Transformer oil degradation & aging diagnostics (DGA, moisture, acidity, etc.)
In a power grid, transformers serve a huge function, metaphorically speaking, as keystones in which a central part of the whole electrical system interlinks, while for their functionality, transformer oil is the component that sustains the system. Over time, transformer oil, like everything else, will deteriorate, compromising its insulation and cooling capabilities and causing a transformer failure. This is a para-disaster situation not just for the grid but for the whole power system.
In the case of transformer oil, the use of diagnostic indicators that look for Dissolved Gas Analysis (DGA), moisture, and acidity, and other particulars like tiering, overheating, or moisture invasion, helps a great deal in ascertaining the moisture inside the transformer. While it may seem challenging, making use of diagnostic indicators makes it much easier, enabling proactive planning, which can save a company a lot of money in case the transformers break.
The aim of this article is analyze analyze condition monitoring to describe it in the most simplified manner. You will be delivered the understanding of how to assess risk and mitigate it through predicting and using accurate figures and answers to rough transformer operation, using the parameters such as DGA and moisture, to describe a safer level of transformer operation.
Why is Transformer Oil Diagnosis So Important?
The stability as well as safety of electrical equipment depends directly on the performance of transformer oil. So, how do you assess the status of the health of transformer oil? If the oil is “aging”, what exactly is the issue? What actions do you take?
Key Issues That Arise from Degraded Transformer Oil:
- Deterioration of the Oil Breakdown: The primary function of transformer oil is insulation. As oil loses its insulating properties, the potential for electric arcing, short circuits, and ignition of fires greatly increases.
- Overheating: The oil, which is supposed to dissipate the heat, no longer performs its function, so the internal temperature begins to rise. The result of this is that efficiency, as well as the life span of the transformer, is greatly diminished. However, the greatest problem stems from the fact that failure of the transformer is now a real possibility.
- Moisture Infiltration: Moisture present in the oil poses a major hazard to the functioning of the transformer because oil, which is supposed to act as an insulator and is now protective of internal equipment and the transformer casing, is a major concern as it leads to the loss of oil.
- Increased Maintenance Costs: The effect of oil degradation, which is pollution in oil mechanical engineering, leads to an increase in expenses as well as unproductiveness.
With the oil diagnostic, it will be possible to shed light on questions like, How healthy is my transformer? And if it’s not healthy, what is the problem? What is the level of severity, and what are the possible next steps?
Transformer Oil Degradation / Aging Diagnostics Core Techniques
Understanding the oil diagnostic parameters while conducting transformer maintenance is crucial for their maintenance process. Hence, each oil diagnostic parameter configuration, along with each letter, is explained below. This is aimed at assisting in the geometric interpretation of oil degradation and possible malfunctions.
1. How to Interpret DGA, Acidity, Moisture, and Other Data?
Dissolved Gas Analysis (DGA) The “Microscope” for Transformer Faults
DGA analyzes gases dissolved in transformer oil, like hydrogen, methane, ethylene, and acetylene, for internal transformer fault diagnosis. Each gas increases in concentration to show a different fault and its severity.
- Hydrogen(H₂) discharge: Relates to either partial discharge or arc discharge. Concentration ranging between 50 to 200 ppm demonstrates severe internal discharge issues, in estimation.
- Methane (CH₄): Indicate overheating, especially at a temperature of 300 °C or higher. A concentration of 100 ppm shows methane to be a significant overheating issue.
- Ethylene (C₂H₄): Overheating gas is most common with Ethylene and is especially associated when the temperature is higher than 150 °C. Concentration less than 50 ppm provides a warning sign.
- Acetylene (C₂H₂): Used to indicate arc discharge, a sign of discharge, especially when the concentration is higher than 1 ppm, requires immediate shutdown and inspection.
Important Gas Thresholds (According to IEC 60599):
- Hydrogen > 50 ppm: Partial discharge potential
- Ethylene > 50 ppm: Overheating potential
- Acetylene > 1 ppm: Arc discharge risk (urgent inspection required)
Moisture Content – The Primary Killer of Insulation Systems
Moisture in transformer oil is one of the most detrimental factors in the degradation of oil due to the insulation performance of oil and water mixtures. Water content should ideally be kept to below 0.05%, as anything higher poses a significant risk to insulation performance:
- > 0.1%: High moisture content, affecting insulation and causing partial discharge.
- > 0.3%: Critical moisture levels that require immediate dewatering treatment to avoid equipment failure.
Acid Value – The “Barometer” of Oil Aging
Oxidation of oil contributes to aging and increasing acid value. High acidity levels can corrode internal oil transformer parts and diminish oil effectiveness. The typical acid value of transformer oil is less than 0.01 mg KOH/g.
- > 0.1 mg KOH/g: Significant aging, with reduced insulation properties.
- > 0.2 mg KOH/g: Severe aging, requiring oil replacement or purification.
Other Diagnostic Parameters:
- Soluble Water: Monitor dissolved moisture levels; excess water can decrease insulation performance.
- Viscosity: Increased viscosity due to oil aging can impair cooling efficiency.
- Flashpoint: A reduced flashpoint indicates degraded oil, posing a fire hazard
2. What information do these data trends reveal about internal transformer faults?
Examining these specific indicators of diagnosis individually enables one to hone in on specific activities, which in turn lets one gather extra information about the inner workings of the transformer.
- Discharge or Arc: An excess volume of acetylene means that some form of arc discharge is taking place inside the transformer.
- Overheating: The presence of ethylene or methane in the oil indicates oil that has been thermally degraded, an indication that the transformer is possibly overheating, running under optimal conditions, or there is a problem with the cooling.
- Leaks: An excess volume of moisture means that there is some form of a leak or defective seal, which is allowing the oil to absorb moisture, and thus its insulating efficiency is reduced.
What thresholds should be reached in oil quality data before action is taken?
Oil quality diagnostic data are crucial in defining the health condition of the transformer. Below are common oil quality parameters with their limit values to assist you in determining when intervention is warranted.
Dissolved Gas Analysis (DGA)
Dissolved Gas Analysis is the quickest method of diagnosing internal faults of transformers, because it is the most ‘hands-on’ approach. For every gas concentration change, there is a shift that follows.
| Gas Type | Concentration Threshold | Possible Causes | Solutions |
| Hydrogen (H₂) | > 50 ppm | Partial discharge | Regularly inspect the equipment. If hydrogen concentration exceeds 200 ppm, stop and check. |
| Methane (CH₄) | > 100 ppm | Transformer overheating (>300°C) | Check the cooling system, reduce load or perform oil cooling, and consider oil replacement if it continues. |
| Ethylene (C₂H₄) | > 50 ppm | Transformer overheating (>150°C) | Stop and check, enhance cooling measures, and consider replacing the oil. |
| Acetylene (C₂H₂) | > 1 ppm | Arc discharge | Immediately shut down for repairs, check for severe internal faults to avoid further damage. |
Moisture Content
Excessive moisture content greatly diminishes the insulating properties of the oil, and beyond a certain amount, moisture can incite partial discharges and elevate the probability of malfunction.
| Moisture Content | Possible Causes | Solutions |
| > 0.05% | Affects oil’s insulating properties | Increase monitoring frequency, closely track moisture changes, and consider dehydration treatment when moisture content exceeds 0.1%. |
| > 0.1% | Partial discharge or insulation failure | Start dehydration, reduce load, shut down for repairs, and consider oil replacement if necessary. |
| > 0.3% | Severe degradation of oil insulation properties | Immediately shut down, perform dehydration, and replace oil, inspecting for damage to equipment. |
Acid Value
The acid value indicates the extent of deterioration of the oil. Not a good oil. Not a good oil. Not a good oil. Not a good oil. Not a good oil. Not a good oil.
| Acid Value | Possible Causes | Solutions |
| > 0.05 mg KOH/g | Increased oil oxidation | Regularly monitor acid value changes. If it exceeds 0.1 mg KOH/g, consider purification treatment. |
| > 0.1 mg KOH/g | Severe oxidation, insulation deterioration | Replace the oil or perform purification to avoid affecting transformer operation. |
| > 0.2 mg KOH/g | Severe oil aging, significant insulation deterioration | Immediately replace the oil, conduct detailed repairs, and assess the transformer’s health status. |
What are the most economical and effective solutions for different types of Transformer Oil Aging faults?
Considering the types of faults present in transformer oil, for example, partial discharge, overheating, arc discharge, etc, separate approaches can be taken to deal with the problems. Given below are some of the typical fault types along with the appropriate, economical, and feasible solutions.
Partial Discharge Fault
An example of this would be discharges within an electric transformer, where partial discharges may come before full arc discharges, compromising the integrity of the insulation and leading to more serious complications.
| Symptoms | Possible Causes | Economical and Effective Solutions |
| Increased hydrogen (H₂) concentration, increased moisture content, higher acid value | Partial discharge occurring inside the transformer | Initial: Monitor DGA data and check hydrogen concentration regularly. |
| Increased hydrogen (H₂) concentration, increased moisture content, and higher acid value | ||
| Long-term: If partial discharge is not controlled, replace the oil and conduct a full inspection. |
Overheating Fault
Overheating has been noted as a problem affecting transformer functionality due to active overload, failure of the cooling system, or the presence of aged transformer oil.
| ymptoms | Possible Causes | Economical and Effective Solutions |
| Increased methane (CH₄) and ethylene (C₂H₄) concentrations, increased moisture content, and higher acid value | Transformer overheating (possible overload or cooling system failure) | Initial: Reduce load and optimize the cooling system. |
| Mid-term: Check the cooling system, clean or replace cooling equipment. | ||
| Long-term: If overheating persists, consider oil replacement and conduct in-depth fault diagnostics. |
Arc Discharge Fault
Arc discharge usually indicates a serious fault inside the transformer, potentially damaging metal parts and insulating materials.
| Symptoms | Possible Causes | Economical and Effective Solutions |
| Acetylene (C₂H₂) concentration exceeds 1 ppm, sharp increase in acid value, possibly accompanied by abnormal temperature | Arc discharge, often indicating a serious fault | Initial: If acetylene concentration exceeds 1 ppm, immediately shut down and inspect, check for arc discharge sources. |
| Mid-term: If the arc discharge is not controlled, replace the oil and perform an internal inspection. | ||
| Long-term: If necessary, perform thorough repairs, inspect for damaged insulation materials or metal parts. |
Oil Aging Fault
As the oil ages, its performance gradually deteriorates, manifested as increased viscosity, lower flashpoint, higher acid value, and increased moisture content.
| Symptoms | Possible Causes | Economical and Effective Solutions |
| Increased oil viscosity, lowered flashpoint, higher acid value, increased moisture content | Oil aging, contamination, oxidation, or water infiltration | Initial: Purify the oil through filtration and dehydration to restore oil performance. |
| Mid-term: Regularly replace the oil and control the aging speed. | ||
| Long-term: Establish an oil health index to predict oil aging trends and reduce unplanned shutdowns. |
How to predict how long the transformer can continue to operate safely based on current oil aging trends?
To find how much longer transformer equipment could function at a safe level, data from DGA, moisture, acid value, etc., needs to be assessed. This data could then be used to estimate how much longer the oil could last, potentially. The following are the steps you would take to make this estimation.
- DGA gases, acid value, and moisture level data are captured and compared to historical data sets to predict the oil aging value. More specifically, if between 20-25% of ethane gas is captured and methane is captured at 10% concentration, then an analysis to observe the rate at which the levels are increasing is documented. If it is determined that the methane and ethylene gases per month increase to 10% over the rate documented before 3 months, it is assumed the oil is aging much faster.
- Use the Arrhenius model as a starting point to calculate how much more oil is left in the equipment. An estimation could be made on the life of the oil determined from the acid value of 3 to 5 years.
- Any moisture value of oil above 0.1% denotes the rate at which aging is taking place as well as how much the insulation is degrading. If moisture level checks are conducted, then risks could be greatly minimized.
- When the equipment is used and analyzed, temperature and factors like load are considered, and then derivative evaluations made from regular oil records determine the value of oil that is useful conclusively. This will provide you with a more accurate estimate of the oil’s useful life left.
How to establish an early warning system to avoid unplanned shutdowns or catastrophic failures?
The result of failing to put advanced notification systems in place could result in an avoidable system breakdown or huge loss, which is far more loss than what the system costs. The case in point is the following set of recommendations:
- Data and Activity Controlled Systems
- Benchmark Standards and Determination of Limit Analytics
- Self-Repeating Action Functionality
- Routine Strategic Maintenance and Assessment
Conclusion
Diagnostic analysis of transformer oil, such as DGA, moisture, gas content, dissolved water, acetylene, moisture, sores, and these paint a rich picture of the condition of the transformer. The ability to recognize and understand ‘warning signs’ of a condition, and establish a condition-based operational trigger that can determine the remaining useful life of the system (RUL) is a necessity. It can save a transformer from operating under stressful and failure-induced conditions. It would be deep the way infrastructure, and best management practices put the reliance, take, and avoid the risk condition, as well as the time based operational trigger formulated, of adherence put operational adherence to the best known practices, of and the best gripped practices mulled, just right by the spent safety and operational efficient practices, and saved the years operational of spent spent procedures years active.
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