You have seen this type of transparent box with blue crystals like substance inside. These blue crystals are silica gel.
So what is the purpose of using this in the transformer. Lets us first understand what silica gel does .
Silica gel when exposed to moist air or simply when it reacts with water , it absorbs water depending upon its ablity. Like you have seen when you wash your cloth, the cloth absorbs water.
When silica gel crystals absorbs water/moisture from the air, it loses its colour and changes to pink. See below figure.
As we all know, transformer conservator does the breathing means like we human do breathing ..takes the air during light load and expels the air during heavy load.
When it takes the air from outside , silica gel in the box connected to conservator ensure only dry air reaches in the conservator tank.
Mositure is absorbed by the silica gel crystals.
Ao why we want to stop the moisture going into the conservator. Simple , we want mositure does not mix with transformer oil . If transformer oil takes the mositure , than its dielectric strength will decrease.
As we all have seen before transformer commissioning, oil filteration of transformer oil is done to remove any moisture and impurities in the transformer oil.
Silica gel reuse
Aa you have read above, silica gel changes its colour from blue to pink after absorbing moisture. So how can we use again. Its simple.
Heat silica gel crystals by heat air blower or in microwave until silica gel crystals again changes to blue colour. Now the crystals are ready to absorb moisture, put back in the the box and fit to transformer.