The power transformer is the pivotal device of the power system. Insulation is the major component, which plays an important role in the life expectancy of the transformer. DGA of insulating oil is universally used and considered as an important indicator of a transformer’s overall condition all over the world. Power transformer windings are most commonly insulated with multiple layers of insulating cellulosic paper and immersed in mineral insulating oil. Transformer life known to us is based on the designed parameter with respect to normal operation and climate conditions. To determine the performance and aging of the asset, insulation behaviour is a main indicator. Transformer failure may be avoided by monitoring the condition of the oil in an operational unit and, based on the results, corrective action may be taken. Transformer oil contains about 70% of diagnostic information. The variations in different oil characteristics may therefore be used to identify/detect the type of incipient failure in the transformer. Several methods are available for the interpretation of laboratory results, such as those recommended in IEC Standard 60599 and IEEE Standard C57.104-1991. Currently, there are several methods developed to do the interpretation of the fault type from the dissolved gases data. In this article, the five methods and their advantages and disadvantages of interpretation of the fault gases are studied.
Transformer Oil
Oil used for insulation in transformers is mineral oil and it is obtained by refining crude petroleum. Animal oils and vegetable oils are not used for this purpose as these form fatty acids on heating, which are corrosive for the cellulosic paper used in insulation. Mineral oils were in use as liquid dielectrics in electrical equipment for over hundred years now. Despite the availability of a variety of synthetic oils, with far more superior properties, mineral oils held its way, due to their abundant availability and economy.
Three properties that are fundamental to use of mineral oil as dielectric are:
- High insulating property,
• Good oxidative and ageing stability and good heat transferability.
Insulating Oil Quality
The condition of the oil greatly affects the performance and the service life of transformers. A combination of electrical, physical and chemical tests is performed to measure the change in the electrical properties, extent of contamination, and the degree of deterioration in the insulating oil. The results are used to establish preventive maintenance procedures, to avoid costly shutdowns and premature equipment failure, and extend the service life of the equipment. There is a multitude of tests available for insulating oil. Threshold levels for these tests are specified in ASTM D3487 for new oils and IEEE Guide 637-1985 for service oils.
Dissolved Gas Analysis (DGA)
DGA has become a very popular technique for monitoring the overall health of a transformer. By analyzing oil sample for dissolved gas content it is possible to assess the condition of the equipment and detecting faults at an early stage. Dissolved gas analysis, like a blood test or a scanner examination of the human body, can warn about an impendent problem, gives an early diagnosis, and increases the chances of finding the appropriate cure. So, this technique is a very efficient fault diagnostic technique for transformer – and lots of approaches have emerged to analyse the results. The increasing dissolved gases in DGA not only indicates the fault inside the transformer, but also points to insufficient cooling system. The gases in oil tests commonly evaluate the concentration of hydrogen (H2), methane (CH4), acetylene (C2H4), ethylene (C2H2), ethane (C2H4), carbon monoxide (CO), carbon dioxide (CO2), nitrogen (N2), and oxygen (O2).
Information from the analysis of gasses dissolved in insulating oil is one of the most valuable tools in evaluating the health of a transformer – and has become an integral part of preventive maintenance programs. Data from DGA can provide:
- warning in advance of developing faults
• monitoring of the rate of fault development
• confirmation on the presence of faults
• checks on new and repaired units
• assistance to the convenient scheduling of repairs
• allowance to the monitoring of units under temporary overload.
Operating Procedure for Dissolved Gas Analysis (DGA)
Following major steps are followed to provide the analysis using DGA:
Detection: The generation of any gas above the normal level is detected first, and then appropriate guidelines are utilized so that possible abnormality can be recognized at the earliest so that damage to the transformer could be minimized.
Evaluation: The impact of the abnormality or fault on the serviceability is evaluated by using a specified set of guidelines and recommendation.
Action: Lastly the action is recommendation, which begins with increased surveillance and confirming or supplementary analysis – and leads to either a determination of load sensitivity, reducing the loads on the transformer, or actually removing the unit from service.
The earliest possible detection of gases is required for the fault gas analysis technique to be successful. The following methods are used for detecting fault gases:
- Direct measurement of the amount of combustible gas in the gas space or relay (Total Combustible Gas (TCG)).
• Direct measurement of the amount of combustible gas dissolved in the oil (gas-in-oil monitors).
• Chromatographic separation and analysis for the individual components in a gas mixture extracted from a sample of the transformer oil or a sample of the transformer gas space.
Incipient Faults And Faults Gases
The operating principle of transformers is based on the slight albeit harmless deterioration of the insulation that accompanies incipient faults, in the form of arcs or peaks resulting from dielectric breakdown of weak or overstressed parts of the insulation, or hot spots due to abnormally high current densities in conductors. Whatever the cause, these stresses will result in the chemical breakdown of some of the oil or cellulose molecules constituting the dielectric insulation.
Gases that are produced in transformer oil can be divided into three groups as follows:
A. Hydrocarbon and hydrogen
- Methane (CH4)
• Ethane (C2H6)
• Ethylene (C2H4)
• Acetylene (C2H2)
• Hydrogen (H2)B. Carbon oxides
- Carbon monoxide (CO)
• Carbon dioxide (CO2)C. Non fault gases
- Nitrogen (N2)
• Oxygen (O2)
Gases, which are produced by the degradation of oil as a result of elevated temperatures, may be caused by several factors as Severe overloading, Lighting, Switching transients, Mechanical flaws, Chemical decomposition of oil or insulation, Overheated areas of the windings and Bad connections which have a high contact resistance.
Conditioning Monitoring Utilizing Total Dissolved Combustible Gas (Tdcg) In Oil
The previous dissolved gas analysis record is very important for determining whether the transformer is behaving normally or not. A four-level criterion has been developed to classify risks to transformers, when there is no previous dissolved gas history, for continued operation at various combustible gas levels. The criterion uses both concentration gases as shown in table 3. This table is used to make the original assessment of a gassing condition on a new or recently repaired transformer or is used if there are no previous tests on the transformer for dissolved gases or if there is no recent history.
Table.3 lists the dissolved gas concentrations for the individual gases and TDCG for conditions 1 through 4. The four conditions are described below:
Condition 1: TDCG below this level indicates the transformer is operating satisfactorily. Any individual combustible gas exceeding specified levels should prompt additional investigation.
Condition 2: TDCG within this range indicates greater than normal combustible gas level. Any individual combustible gas exceeding specified levels should prompt additional investigation. Action should be taken to establish a trend. Fault(s) may be present.
Condition 3: TDCG within this range indicates a high level of decomposition. Any individual combustible gas exceeding specified levels should prompt additional investigation. Immediate action should be taken to establish a trend. Fault(s) are probably present.
Condition 4: TDCG within this range indicates excessive decomposition. Continued operation could result in failure of the transformer. Proceed immediately and with caution. Table 3 is only suitable if no previous tests on the transformer for dissolved gas analysis have been made or that no recent history exists. If previous history of the transformer is there, then this table only determines the situation which is stable or unstable. For such type of conditions we will consider the TCG and gassing rates for providing the necessary analysis.
Methods Of Fault Gas Detection
Insulating oils under abnormal electrical or thermal stress breakdown to liberate small quantities of gases. The composition of these gases is dependent upon type of fault. By means of DGA, it is possible to distinguish fault such as partial discharge (corona), overheating, and arcing in a great variety of oil filled equipment. DGA can give early diagnosis and increase the chances of finding the appropriate cure. There are many methods in DGA. In this article, six of the more commonly used methods were studied:
Roger ratio method
The Roger’s method utilizes four gas ratios: CH4/H2, C2H6/CH4, C2H4/C2H6 and C2H2/C2H4. Diagnosis of faults is accomplished via a simple coding scheme based on ranges of the ratios as shown in tables 4(a) and 4(b) below.
The combination of the coding gives 12 different types of transformer faults. The type of faults based on the code is shown in table 5 below.
IEC ratio method
This method originated from the Roger’s Ratio method, except that the ratio C2H4/CH4 was dropped since it only indicated a limited temperature range of decomposition. Although IEC three-ratio method is widely used in transformer fault diagnosis, but because the number of code combination is larger than fault type number, no matching often occurs in the diagnosis. Here, the remaining three gas ratios have different ranges of code as compared to the Roger’s ratio method and they are shown in table 4(b). The faults are divided into nine different types as listed in table 6.
Duval Traingle Method
The Duval Triangle diagnostic method for oil-insulated transformers was first developed in 1974 by Michel Duval of Hydro Quebec’s Institute of Research (IREQ). This method has been proved to be accurate and dependable over many years and is now becoming popular for the fault diagnosis. In this method uses three hydrocarbon gases only (C2H2, C2H4 and CH4). These three gases correspond to the increasing levels of energy necessary to generate gases in transformers in service. The Triangle method is indicated in Figure 1.
Fig. 1: Duval Triangle…
Triangle Coordinates
With x= [C2H2]; y= [C2H4]; z= [CH4] in ppm
In addition to the 6 zones of individual faults mentioned in Table 8 (PD, D1, D2, T1, T2 or T3), an intermediate zone DT has been attributed to mixtures of electrical and thermal faults in the transformer.
Key gas method
This is one of the most frequently used diagnostic tools and, unfortunately, one of the weakest in our arsenal. The combination of frequent use and poor diagnostic capability unite with the result being the source of a significant number of misdiagnoses in the field. The dependence on temperature of the types of oil and cellulose decomposition gases provides the basis for the qualitative determination of fault types from the gases that are typical, or predominant, at various temperatures. These significant gases and proportions are called key gases.
The IEEE guide Key Gas Method offers diagnosis through calculating the relative proportions (in percent) of these key gases to the rest of the gases in the transformer. The proportions indicate the general fault type – and these fault types with their relative proportions of gases (in percent) are followed in figure 2.
Conclusion
As the population of transformers in service increases and their operating time extends, much attention has been focused on their availability and reliability. Most of the decay products that progressively damage the properties of oil-paper insulation in power transformers result from secondary chemical reactions between decomposed molecule under the impact of electrical, chemical and thermal stresses. In this article, a brief analysis has been made about commonly used five types of DGA methods that find use in the DGA for power transformers.
It was found that the Duval-Triangle method is the best, because, it always provides a diagnosis, with a low percentage of wrong diagnoses as compared to all methods, then Ratio methods and the Key-Gas method, because, it often provides wrong diagnoses. Those methods using specific codes in their interpretation are more accurate if they make a prediction. Further application of these techniques as a monitor during the factory proving tests of power transformers, is being developed employing very much greater detection sensitivities than used in the field.
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