Transformer Oil and Dissolved Gas Analysis
Ten gases and moisture in oil, measured individually, at the transformer. The answer before the crew leaves the site, not a week later.
The transformer is already telling you
You cannot open a loaded power transformer to see what is happening inside it. You do not need to. Every fault inside the tank leaves evidence dissolved in the oil.
Heat cracks the insulating oil into light hydrocarbon gases. An arc produces acetylene. When the paper insulation is involved rather than just the oil, carbon monoxide and carbon dioxide appear. Hydrogen turns up in very nearly every fault there is. Read the gases and you can tell not only that something is wrong, but what kind of wrong, and often how urgent.
That reading is dissolved gas analysis, and it is the single most informative test performed on a transformer. The usual problem is not whether to do it. It is how long the answer takes.
What each gas is telling you
| Hydrogen, H2 | Involved in nearly every fault of the insulation system. The general alarm bell. |
| Acetylene, C2H2 | High energy arcing. The gas nobody wants to see appear. |
| Methane CH4, Ethane C2H6, Ethylene C2H4 | Thermal faults. The mix and the trend classify the severity of overheating. |
| Carbon monoxide CO, Carbon dioxide CO2 | The cellulosic paper insulation is involved, not just the oil. That changes the seriousness of the finding. |
| Oxygen O2, Nitrogen N2 | Excessive ageing, or a leak in the sealing of a hermetic transformer. |
| Propane C3H8 | Measured individually rather than lumped with the others. |
| Moisture in oil, H2O | Water content, measured directly rather than inferred. |
It is the pattern across gases, not any single number, that separates a hot connection from partial discharge from an arc. Each of these is measured separately.
Where portable DGA earns its place
Laboratory DGA means drawing a sample, shipping it and waiting. Online DGA gives continuous data, but only pays for itself on transformers big enough to justify the installed cost. Between those two sits most of the fleet.
- Confirming an alarm. When an online monitor or a Buchholz relay raises an alert, this is how a crew finds out whether it is real before an outage decision gets made.
- Fleets of smaller transformers where a permanent monitor on each unit would never be commercially viable.
- Other oil-filled equipment, including measuring and protection transformers, CTs and PTs, and circuit breakers.
- During installation or repair, where you want the condition before and after the work rather than weeks afterward.
- Gas samples from Buchholz relays.
Featured instrument: MTE HYDROCAL 1011 genX P
A portable dissolved gas analyzer in a sealed case, about 30 pounds, built by MTE Meter Test Equipment of Zug, Switzerland, with manufacturing in Germany.
- Measures ten gases individually, plus moisture in oil
- No carrier gases and no calibration gas bottles to store, ship or replace
- Seven inch colour touchscreen with videos that walk the operator through each step, or run it from any browser over the built-in webserver
- Integrated thermal printer for a field report before the crew leaves
- Results stored on board and in the HydroSOFT Hybrid App, alongside data from MTE online monitors on the same transformer
- Runs on 88 to 264 V AC, so a truck inverter handles it. Optional battery pack gives about ten measurements away from mains power
How it works
Gas is separated from the oil by a headspace extraction system built for portable use, following IEC 60567, with extraction improved by bubbling the target gas out under negative pressure. Three sensing technologies then do the measuring. Near infrared handles the hydrocarbons and the carbon oxides. A micro-electronic sensor reads hydrogen, oxygen and nitrogen. A thin-film capacitive sensor measures moisture.
The sample is 100 ml of oil. A full analysis including oxygen and nitrogen takes about an hour on the first run and less on the ones that follow. Replaceable oil filters keep droplets out of the measuring system, and the instrument checks the filters before a test starts rather than after.
What a portable will and will not do
A laboratory has controlled conditions, standardised processes and chemists. It will always be the more precise answer, and a lab sample is still worth drawing for the record.
A portable analyzer is a field screening and decision support tool. It tells a crew standing at the transformer whether gas levels are normal or materially elevated, and whether this year looks like last year. That is the question in front of them at that moment, and waiting a week to answer it has a cost of its own.
Measurement ranges and accuracy
| Measurement | Range | Accuracy of gas measurement |
|---|---|---|
| Hydrogen H2 | 0 to 10,000 ppm | 10% or 20 ppm |
| Carbon monoxide CO | 0 to 10,000 ppm | 10% or 5 ppm |
| Carbon dioxide CO2 | 0 to 20,000 ppm | 10% or 5 ppm |
| Methane CH4 | 0 to 10,000 ppm | 10% or 5 ppm |
| Acetylene C2H2 | 0 to 10,000 ppm | 10% or 5 ppm |
| Ethylene C2H4 | 0 to 10,000 ppm | 10% or 5 ppm |
| Ethane C2H6 | 0 to 10,000 ppm | 10% or 5 ppm |
| Propane C3H8 | 0 to 5,000 ppm | 15% or 20 ppm |
| Oxygen O2 | 0 to 50,000 ppm | 10% or 500 ppm |
| Nitrogen N2 | 0 to 150,000 ppm | 10% or 1,500 ppm |
| Moisture in oil H2O | 0 to 150 ppm | 3% and 3 ppm |
Accuracy figures are stated as the greater of the percentage or the fixed value. Gas extraction accuracy is 8% across the gases. Size 447 x 297 x 187 mm, approximately 13.4 kg. IP68 closed, IP40 open. Operating range -10 to +50 C.
Equipment, education, service
ProgUSA has supplied test and measurement equipment to electric utilities, NETA firms and industrial customers since 2005. We represent MTE Meter Test Equipment in the United States with demonstration inventory, applications support, training and calibration coordination.
We put instruments in front of customers on their own assets. If you own oil-filled equipment and you want to see this run on your transformer, ask us.