Why purify transformer oil?

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. particles contamination will also affect the function of insulating oil. So we need to clean used transformer oil.