I came across a post about nitrogen filling for liquid-filled transformers, but one question wasn’t addressed: Can this procedure be performed on an energized transformer?
The reason I’m asking relates to transformer liquid sampling. In a recent course, I learned that sampling should never be done under negative pressure. ASTM D-923 states not to sample under negative pressure and provides a method to determine if negative pressure exists, but it doesn’t offer guidance on introducing positive pressure. It almost reads that you should abort the liquid sample.
During the course, they mentioned that if negative pressure is present, you should fill the transformer with dry nitrogen to raise the pressure to an acceptable range (typically 1–3 PSI).
So my questions are:
Can this nitrogen filling be done while the transformer is energized, assuming there are no issues and you have the proper valves (e.g., purge valve/nitrogen fill valve), tools, etc.?
If yes, what is the typical procedure?
Are there any standards or documented practices you’re aware of for performing this work?
Obviously, it makes sense that doing any work on a transformer is best and safest when performed de-energized, however, sometimes that option is not easily available to do; just like conducting liquid sampling, they introduce external sampling ports so it can be done when the transformer is energized.
Yes, this can be safely done and usually is done with the transformer energized. Its really common to see transformers with auto nitrogen systems that will activly keep the transformer at positive pressure.
Transformer energized, if protected by transformer differential and/or high side overload: make Sudden Pressure protection Non-Auto, fill with nitrogen to 2-3psig, make SP Auto.
As others have stated, it shouldn’t be an issue as long as you are keeping clear from energized components. Sometimes the fill port can be right above the secondary terminals. Also be sure to disable any pressure relays to prevent tripping the transformer offline.
In my application, the transformers are substation medium voltage < 5MVA.
The sampling ports are located externally with the live components enclosed in air terminal chambers or throats.
See image below of an example similar transformer.
That’s good advice to have the sudden pressure relay disabled before you fill with nitrogen. I don’t think we have an easy switch to disable, so lifting the wires in the control cabinet will most likely be the solution to temporary disable the sudden gas.
We do have external pad mount transformers, that do require isolations in order to sample.
Are you aware of any resources that describe the steps of nitrogen filling?
It’s recommended that you reference the installation or maintenance manual for the specific transformer you are filling. Each one comes with its own nuances.
It is always good to check with the manufacturer, but with the size of the transformer and the amount of pressure you are inserting, the sudden pressure should never release. Have you seen the springs in those? They will injury you significantly if not kill you if you try to disassemble. They are like air brake cannisters on a semi. It takes a lot of “sudden pressure” for them to activate.
Transformer can have Nitrogen filled while energized. It is very important to sample with positive pressure to prevent inducing cavitation induced heat spots in the transformer that can increase laminar heating leading to potential damage to the transformer (i.e. air pockets next to the insulation removing thermo-cooling of the coils).
I would verify how your sudden pressure relief valve operates as most tend to ignore slow pressure increase versus a sudden 3-7 psi increase in pressure which is not needed when filling nitrogen into the tank as you only need ~1 psi of pressure for sampling.
Lastly, if you have closed door padmount transformers that require shutdown due to clearances, I recommend looking into SD Meyers (or other similar manufacturer setups) external sampling kits that can be installed in about 4 hours by a qualified team (I have a team available if needed). You can find their kit here:
