why is it important to have purified oil in a transformer?

why is it important to have purified oil in a transformer?

 Dissolved gasses in transformer oil can cause arcing, corona discharges, and overheating--reducing the electrical efficiency and lifetime of the transformer. Likewise, water contamination at levels as low as 30 ppm (parts per million) can adversely affect the insulating strength of the oil. With ever-increasing standards for energy efficiency of power distribution transformers, the need to effectively degas will become even more important in the future. Dissolved gas is different from gas bubbles; it’s distributed in the oil as individual molecules or as clusters of molecules which are invisible to the naked eye. The way to get the gas out is to reduce the pressure. It’s similar to carbonation in soda which is invisible until the cap is removed. Once the bottle is opened, the pressure is lowered and some of the carbon dioxide gas comes out as bubbles. Likewise to remove the dissolved gas in transformer oil, you must lower the pressure enough so that the molecules collect into bubbles; the bubbles expand due to the natural tendency of gasses to increase their volume as pressure is reduced; and finally, the bubbles rise to the surface or are forced through a coalescer where the gas can be pumped away. To ensure that degassing is happening effectively and to know when the oil has been sufficiently degassed, a process based on pressure measurement is much more reliable than one based simply on time. When filling a transformer, moisture in the winding insulation and other components will quickly degrade the oil quality. With the proper instrumentation in place, monitoring pressure during the pre-fill pump-out indicates when the water or other contamination by-products have been removed. It is much easier to remove traces of moisture from the transformer before oil filling than after.