Transformer oil sampling under full-load conditions?

[eerickson]

All, we've been reviewing our safety procedures and standard practices for taking oil samples on our pad mounted 4.16 kV/0.48 kV transformers. Is there any reason why the sample should be take under full-load conditions?

 

[ScottyUK]

I would say it should be taken under reasonable load conditions just to get good oil circulation and a representative sample. I guess you could argue that any fault which is load-dependent will be at its worst under full load, but any gas evolved will still be present in the oil when load drops back down.


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If we learn from our mistakes I'm getting a great education!

 

[Marmite]

I agree with Scotty.
The sample should be taken on load or very soon after de-energising to ensure that contaminants including water are in suspension, rather than settling in the tank bottom. Short of attaching a load bank at each transformer it might prove difficult to apply full load in any case.
Regards
Marmite

 

[Zogzog]

While I agree from a sample quality aspect you need to consider safety concerns for smapling these types energized, you are required to fill out an EEWP per the NFPA 70E for these samples and wear proper arc flash and possibly shock protection if the sample valve is inside the RAB.

Pad mounted transformer oil sampling with the sample valve inside the cabinet is one of the toughest tasks to comply with the 70E.

Best solution, if you are the owner, is to install exterior sample valves, there are several aftermarket modification kits available because of the 70E issues.

 

[trevorm]

To get accurate analysis of oil samples, the temperature the sample is taken at is important.

I may be out of line on NFPA70E as it is not applicable in the country I operate in, if so I apologize in advance.
My understanding is that where the work is done by qualified persons related to the task, exceptions can be made to the Energized Electrical Work Permit provided there are appropriate safe work practices (Article 130.1 (A)(3))pg 24 of 70E.

If I was doing the work, I would start with the Hazard / Risk Evaluation Procedure on pg 107.
Most industrial transformers have enclosed metal terminal boxes, in this case approach boundaries are not an issue as there are no exposed conductors.
Where LV terminals are exposed, they are usually taped up, thus reducing the risk. At 4.16kV the RAB would be 660.4mm.
If any of the terminals are exposed, the Approach Boundaries of Table 130.2(C) come into play.

 

[eerickson]

Thanks Scotty, I agree the oil sample should reflect the transformer's condition regardless of whether the sample was taken at full-load or no-load condition.

 

[ScottyUK]

Just make sure that there is enough warmth in the transformer to create convection currents within the oil - sampling a dead pocket of oil isn't usually very informative, or at least it is very informative but the information is not indicative of true conditions.


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If we learn from our mistakes I'm getting a great education!

 

[Ceast]

2 considerations. Is the the transformer pressure positive? Secondly is the ambient humidity low. If the answer is yes to both then pull a sample.

EastB