What is the use of the transformers in electricity?

Large transformers to be used indoors must use a nonflammable liquid or be Dry Type, i.e., having no fluid. Prior to about 1970, polychlorinated biphenyl (PCB) was often used as a dielectric fluid since it was not flammable. However, under incomplete combustion, PCBs can form highly toxic products, furans, etc. Due to the stability of PCB and its environmental accumulation, it has not been permitted in new equipment since late 1960's in the United States. Today, nontoxic, stable silicone-based or fluorinated hydrocarbons may be used, where the added expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Other less-flammable fluids such as canola oil may be used, but all fire-resistant fluids have various drawbacks in performance, cost, or toxicity compared with mineral oil.