In transformer diagnostics, stray gassing is a mysterious and often misunderstood issue. Dissolved gas analysis was pioneered by giants such as Rogers and Duval, the former who used to work for my father in the 80s/90s. This essentially statistical analysis of faults compared to gases generated produced observable and repeatable patterns between the gases termed ratios. This allows confident prediction of faults with amazing accuracy. These are some of the most commonly used techniques worldwide in laboratories to diagnose dissolved gas analysis data on transformer samples. Based on dissolved gas analysis data you could safely identify a problem, plan for treating it and open up the transformer and often observe the evidence from the gases produced. This means the very expensive invasive view inside the transformer could be avoided unless absolutely necessary. If you imagine the electrical engineer as a surgeon, you only want to cut open the patient if there is a known reason such as a mass on the x ray etc. You always need something to identify the problem rather than just cutting open everyone.

“There were some findings that just couldn’t be unexplained “

However, there were some unexplained cases that just didn’t fit the pattern. There were some strange findings that just couldn’t be explained. Transformer oil samples with clear evidence of fault gases, but when the transformer is opened up nothing is wrong. In our surgeon analogy, imagine having blood test results and x rays confirming cancer and then opening up the patient to find only healthy tissue. It must have been very confusing for these early pioneers and they terms these strange cases “stray gassing”. This post will dive deep into the chemistry of stray gassing, exploring the history of its discovery, why it occurs, what it signifies, and how you can best navigate this perplexing issue in transformer management.

What is Stray Gassing?

In transformers, dissolved gas analysis (DGA) is the go-to method for assessing the condition of insulation oils. Gases such as hydrogen, methane, ethylene, and acetylene can indicate different types of faults when present in excess quantities. But occasionally, gases like hydrogen, ethylene and ethane are detected at elevated levels without any corresponding fault in the transformer. This is what we call stray gassing.

Stray gassing refers to the formation of dissolved gases in the transformer’s insulating oil under operating conditions, but without any actual electrical or thermal fault being present. This anomaly is usually seen at lower temperatures (below 200°C), where chemical reactions still take place, but not due to arcing, overheating, or other typical fault conditions.

When was stray gassing first discovered?

The concept of stray gassing in transformers was first observed in the 1970s. At that time, transformer operators and engineers noticed unexpected gas formations during dissolved gas analysis (DGA), despite no apparent signs of transformer faults. As transformer monitoring techniques improved, particularly through the use of DGA as a diagnostic tool, it became clear that gases like hydrogen, methane, ethylene and ethane could sometimes accumulate in the oil without any associated thermal or electrical fault.

The phenomenon initially puzzled engineers and researchers, as the gases typically indicated fault conditions such as overheating, partial discharges, or arcing. However, in these instances, the transformers were functioning normally with no evidence of internal failure. This led to the coining of the term stray gassing to describe these occurrences, where gas formation happens under lower-temperature conditions without fault, often due to the oil’s chemistry and operating conditions rather than actual transformer defects.

Over the years, research has delved into the mechanisms behind stray gassing, identifying factors like oxidation, catalytic reactions (particularly involving copper), and the influence of oil inhibitors. As the understanding of transformer oil degradation processes improved, the recognition and management of stray gassing became a critical part of routine transformer maintenance and oil analysis. In fact Duval corrected his well known triangle for a pentagon to factor for this very issue.

Since its discovery, stray gassing has been a topic of ongoing research in the field of transformer diagnostics, helping engineers and operators better distinguish between benign gas formation and more serious fault conditions.

The Chemistry of Stray Gassing

To understand stray gassing fully, we need to dig into the molecular interactions within the transformer oil. The main components of transformer oil are hydrocarbons—chains of hydrogen and carbon atoms. When these hydrocarbons are subjected to heat, oxygen, and electric stress, they can break down into smaller molecules, giving rise to dissolved gases.

Hydrocarbon Structure

The typical insulating oils used in transformers are mineral oils composed mainly of three types of hydrocarbons:

1. Paraffins (alkanes) – Straight or branched chains of carbon and hydrogen.
2. Naphthenes (cycloalkanes) – Ring-structured hydrocarbons.
3. Aromatics – Hydrocarbons that contain conjugated double bonds in a ring structure, like benzene.

These hydrocarbon chains are susceptible to cracking, especially under the influence of heat and electrical stress. The longer the chain, the more prone it is to breakage and equally depending on the bonding patterns eg double bonds the more likely too.

Before we explain stray gassing (which is when there is no fault) we need to understand the gases formed when there is a fault first. So let’s recap these.

Key Gases and Their Origins

When hydrocarbons break down in transformer oil, the gases that form can indicate specific processes:

• Hydrogen (H₂): Usually associated with low-energy partial discharges or corona effects.
• Methane (CH₄): Results from the breakdown of simple hydrocarbons under moderate thermal stress (150-300°C).
• Ethylene (C₂H₄): Often forms at higher temperatures (300-700°C), linked to moderate thermal faults.
• Ethane (C₂H₆): Commonly seen at lower temperatures, indicating less severe thermal degradation.
• Acetylene (C₂H₂): A key indicator of electrical arcing (temperatures > 700°C).

In stray gassing, the gases of concern are usually hydrogen, methane, ethylene and ethane, but I am yet to see acetylene formed this way (though research may make this statement incorrect in the future).

Why Does Stray Gassing Occur?

Unlike fault-driven gas generation, stray gassing does not stem from catastrophic thermal or electrical breakdowns. Instead, it is driven by more subtle chemical processes in the oil. If any of the electrical engineers have read any of my posts on topics like varnishing in lube oils you will be aware there is a lot of overlap in these topics. Unfortunately electrical and mechanical engineer research bodies don’t in my experience tend to share between each other and operate in silos. Explaining this phenomenon would have been much earlier if there had been discussions between the lube oil and electrical oil industries. I will explain further now as several factors can contribute to stray gassing:

1. Oxidation Reactions

One of the leading theories behind stray gassing is that it results from slow, low-energy oxidation of the hydrocarbon molecules in the oil. In the presence of oxygen, hydrocarbons undergo autoxidation, where free radicals are formed and react with oxygen to break the hydrocarbon chains, producing small gas molecules. This reaction occurs even at moderate operating temperatures, particularly in the range of 90°C to 120°C.

In these reactions, paraffins and naphthenes are especially prone to producing gases like hydrogen, methane and ethane. In natural esters ethane is quite common linked to its linolenic content. In most cases initiation phase of oxidation forms alkyl peroxides, which further degrade into volatile products like methane, ethane and sometimes ethylene. Equally hydrogen free radicals occur by oxidative stresses cleaving hydrogens from the molecules.

2. Catalysis by Copper

Anyone who has seen my posts on RPVOT knows that copper is a great oxidation catalyst and your transformer is basically a large scale RPVOT cup full of copper and oil. Copper components inside transformers can act as catalysts, accelerating the breakdown of oil. This catalytic action can promote gas formation even in the absence of high temperatures or fault conditions. The copper surface encourages hydrocarbon degradation reactions, which lead to the generation of gases, particularly at lower temperatures as catalysts by definition lower the energy threshold of the reaction. Interestingly, copper oxides or other metal oxides present on transformer components can enhance these reactions.

3. Effect of Oil Inhibitors

Many transformer oils are treated with inhibitors like butylated hydroxytoluene (BHT) to prevent oxidation. However, under certain conditions, the breakdown of these inhibitors themselves can contribute to stray gassing. If the inhibitor concentration drops below optimal levels, the remaining hydrocarbons in the oil may become more susceptible to oxidation and breakdown.

4. Thermal Cycling

Another common cause of stray gassing is thermal cycling, where the transformer is subjected to repeated heating and cooling over its operating life. This cycling can cause minor breakdowns in the oil molecules without causing any obvious or severe fault in the transformer. Over time, these cycles lead to the accumulation of gases, without creating enough heat to burn off the gas or cause a more serious issue.

How to Distinguish Stray Gassing from Fault Conditions?

You want your diagnostic test to be as accurate as possible so you simply don’t want scenarios like seen with stray gases where they can be mistaken for catastrophic faults. Since stray gassing can produce gases commonly associated with fault conditions, differentiating between the two is critical to ensuring the proper management of transformer health. Here are some key approaches to make that distinction:

1. Gas Ratios

DGA relies on various gas ratios to differentiate between thermal faults, electrical faults, and stray gassing. For example:

• Ethylene/Ethane Ratio: In fault conditions, ethylene is typically much higher than ethane, especially in thermal faults at high temperatures. In stray gassing, the ratio tends to be much lower.
• Hydrogen and Methane: Stray gassing produces only small amounts of hydrogen and methane compared to what would be expected in a fault situation and although can get to quite high numbers (especially for hydrogen) the rate tends to be fairly constant rather than in faults where the trend will be rising possibly exponentially in serious faults.

Comparing the specific gas concentrations, trends in gas production rate and ratios can help pinpoint whether stray gassing or a fault is occurring.

2. Temperature Correlation

Stray gassing generally occurs at temperatures below 150°C, while gas generation from faults typically begins at much higher temperatures. If the transformer has a stable operating temperature and no evidence of hotspots with all points below 150°C but still shows elevated gases, it’s more likely due to stray gassing than a fault. This doesn’t rule out temporary hotspots but confirmation tests such as thermography can help with this non-invasively.

3. Time and Stability

Stray gassing is often a slow and consistent process. If gas levels remain relatively stable and only slowly rise over time and do not increase significantly, it could indicate stray gassing rather than an escalating fault condition.

4. Investigating Transformer Load History

Reviewing the transformer’s load and thermal history is critical in distinguishing between stray gassing and genuine faults. Load fluctuations or seasonal temperature changes that don’t correspond with significant shifts in gas levels can point towards stray gassing. For instance hydrogen is generated after energisation or re-energisation which is normal too and so using this information in context with the DGA data can help inform the relevance of eg a sudden hydrogen rise.

Management and Mitigation of Stray Gassing

So, what should you do if you detect stray gassing in your transformer? After all when you get a certain concentration of fault gases these can become a flammable situation even without a fault causing them.

1. Continuous Monitoring

If stray gassing is suspected, implementing more frequent DGA tests can help you track the trend of gas levels. If gas levels remain steady and are below concerning thresholds, the transformer may continue operating without intervention. However, if gas levels begin to rise or additional gases such as acetylene appear, it may indicate an evolving fault, necessitating further investigation. Further to this very high long term gas levels should trigger a flash point analysis to be performed on the oil to establish if the gases are high enough to be explosive.

2. Oil Filtration and Reconditioning

Oxidation is one of the primary causes of stray gassing, so improving the oil’s resistance to oxidation can mitigate the issue. Periodic filtration and reconditioning of the transformer oil, including replenishing inhibitors, can help slow down gas formation. You may be interested to use our SAIL analysis for soluble and insoluble lacquers to determine how much as a percent each fraction of the oil is in its life so that you can identify not only varnish as a source of overheating as it’s an insulator, but also to determine the risk for stray gassing as the oil degrades.

3. Temperature Control

Managing the transformer’s operating temperature can also help reduce stray gassing. If stray gassing is attributed to thermal cycling or excessive heating, optimising the cooling system or adjusting the load may minimise gas generation.

4. Routine Transformer Inspection

Inspecting internal components for corrosion, oxidation, or oil degradation can reveal potential causes of stray gassing. Any signs of copper corrosion or excessive varnishing on internal parts might indicate catalysed reactions that contribute to gas generation. Correcting these issues early can help reduce stray gassing and extend the transformer’s operational life. These usually will be done on a major overhaul but if you have to do anything invasive for any reason always use your eyes to look for these issues even if working on a different issue.

Conclusion

Stray gassing in transformers presents a unique diagnostic challenge. While it can be alarming to see elevated gas levels, it’s essential to differentiate between benign stray gassing and fault-related gas generation. By understanding the chemistry of the oil, the role of oxidation, catalysis, and thermal effects, you can make more informed decisions about transformer maintenance.

Regular DGA testing, combined with a thorough understanding of the gases involved, is your best tool for distinguishing between fault conditions and stray gassing. If you’re dealing with unexplained gas levels in your transformer, don’t hesitate to reach out to our lab for a detailed analysis. Press the ‘Contact Us’ button to discuss how we can support your transformer monitoring and testing needs!