Richard Huber, P.E.
Richard Huber Engineering, Ltd.
The intent of this article is to provide an overview of the more common techniques used for dissolved gas analysis (DGA) of mineral oil.
Types of gases created during transformer faults
There are several key gases produced during transformer faults. The type of gas and the quantity depends on the materials involved in the fault, the energy dissipated at the fault location and the solubility of the gases in oil. The most common gases found in mineral oil filled transformers are:
- Hydrogen H2
- Methane CH4
- Ethane C2H6
- Ethylene C2H4
- Acetylene C2H2
- Carbon Dioxide CO2
- Carbon Monoxide CO
Other less commonly reported gases are:
- Propane C3H8
- Propylene C3H6
The latter two gases are produced at low temperatures and with an increase in heating will produce the more commonly reported methane, ethane, and ethylene.
The more common gases and the conditions under which they are produced are shown in Table 1. Primary products are shown in bold/blue type.
The gases normally encountered as a transformer ages are hydrogen (H2), methane (CH4), ethane (C2H6), carbon dioxide (CO2) and carbon monoxide (CO). In addition, the decomposition of the paper will produce water (H2O). Some of these same gases are produced under fault conditions, but in larger quantities. The general fault categories are corona, low temperature heating (<300°C), high temperature heating (300<T<700°C) and arcing.
Corona in oil will generate large amounts of hydrogen gas (H2) and perhaps small amounts of methane (CH4). The methane could also be part of the normal mixture of dissolved gasses in the transformer. It is produced at fairly low temperature and may be part of the normal aging process for a particular transformer. If paper is involved in the corona activity, carbon monoxide (CO) and carbon dioxide (CO2) will also be produced in approximately equal amounts.
Heating below approximately 300°C will generate significant amounts of methane (CH4), ethane (C2H6) and if paper is involved some carbon dioxide (CO2) with lesser amounts of carbon monoxide (CO). There may also be a small amount of hydrogen that is part of the normal ageing process of insulating oil. If the fault temperature increases above 300°C, ethylene (C2H4) will become the dominant gas along with hydrogen (H2) and lesser amounts of methane (CH4) and ethane (C2H6). At these temperatures, carbon monoxide (CO) will be the predominant gas generated from the paper with lesser amounts of carbon dioxide (CO2). Fault temperatures above 700°C will create high quantities of acetylene (C2H2) and some hydrogen (H2). This type of fault usually involves electrical arcing. Paper is often not involved as it either was not near the fault to start with or it was already disintegrated in the region of the fault. If paper is involved, the main gas generated from this material will be carbon monoxide (CO).
Methods of analysis and interpretation
There are many methods used to analyze the significance of the various gases and their concentrations. Some of the more common techniques are:
- Dornenburg ratios
- Rogers ratios
- Key gas method
- Gas limits
- Duval triangle
Dornenburg ratios
One of the earliest methods of analysis was developed by Dornenburg who initially used the two ratios CH4/H2 and C2H2/C2H4. This analysis technique was later expanded to use the two additional ratios C2H2/CH4 and C2H6/C2H2. Using this method the analyst can distinguish three different types of faults: corona, thermal deterioration and arcing.
Rogers ratios
Rogers expanded on the work of Dornenburg by incorporating the effect that fault temperature has on the generation of fault gases. The ratios used were: CH4/H2, C2H6/CH4, C2H4/C2H6 and C2H2/C2H4. Today the method has been modified as it appears in IEEE C57-104 and IEC 60559 where diagnostic charts can be found.
Key gas method
This method uses selected gases and their relative concentrations to allow the user to quickly identify the possible fault within the transformer. The gases used in this method are hydrogen (H2), ethylene (C2H4), carbon monoxide (CO), and acetylene (C2H2). The general diagnosis based on these gases is as follows:
- H2 – Low energy discharges, corona.
- C2H4 - Thermal decomposition of the oil
- CO – Thermal decomposition of cellulose
- C2H2 – Electrical arcing.
This method, although used frequently, is not very accurate and can lead to misdiagnosis of the problem.
Gas limits
Another method that can be used to analyze gases contained in insulating oils is the use of concentration limits for individual gases and total dissolved combustible gases (TDCG). Similar to the key gas method, this method is usually used to first alert an operator of a potential problem. The limits for each gas and for TDCG are often set by knowledgeable personnel familiar with the equipment in their system. Hence the literature contains many different values for the gas concentration limits. Examples are shown in Table 2.
The significance of these values has to be tempered by the age of the transformer, the volume of oil involved and the type of transformer, in particular whether it is sealed or free breathing.
Duval triangle
This method was first developed in 1974 by Michel Duval. It is now a part of IEC 60599 and is one of the most accurate methods of analyzing dissolved gases in insulating oil. The Duval triangle method uses the concentrations of three gases, CH4, C2H4 and C2H2. The percentage of each relative to the sum of the concentration of all three gases is plotted along the sides of a triangle as shown in Figure 1.
The regions shown in the triangle correspond to the conditions shown in Table 3. The location of the boundaries is based on gas analysis and inspections over a 60-year period.
Analyzing other gases
The thermal decomposition of cellulose produces carbon dioxide (CO2), carbon monoxide (CO) and water (H2O). The ratio of the two gases CO2/CO changes as the temperature of the fault increases. At low temperature (<150°C), the generation of CO2 dominates and the ratio of the two gases will generally be >10. At higher temperatures above 250°C the generation of CO dominates reducing the ratio to <3. Because of the generally large concentration of CO2 and CO in transformer oil under normal circumstances, the use of these gases as an indicator of a fault should be done with care.
Oxygen (O2) and nitrogen (N) are often found as part of the dissolved gases in transformer oil. They are generally present if the transformer is free breathing. If the transformer is sealed or nitrogen blanketed, there should be virtually no oxygen dissolved in the oil. In sealed units, the ratio of oxygen to nitrogen approaching that of air would indicate a leak. Another source of the oxygen and nitrogen can be contamination of the oil sample. This would indicate improper sampling, transporting or lab testing techniques.
Factors affecting gas concentrations and diagnosis
Some of the factors that affect the concentration of gas in transformer oil have been mentioned in the preceding paragraphs. To summarize, they are: fault severity, gas solubility, and oil temperature.
The type of transformer is also important. Core type versus shell type affects gas generation rates. Shell types are often hotter hence generate more gas. Also sealed versus free breathing transformers affect the gases present in the oil. Sealed transformers should have little or no oxygen or water from outside sources. If these items do exist they may indicate that the transformer has a leak.
There is also the possibility of leaks from the tap changer compartment contaminating the oil in the main tank. This would show up as higher than normal arcing byproducts, that is, hydrogen, acetylene, ethane and ethylene. Contaminants from gaskets, paper and oil processing equipment can also affect gas concentrations. At times one finds volatile materials used in the fabrication of the gasket material or the paper can become dissolved in the oil and affect DGA results.
Laboratory test accuracy can affect the diagnosis. If the test results have a variation greater than +/-15%, then the diagnosis will likely be inaccurate.
Summary
Over the years the methodology for dissolved gas analysis (DGA) of mineral oil has become increasingly more sophisticated, but all methods benefit from the input of a knowledgeable specialist. A combination of the methods cited should be a vital part of all maintenance programs that deal with aging oil filled transformers.
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