An oil-filled transformer insulation system consists of insulating oil and cellulose (paper) materials. Under normal use, transformer insulation deteriorates and generates certain combustible and noncombustible gases. This effect becomes more pronounced when the transformer insulation is exposed to higher temperatures.
When cellulose insulation (i.e., winding insulation) is overheated to temperatures as low as 140°C, carbon monoxide (CO), carbon dioxide (CO2), and some hydrogen (H) or methane (CH4) are liberated. The rate at which these gases are liberated depends exponentially on the temperature and directly on the volume of the insulation at that temperature.
When insulating oil is overheated to temperatures up to 500°C, ethylene (C2H4), ethane (C2H6), and methane (CH4) are liberated. When oil is heated to extreme temperatures, such as an electrical arc, hydrogen (H) and acetylene (C2H4) are liberated in addition to the above-mentioned gases.
The main cause of gas formation in a transformer is due to the heating of paper and oil insulation and electrical problems inside the transformer tank. The electrical problems can be low-energy phenomena, such as corona, or high-energy phenomena, such as an electrical arc.
The detection, analysis, and identification of these gases can be very helpful in determining the condition of the transformer. Establishing baseline data as a reference point for new transformers and then comparing it with future routine maintenance test results is a key element in the application of this test method.
However, monitoring the condition of a transformer using this method can start at any time even if the reference data is not available. There are two methods for detecting these gases:
- dissolved gas analysis (DGA), and
- total combustible gas analysis (TCGA).
DGA Analysis of Transformer Oil
Dissolved gas analysis of transformer oil (DGA) is basically a laboratory test. It is used to determine the content of gases in the oil. In this test, an oil sample taken from the transformer is subjected to a vacuum to remove the combustible gases. These gases are then passed through a Gas Chromatograph for measurement and analysis of each component. The quantity of each gas is given in part per million (ppm) or percent of the total gas present. The analysis of each gas present provides a useful tool in determining the condition of the transformer.
Total Combustible Gas Analysis (TCGA)
TCG can be determined in the field or analyzed in the laboratory from a sample of gas drawn from the gas space above the oil. The equipment used for measuring TCG is a Wheatstone bridge circuit. A combination of air and the combustible gas sample is passed over a resistor where catalytic burning takes place on the resistor, which causes a proportional change in resistance. Based on the change in resistance of the resistor, the TCG is measured in percent.
Comparing the Two Methods
Total fault gas analysis (TCGA) is probably the most widely used in the field. Its major advantages are that it is quick and can be used in the field or may be used to continuously monitor the transformer.
Its disadvantages are that it provides only a single value of oil combustible gases and does not identify or quantify what gases are present, and detect gases that are available freely in the free space above the oil.
As shown in Table 1, some of these gases are very soluble in oil and may not be liberated to the free space until the oil is fully saturated with the gas. Also, the solubility of these gases varies with temperature and pressure.
The DGA is the most informative method of detecting combustible gases. Although this is a laboratory method, it provides the earliest possible detection of any abnormal conditions in the transformer. Since the diffusion of gases from liquid to gaseous space takes time, the TCGA method is not as sensitive as the DGA method and serious equipment damage could occur undetected if only the TCGA method is employed in assessing the serviceability of the transformer.
Interpretation of Gas Analysis
As discussed earlier, the decomposition of paper insulation produces CO, CO2, and water vapor at temperatures much lower than that for the decomposition of oil. This is because the paper begins to degrade at lower temperatures than the oil and its gaseous byproducts are found at normal operating temperatures in the transformer.
A transformer that operates at or near its nameplate rating normally generates several hundred ppm of CO and several thousand ppm of CO2 without excessive hot spots. The decomposition of oil at temperatures from 150°C to 500°C produces large quantities of hydrogen and methane, and trace quantities of ethylene and ethane.
As the oil temperature increases to modest temperatures, more hydrogen gas is liberated than methane, higher quantities of ethane, and ethylene. As the temperature increases further, increasing quantities of hydrogen and ethylene are produced.
Low-level intermittent arcing and partial discharges (corona) produce mainly hydrogen, with small quantities of methane and trace quantities of acetylene. The quantities of acetylene become pronounced only when high-intensity arcing (700°C–1800°C) occurs inside the transformer tank.
The success of fault gas analysis is based on detecting the combustible gases at the earliest possible time and then taking steps to correct the problem. The Institute of Electrical and Electronic Engineers (IEEE) Standard C57.104-1991, “IEEE guide for the interpretation of gases generated in oil-immersed transformers,” suggests the following procedure for the detection and analysis of combustible gases.
- Direct measurement of the amount of TCG in the gas space above the oil and the rate of generation of these gases.
- Direct measurement of the amount of combustible gases dissolved in the oil (gas-in-oil) and the rate of generation of these gases.
- Gas chromatographic separation and analysis of individual gases and the rate of generation of each gas.
Assessing the Transformer Condition Using TCGA
A new transformer should be tested within a week after putting it in service. If it is not gassing and does not start gassing, subsequent tests should be made progressively increasing intervals until the 12-month normal interval is reached.
When sudden increases in the combustible gas quantities or generating rates in the gas space of an operating transformer occur and the internal fault is suspected, IEEE Standard C57.104-1991 recommends the procedure to be used as shown in Table 2.
Assessing the Transformer Condition Using DGA
As stated earlier, a new transformer should be tested for combustible gases within a week after energization and continued with testing until the 12-month normal interval is reached. However, if there is no previous DGA history on the transformer, then it can be difficult to determine whether a transformer is operating normally or not.
The IEEE Standard C57.104-1991 has established a four-level criterion to classify risk to transformers, when there is no previous DGA history, for continued use at various combustible gas levels. The IEEE criterion is shown in Table 3 below, which shows the individual gases and TDGA.
Here, it is assumed that no previous tests on the transformer for DGA have been made or that no recent history exists. If a previous analysis exists, it should be reviewed to determine if the situation is stable or unstable.
An ASTM round robin indicated variability in gas analysis between laboratories. This should be considered when having gas analyses made by different laboratories.
Condition 1: TDCG below this level indicates the transformer is operating satisfactorily. Any individual combustible gas exceeding a specified level should prompt additional investigation.
Condition 2: TDCG within this range indicates a greater than normal combustible gas level. Any individual combustible gas exceeding specified levels should prompt additional investigation.
Condition 3: TDCG within this range indicates a higher level of decomposition. Any individual combustible gas exceeding specified levels should be taken to establish a trend. Faults are probably present inside the transformer. Take action as specified in Tables 2 and 4.
Condition 4: TDCG within this range indicates excessive decomposition. Continued operation could fail the transformer. Proceed immediately and with caution with actions specified in Tables 2 and 4. Faults are probably present in the transformer.
The numbers shown in Table 3 are in parts of gas per million parts of oil (ppm) volumetrically and are based on a large power transformer with several thousand gallons of oil. With a smaller oil volume, the same volume of gas will give a higher gas concentration. Small distribution transformers and voltage regulators may contain combustible gases because of the operation of internal expulsion fuses or load break switches.
The status codes in Table 3 are also not applicable to other apparatus in which load break switches operate under oil.
The TDCG value specified in the Table 3 does not include CO2, which is not a combustible gas.
Action Based on TDCG
When sudden increases in the dissolved gas quantity of the oil in a normally operating transformer is noted and an internal fault is suspected, the IEEE Standard C57.104-1991 recommends the procedure shown in Table 4. The IEEE procedure recommends the initial sampling intervals and operating procedure for various levels of TDCG and TDCG rate. An increasing gas generating rate means that the problem in the transformer may be severe and therefore a shorter sampling interval is recommended.
Fault Types and Associated Key Gases
After the DGA has been obtained, then the next step is to determine the condition of the transformer. The evaluation has been simplified by looking at key gases and the associated condition as specified in Table 5.
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