Dissolved gas analysis (DGA) is the most important tool in determining transformer conditions. It is the first indicator of a transformer problem and can identify the insulating state and deteriorating oil, overheating, hot spots, partial discharge, and arcing. Dissolved gas analysis begins with sending a sample of transformer oil to a commercial laboratory for testing. The most important indicators are individual and total flammable gas (TCG) whose rate of increase is based on IEC 60599 and IEEE C 57-104 standards.
1. Understanding DGA
Definition of DGA: "Transformer condition analysis based on the amount of dissolved gas in transformer oil". DGA testing is one of the preventive maintenance steps that must be done with testing intervals at least once a year (annually). This is done by taking an oil sample from the transformer unit and then the dissolved gas is extracted to identify its individual components. The DGA test will provide information regarding the overall health and quality of the transformer work. The advantage of the DGA test is early detection of failure phenomena that exist in the transformer being tested Weaknesses of the DGA test: It requires a high level of purity of the oil sample being tested
2. Stages of Dissolved Gas Analysis
The DGA test steps begin with taking the oil sample from the transformer. Then the gas extraction is carried out to determine the dissolved gas content in the oil. Furthermore, the gas content data that has been obtained are interpreted to be analyzed and the types of disturbances that have occurred are known so that further action can be taken.
3. Gas Extraction Method
There are 2 types of gas extraction methods in the dissolved gas test, namely:
Gas Cromatograph
The technique of separating certain substances from a compound compound based on their level of evaporation (volatility).
Figure 1. Gas Process Cromatograph
Photoacoustic Spectroscopy
With electromagnetic wave radiation in determining dissolved gas concentrations.
Figure 2. Photoacoustic Spectroscopy Process
4. Gas Level Limit (IEEE C57.104-1991)
4 DGA conditions guide to classify the transformer condition if there is no previous DGA data, according to IEEE C57-104 standard. The guide uses a combination of individual gases and the total flammable gas concentration as an indicator. It is not universally accepted and is only one of the tools used to evaluate dissolved gases in transformers. The four IEEE conditions have been defined directly below, and the gas levels in table 4.1 follow the definitions.
Table 1. Limits of Gas Content
Table 2. Operating Procedures
Condition 1: The total flammable dissolved gas (TDCG) above indicates the transformer is operating properly. Each individual flammable gas exceeding the level specified in table 2 shall have other investigations.
Condition 2: TDCG in this range indicates a level of flammable gas is greater than normal. Any individual flammable gases that exceed a certain level in table 2 should be another investigation. Mistakes may be present. Collect DGA samples at least frequently enough to calculate the number of gas forces per day for each gas. (See table 2 for recommended sampling frequency and measures)
Condition 3: TDCG in this range indicates a high degree of decomposition
insulating cellulose and / or oil. Any individual flammable gases that exceed the levels specified in table 1 shall have other investigations. An error or glitch that may be present. Collect DGA samples at least frequently enough to calculate the amount of gas generation per day for each gas.
Condition 4: TDCG in this range indicates excessive decomposition
insulating cellulose and / or oil. Continued operation can result in transformer failure.
5. Data Interpretation Method
1. Key gas method
The gas key method has been defined in the IEEE. The key gases that have been formed by the degradation of oil and paper insulation are hydrogen (H2), methane (CH4), ethane (C2H6), ethylene (C2H4), acetylene (H2C2), carbon monoxide (CO), and oxygen (O2). Except for carbon monoxide and oxygen, all of these gases are formed from the degradation of the oil itself. Carbon monoxide (CO), carbon dioxide (CO2), and oxygen are formed from the degradation of insulating cellulose (paper). Carbon dioxide, oxygen, nitrogen (N2), and moisture can also be absorbed from the air if there is oil or air, or if there is a leak in the tank. Gas key method,
Thermal - Oil
The products of deterioration include ethylene and methane, coupled with a small presence of hydrogen and methane. Key Gas: Ethylene
Figure 3. Thermal oil graph
Thermal - Celullose
Large number of CO and CO2 compounds are formed from excess heat in cellulose paper. Key gas: Carbon monoxide
Figure 4. Graph of thermal celullose
Electrical - Korona
Low-energy electric discharge produces hydrogen and methane, with small amounts of ethane and ethylene. Key gas: Hydrogen
Figure 5. Electrical corona graph
Electrical - Arcing
Large amounts of hydrogen and acetylene with small amounts of methane and ethylene occur during arcing. Key gas: acetylene
Figure 6. Electrical arcing graph
2. Roger Ratio Method
Is one additional method that can be used to interpret what happens based on the composition of the dissolved gas in the insulating oil. Roger ratio method is to compare the amount of different gases by dividing one gas with another, this forms a ratio between one gas and another.This method uses the ratio of three gases, namely C2H2 / C2H4, CH4 / H2 and C2H4 / C2H6. This method is used for disturbance analysis not to detect interference, therefore interference must have been detected using the IEEE limit.
Some notes regarding the interpretation of the Roger ratio table:
1. There is a tendency for the C2H2 / C2H4 ratio to increase from 0.1 to> 3 and the C2H4 / C2H6 ratio to increase from 1-3 to> 3 due to the increase in spark intensity. So that the initial code is no longer 0 0 0 but 1 0 1
2. The majority of the gas generated is produced by the paper decomposition process, so that the number 0 appears in the Roger ratio code.
3. This failure condition is indicated by an increase in fault gas concentration. CH4 / H2 is normally a value of 1, however this value depends on various factors such as the condition of the conservator, N2 blanket, oil temperature and oil quality.
4. The increase in the value of C2H2 (more than the detected value), generally indicates a hot-spot with a temperature of more than 7000 C, causing arching to occur in the transformer. If the concentration and the rate of formation of acetylene gas increases, the transformer must be immediately repaired (de-energized). If operated further the conditions will be very dangerous.
5. Transformers with OLTC (On-Load Tap Changer) may show the code 2 0 2 or 1 0 2 depending on the amount of oil exchange between the tap changer tank and the main tank.
Table 3. Roger Ratio analysis
2. Duval Triangle Method
Michael Duval of Hydro Quebec developed this method in the 1960s using thousands of DGA databases and a diagnosis of transformer problems. Recently, this method was included in Transformer Oil Analyst Software version 4 (TOA 4), developed by Delta X Research and used by many in the utility industry to diagnose transformer problems. This method has been proven to be accurate and reliable over the years and is now gaining in popularity. The methods and how to use them are described below.
Figure 7. Duval Triangle
PD = Partial Discharge
T1 = Thermal Fault Less than 300 ° C
T2 = Thermal Fault Between 300 ° C and 700 ° C
T3 = Thermal Fault Greater than 700 ° C
D1 = Low Energy Discharge (Sparking)
D2 = High Energy Discharge (Arcing)
DT = Mix of Thermal and Electrical Faults
Table 4. Limits for Individual Gas
How to use the Duval Triangle
First determine if there is a problem using the IEEE method. At least one of the hydrocarbon or hydrogen (H2) gases must be in the IEEE 3 condition, and increased to the generation rate (G2) from the table above, before the problem is confirmed. To use the table above without the IEEE method, at least one of the individual gases must be at or above L1 level
and the gas generation rate is at least at G2. The L1 limit and gas rise rate from the table above are more reliable than the IEEE method, but one should use both methods to ensure that there is a problem. If there is a sudden increase in H2 with only carbon monoxide (CO) and carbon dioxide (CO2) and little or no hydrocarbon gases, use section 17 (CO2 / CO ratio) below to determine whether cellulose insulation is damaged due to overheating.
Once the problem has been determined it exists, use the total accumulated sum of the three Duval gas triangles and plot the percentage of the total on the triangles to arrive at a diagnosis. Also, calculate the amount of the three gases used in the Duval Triangle, which were produced since the gas increase suddenly started. Subtracting from the amount of gas produced before the sudden increase will give the amount of gas produced because the error started. Detailed instructions and examples are shown below.
Take the amount (ppm) of methane (CH4) in DGA and subtract the amount of CH4 from the previous DGA, before the sudden increase in gas. This will give you the amount of methane generated as the problem started.
Repeat this process for the two remaining gases, ethylene (C2H4) and acetylene (C2H2).
Add the three numbers (differences) obtained by process la
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