purpose of nitrogen injection in transformer

[DmanD]

dear pals,
i am curious about nitrogen injection in a transformer under vacuum. it is generally being done in the oil cooled transformers, after draining the oil.

the transformer is subjected to impedance heating under vacuum. then impedance heating is stopped and nitrogen is injected. after leaving the transformer with injected nitrogen for 24 hours minimum, it is vacuumed again. then the new or filtered oil is pushed into the transformer.

could anyone please explain the purpose of injecting transformer with nitrogen?

dman

 

[MacGyverS2000]

My off the cuff guess would be to prevent corrosion, prevent standard air moisture from creeping into parts, like the core plate sandwich.

 

[dpc]

As MacGyver says, nitrogen is used to displace air and keep moisture out of the transformer.

Nitrogen is commonly used on larger power transformers while they are in service for the same reason (non-conservator type). The nitrogen gas above the oil keeps out air/moisture.

 

[xabproject]

Not always Nitrogen!
Dry air is also used. It is the dryness that is more important (dew point <minus 51 deg C (!), slight positive pressure of about 0.2 bar). Normally oil filling immediately follows vacuuming/dryout unless the oil filling process is not ready for some or the other reason. If transformer insulation is allowed to cool with moist air/gas, the moisture gets loaded back into the insulation!
Gas cooling is preferred as the cooling rate is slower/controlled, liquid admission would result in quenching! Warming gas is cheaper than warming oil, before admission - if thermal shock is to be avoided!
There is no step such as 'passsivation' in or on transformer metal/insulation. The period of 24 hrs or so is for the insulation to throw out deeply settled moisture. Some prefer to perform a dew point test at this stage.
However, a chemically 'inert' - clinically 'safer' gas if available cheaply and readily could be someone's personal preference.

 

[Dopler]

Some transformer uses in operating conditions nitrogen to have positive pressure inside the tank, to avoid entrance of moisture.

 

[orm2]

When the oil is removed from the transformer, the inside of the transformer acts like a sponge and absorbs any moisture from the air. This is unwanted. The hydrogen gas will not contain any moisture and keep the air out. If the windings are removed from the case of the transformer i.e. for refurb, they are usually placed in a large oven to stop the ingress of moisture.

 

[ScottyUK]

Hydrogen? I guess your fingers and brain have got out of step with each other, and you actually mean nitrogen? Otherwise, can you expand on why you would choose to introduce a flammable and potentially explosive gas into a transformer tank when there are safer alternatives?




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[orm2]

looks like they did! nitrogen is what I mean

 

[electricpete]

I agree with the above. In field processing of power transformers that I have seen, when the vacuum process is interrupted for any period of time, the transformer is filled with dry air approx 5 psig to avoid any possibility of inleakage.

Also, the air supply valve can be closed and air supply disconnected to conduct a leak check by observing pressure change over a period of perhaps 24 hours, and correcting for effect of temperature changes.

By the way, if for some reason it becomes necessary to enter a transformer after having been pressurized with nitrogen, it would be necessary to purge/ventilate with dry air to establish a safe oxygen rich atmosphere for people.

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