Collecting transformer dielectric fluid samples
Author: Lawrence Kirchner
Transformer dielectric fluid samples can provide early detection of mechanical, electrical, or thermal problems inside a transformer. Problems detected using this information can often be repaired and a costly catastrophic failure possibly avoided. Tracking this information can be useful when trying to determine the remaining life of the unit, as well as in determining necessity for next scheduled maintenance.
The following proposed procedure is designed to address accuracy, consistency, personal safety, and environmental concerns related to this diagnostic tool.
Equipment required per transformer
- Latex gloves
- (1) syringe (75cc capacity minimum)
- (2) 200 cc clean lab quality jars (glass preferred) with lids
- (1) 500 cc clean lab quality jar (glass preferred) with lid
- Lint free cotton rags
- Hand tools
- Small bucket
- Polyurethane hose to fit syringe and/or sample port nipple
- Oil diapers
- Miscellaneous plumbing adapters such as nipples and reducers. No Galvanized fitting shall be used. Galvanized fittings can contaminate the sample.
Safety: Fluid may be hot. Avoid direct contact with skin. Pressure at the bottom of the tank is higher than the tank gauge pressure indicates due to the weight of the fluid. Open valve slowly to avoid getting splashed. Wear safety glasses with side shields.
Environmental: Avoid spilling any fluid on the ground. Use buckets, diapers, and rags.
Transformer: Unit pressure gauge must show positive pressure before opening any valve on the bottom of the tank. This is to avoid introducing an air bubble into the transformer that could initiate a dielectric failure. If the unit is showing zero psi or less, (only possible in nitrogen blanketed design) dry nitrogen must be added to raise tank pressure to approximately one psi.
Samples: Never take samples during rain or snow. A sample should represent the average content of the transformer. If there is a small amount of debris or water in the transformer, it will most likely be in the bottom of the tank, near the valve. A contaminated sample does not represent the majority of fluid in the transformer. This is why it is necessary to thoroughly clean and flush the valve before final collection. A Dissolved Gas Analysis (DGA) sample contaminated with outside air will indicate oxygen and/or nitrogen inside the unit when this is not the case. Samples collected incorrectly can generate misleading information. Laboratory and collection costs are expensive; following proper procedures can avoid costly re-sampling and re-testing.
Process for Dissolved Gas Analysis (DGA) sample collection
A sample of the fluid is extracted from the bottom of the transformer unit using a syringe, taking extreme caution not to introduce any outside air into the sample. The lab technician extracts and then analyzes any gasses taken out of the fluid. Both a syringe sample as well as bottle samples should be taken.
1. The sample needs to be taken from the bottom transformer valve. It is not advisable to use the sample port (if provided). Verify that the valve is closed and remove valve end-plug. Wipe the cavity clean with a dry, clean, lint free rag. Open valve slowly and thoroughly flush cavity by draining 1-2 gallons of fluid into a bucket.
2. Install end-plug with sample nipple. Connect the syringe between the syringe and nipple using polyurethane tubing. Open the valve very slowly, allowing transformer pressure to push the syringe plunger out; never pull the plunger out. Fill syringe to about 10cc and then close the valve. Eject all air bubbles from the syringe and tubing. Open valve and allow tank pressure to push approximately 65cc into syringe. Close valve. Extract any remaining bubbles, but retain a minimum of 50cc for lab analysis. It is not uncommon for bubbles to start developing in the sample hours after taken. DO NOT extract these bubbles. This is the gas that will be analyzed.
3. Package carefully. Attach a “Do not Refrigerate” label to the box. Store sample in a location that is not excessively hot or cold.
4. The sample should be shipped to the testing laboratory within 18 hours. Shipping paperwork must be filled out properly and completely.
Process for chemical makeup (water/particulates/dielectric) sample collection
- Fluid samples are simply taken from the bottom of the unit and stored in a clean, dry, sealed lab jar(s). The sample needs to be taken from the bottom transformer valve. It is not advisable to use the sample port (if provided). Verify that the valve is closed and remove valve end plug. Wipe the cavity clean with a dry, clean, lint free rag. Install end plug with sample nipple. Open valve slowly and thoroughly flush cavity by draining one to two gallons of fluid into a bucket.
- Fill (2) 200cc and (1) 500cc containers to approximately 90% capacity and seal with lids.
- The sample should be shipped to the testing laboratory within 18 hours. Shipping paperwork must be filled out properly and completely.