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Wind blown contaminants, such as biological matter at 400 kV Aurora Substation, can settle onto sides of DDG slots and must be washed down by spraying before analysis.
Photos: INMR © any objects in the path of wind and therefore offers a valid simulation of how contamination deposits on insulator surfaces.
Not surprisingly, the volume of material collected in each receptacle depends mainly on wind direction and is obviously greatest when it blows directly into the slot and lowest when it comes from a perpendicular angle. This means that the DDG is also effective in indicating from where contamination most impacts a particular substation and thereby helps identify local pollution sources of greatest concern.
Conversely, wind tunnel tests to see if contaminants could be blown out of a DDG confirmed that particles larger than 300 µ m diameter remained unaffected, even at windspeeds up to 90 km / hr. Moreover, very small particles decreased only marginally with no significant impact in measuring net pollution levels – especially if one considers that such high winds will clean insulators of these tiny particles as well.
Watson reports that he conducts his analysis of each DDG by adding
The DDG indicates from which direction contamination most impacts a substation and also identifies the specific pollution sources of greatest concern. enough de-mineralized water( i. e. less than 5 µ S / cm) to make up a solution totalling 500 ml in volume, allowing for any water that may already be in the container from rainfall. After shaking to disolve the contents of each receptacle, he uses a portable meter to measure resulting conductivity, which varies directly according to volume and nature of pollutants inside. The resulting Pollution Index established by the DDG is then taken as the mean of the reading from all four directions, normalized over the 30- day period between readings.
Another application of the DDG is filtering and weighing the resulting solution to assess levels of nonsoluble pollutants( NSDD).
The second aspect of Watson’ s regular visits to test sites across the Western Cape involves taking readings of pollution accumulating on the surfaces of reference insulator strings composed of 7 standard glass shells. Each disc in the string is analyzed at different time intervals
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