The ultimate guide to DGA Interpretation
Dissolved Gas Analysis (DGA) is one of the most important ways to ensure transformer health. Unfortunately, interpreting DGA results isn't easy.
There are lots of ways how the measured numbers can be affected. These have to be accounted for, otherwise the risk of both false alarms and undetected faults is increased.
This blog article cannot replace the lore of seasoned transformer DGA experts. It can - however - give you a first overview on what aspects to think about when interpreting DGA values.
Absolute values
Absolute values
The most salient feature of any DGA interpretation is of course a look at the numbers itself. It's difficult to tell from the numbers alone if a transformer is faulty, but we can compare the numbers to those of other transformers. Multiple sources offer reference data, pictured here is a table from IEEE Std C57.104-2019.
Some explanations are in order: Having an O2/N2 ratio below 0.2 is just code for "sealed transformer". Sealed transformers have naturally higher DGA values because the gasses escape more slowly. So if you know that your transformer isn't sealed, feel free to ignore this ratio and just look at the right side of your chart.
In the end, the 90th percentile is just a rough guideline, not a magical border. If your gasses are increased, its important to see by how much. Values from different transformers can vary by multiple orders of magnitude. So hydrogen values of over 9000 ppm are not unheard of. However, if you reach these, you should run.
The 90th percentile means that 9 out of 10 transformers have DGA values lower than the numbers in the table. This does not mean that any number higher than the 90th percentile indicates a fault. It just means that the probability of a fault is increased compared to an average transformer. The base probability of transformer faults is low, so there is no reason to panic yet.
Watch out if you have a DGA system that measures all seven of these gasses. If each of the gasses has a 10% chance of being increased, and if your gas values are independent of each other (they aren't really, this is just for illustration), then you have a 52.1% base chance of having any of your gasses increased. So don't overrate it if one of them is.
Trends
Trends
Let us create a mental model: At some point, a part of the transformer starts overheating. As a result, gasses are produced at a constant rate. At first, the shape of DGA values over time is an increasing line. However, most transformers are not completely sealed, so some of the produced gas molecules are escaping the oil. The more gas is dissolved in the transformer oil, the more gas is escaping. After a while, gas production and gas escape is in equilibrium. Gas values are not changing anymore. The resulting time series roughly looks like the illustration.
You can use this mental model to improve your DGA interpretations. Some examples:
- Let's say you have slightly increased absolute gas values and are unsure how much you should worry. A strongly increasing trend indicates that your values will be much higher in a few months, so some concern is in order. On the other hand, if the values aren't increasing, you have probably nothing to worry about.
- In another case, absolute gas values are already very high, but not increasing. This could mean that the equilibrium has already been reached. Even though no trend is visible, a fault is likely.
- If both absolute values and trends are low or average, the probability of a (dga-detectable) fault is very low.
Extenuating circumstances
Not all transformers are created equal, so we shouldn't treat them as such. When interpreting DGA values, it's always important to keep the peculiarities of a given transformer in mind. Several features of a transformer can have an influence on DGA values. If gas values can be explained away by these features, you don't have to worry as much. The most important features are listed here:
- Load. A highly loaded transformer will often have increased gas values. This is both a function of current and temperature.
- Volume. If two transformers are otherwise equal but one of them has a higher oil volume, the same amount of gasses is more diluted. Since DGA values are measured in ppm, which is a measure of dilution, they are affected.
- Type of insulating fluid. Not every transformer oil is the same. At the same fault temperature, the ratio of decomposition products is slightly different for paraffinic and naphthenic oils, for instance.
- Inhibitors. Inhibitors are transformer oil additives used to improve the resistance to oxidation. If your transformer oil contains inhibitors, the amount of carbon monoxide (CO) is reduced. To compensate your DGA interpretation, you should mentally increase the number for CO.
- Passivators. Passivators are transformer oil additives used to decrease interactions between the transformer oil and metal surfaces. Passivators can increase gas values, especially hydrogen (H2). So you should mentally decrease your H2 number to compensate if your transformer oil contains passivators.
Comparison to similar transformers
Comparison to similar transformers
If you have multiple similar transformers in the field, you can use them to find outliers. It is unlikely that all of them are faulty, and it's even more unlikely that all of them developed a fault at the same time. Therefore you can lean back as long as all of them exhibit similar DGA behavior, even if the numbers are increased.
Conclusion
The interpretation of DGA values is a lot of work. Multiple aspects have to be taken into account, and different explanations for measured values have to be considered. But when you do, you are rewarded with new insight into your asset.
