Why we used in Nitrogen used in huge type Trandformer

For an inert gas pressure type transformers, nitrogen is used for several reasons.

1) The first is cost, it gets the job done and is low cost and readily available.

2) The purpose of the gas blanket is to minimize oil deterioration. The oil placed in a transformer naturally has a high affinity for oxygen. The oxygen combines with unstable hydrocarbon impurities and forms sludge, various particles, and other undesirable hydrocarbon byproducts. The presence of heat and or water accelerates the process and the end result is excessive electrical stresses in the transformer. Any number of failure mechanisms have been seen, but generally the onset of failure can be thought of as the formation of conducting bridges across the normally oil insulated gaps which exist throughout the transformer.

This getting kind of wordy. Basically nitrogen is used in this type of transformer as an overpressure blanket to keep out oxygen and water; both of which have a detrimental effect on the transformer oil, and ultimately the transformer itself.