How often should transformer oil be filtered?

Annually for 132 KV transformer, once in 2 years for the transformer below 132 KV transformer and in 2 years interval for the transformer above 132 KV transformer.

The Transformer Oil Purifier / Filtration System

removes impurities and restores / improves the electrical, chemical and physical properties of transformer insulating oils. This oil is an integral part of most large power transformers.  It serves two major roles, as a cooling agent and as an electrical insulator between internal components. Contamination of this oil compromises the insulating properties and leads to premature failure of the transformer. This contamination can be in the form of water, dissolved gases, or foreign particles. By processing this oil with a vacuum dehydration unit like the Lushun Transformer Oil Purifier, this contamination can be removed returning the insulating oil to it’s original specifications. The system incorporates high vacuum and mechanical filtration to dehydration, degasification and filter contaminated oil. Engineered with functionality and versatility to meet a wide range of applications with features such as: an inlet pump capable of drawing oil from long distances and low elevations, a unique multi-level vacuum chamber design for rapid degasification, a sophisticated heating control scheme to insure safe and efficient processing of the contaminated oil, and a variable speed outlet pump to return the processed oil to the transformer being serviced. Control, monitoring, and user interaction is accomplished through an industrial grade touchscreen operator interface. All parameters are easily viewed and adjusted through this interface along with viewing of current and historical data.