One major element of a transformer maintenance program is periodic testing of its insulating fluid - and obtaining a good fluid sample starts with knowing where to draw the sample from.
Transformer fluids require sampling from different locations based on their specific gravity (SG).
The specific gravity is of an insulating liquid is simply the ratio of the density of the fluid to the density of water.
SG = d of sample / d of water
Since water has a specific gravity of 1.0, free water in an insulating fluid will migrate to the top or bottom depending on the specific gravity of the fluid. The dielectric breakdown of most insulating fluid is inversely related to the water content, meaning the dielectric strength of the fluid is lowered as its water content increases.
As a general rule of thumb: if the specific gravity of an insulating fluid is greater than 1, sample from the top. If the insulating fluid specific gravity is less than 1, sample from the bottom. Flush two quarts of waste oil through the sample valve before collecting the sample.
For example, if the specific gravity of water is 1.0 and oil is 0.89, the oil will float on top of the water. Sampling from the bottom would provide the most water content in the fluid.
Of course there are exceptions to this rule as the sampling point can change throughout the life of the transformer. Mineral oil transformers with no drain valve are usually accessed and sampled by removing the top cover, for example.
Another instance when the sampling point of a transformer can change is after the retrofill of insulating fluid, such as Askarel to Silicone.
In this case Askarel fluid has a specific gravity of 1.5 and is normally sampled from the top but if the fluid was replaced with silicone, the transformer would now be sampled from the bottom because silicone has a specific gravity less than one.
Specific Gravity of Transformer Insulating Fluids
Below is a table listing the specific gravity of common insulating fluids technicians may encounter in the field:
|Fluid||Specific Gravity||Sample Location|
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