity of Operating HVDC Composite Insulator  
Following the guidelines in literature [18], ESDD and NSDD of the samples were measured based on the method shown in Figure 3: Firstly, moderately purified water was used to clean up the surface of the polluted insulator, then the conductivity of the dirty liquid after filtering was measured, the salt quantity attached to surface of each group sheds was calculated separately, and finally, based on the areas of the insulator surface, the converse salt quantity to ESDD was calculated; the dirty liquid was filtered after cleaning the insulator, then the insoluble materials were left on the filtering paper to dry and were weighed. Lastly, referring to the surface area, the converse of the quantity of insoluble materials to NSDD was calculated.  
In this test, 64 series of measurements were made. Each group was taken for calculating contamination non-uniformity of top surface and bottom surface, windward side and leeward side, high voltage groups and middle groups respectively. The SDD and relative standard deviation error (\(\sigma\)) were calculated by the equation as follows:  
\[\text{SDD}=\frac{\sum\limits_{i=1}^{n}SDD_{i}}{n} \tag{1}\]  
\[\sigma=\sqrt{(\sum\limits_{i=1}^{n}{(SDD_{i}-SDD)}^{2})/(n-1)}/\text{SDD}\times 1 00\% \tag{2}\]  
where \(\text{SDD}_{i}\) is a calculated SDD from test results, and \(n\) is the number of measurements made.  
To better understand contamination non-uniformity of insulators, \(\text{K}_{\text{T/B}}\), \(\text{K}_{\text{W/L}}\), \(\text{K}_{\text{H/M}}\), \(\text{K}^{\star}_{\text{T/B}}\), \(\text{K}^{\star}_{\text{W/L}}\) and \(\text{K}^{\star}_{\text{H/M}}\) were proposed and calculated as follows:  
\[\left\{\begin{array}{c}\text{K}_{\text{T/B}}=\text{NSDD}_{\text{T}}/\text{ NSDD}_{\text{B}}\\ \text{K}_{\text{W/L}}=\text{NSDD}_{\text{W}}/\text{NSDD}_{\text{L}}\\ \text{K}_{\text{H/M}}=\text{NSDD}_{\text{H}}/\text{NSDD}_{\text{M}}\\ \text{K}*_{\text{T/B}}=\text{ESDD}_{\text{T}}/\text{ESDD}_{\text{B}}\\ \text{K}*_{\text{W/L}}=\text{ESDD}_{\text{W}}/\text{ESDD}_{\text{L}}\\ \text{K}*_{\text{H/M}}=\text{ESDD}_{\text{H}}/\text{ESDD}_{\text{M}}\\ \end{array}\right. \tag{3}\]  
where \(\text{NSDD}_{\text{T}}\), \(\text{NSDD}_{\text{B}}\), \(\text{NSDD}_{\text{W}}\), \(\text{NSDD}_{\text{L}}\), \(\text{NSDD}_{\text{H}}\) and \(\text{NSDD}_{\text{M}}\), are the NSDD of top surface, bottom surface, windward side, leeward side, high voltage group and middle group respectively, as is ESDD. The Contamination degree and calculated K are shown in Table 1.  
Figure 3: The flowchart of the measuring of the ESDD and NSDD.  
It can be seen from the table that both K\({}_{\rm T/B}\) and K\({}^{\star}\)\({}_{\rm T/B}\) are around 1/0.51, which means that the amount of contamination on the top surface is 2 times than that of the bottom surface. Thus, K\({}_{\rm W/L}\), K\({}^{\star}\)\({}_{\rm W/L}\), K\({}_{\rm H/M}\), K\({}^{\star}\)\({}_{\rm H/M}\) are 1/1.45, 1/1.46, 1/0.61, 1/0.62 respectively, from this we can get two conclusions, on the one hand, the contamination on insulator surfaces is very uneven, on the other hand, the value of K and K* is almost equal. The above data is very important for guiding insulation design and field anti-pollution works.  
The measurement of contamination non-uniformity on insulator surface in operating HVDC transmission lines is complicated and dangerous work which needs plenty of manpower. But by using wind tunnel simulation method the factors of particle diameter, wind velocity, relative humidity and electric field can also be considered. It may be a useful way for obtaining contamination non-uniformity K, without so much time-consuming, danger and sometimes irregular field pollution measurements. Because the non-uniformity of NSDD is almost equal to that of ESDD, only the non-uniformity of NSDD is considered in the wind tunnel simulation.