During the fault in the electrical network, the large current flowing through the transformer winding cause thermal and electrical stresses and certain gases are produced due to decomposition of the transformer oil.
The analysis of the dissolved gases gives the clearer picture of the healthiness of the transformer oil and the paper insulation. The test conducted for analysis of the transformer insulation condition by examining the transformer oil is called the DGA Test. DGA test must be conducted periodically to ensure the healthiness of the transformer for trouble-free operation of the transformer. The DGA results indicates about the healthiness of the insulation system of the transformer.
Advantages of Dissolved Gas Analysis Test of Transformer Oil
- Current state of insulating material inside the transformer
- Remaining life of the transformer
- Identifies degradation before it lead to failure
- It is a good tool for effective maintenance and replacement strategies.
- It is low cost process
1. Hydrogen (H2)
2. Methane (CH4)
3. Ethane (C2H6),
4. Ethylene (C2H4),
5. Acetylene (C2H3),
6. Carbon Monoxide (CO),
7. Carbon dioxide (CO2),
8. Nitrogen (N2)
10. Oxygen(O2).
The gases is evolved during fault and gets dissolved in the transformer oil. In Dissolved Gas Analysis of transformer oil or DGA analysis of transformer oil the gases dissolved in the transformer oil is extracted from oil and the amount of gas present in the oil is analyzed.
Methods of determining gas amount in transformer oil
The following methods are used to determine the content of the gases in the transformer oil.
- Vacuum gas extraction
- Gas Chronographs
If the temperature of the transformer rises up to 150°C to 300° C. during thermal stress, the hydrogen(H2) and methane((CH4) is produced in a large quantity. If the temperature of the transformer exceeds above 300°C ethelene(C2H4) is produced in a large quantity. Hydrogen(H2) and ethelene (C2H4) are produced if temperature rises above 700°C. The gas generation at different temperature is as shown in below given chart.
If there is a hot spot inside the transformer, the content of ethelene(C2H4) will get increased. The large quantity of carbon monoxide(CO), and carbon dioxide(CO2) in the transformer oil indicates paper insulation quality deterioration.
If the amount of the gases in the transformer value is more than the specified highest limit the following types of fault as shown in the below given table can be detected.
S.N
|
Parameters
|
Indicative fault
|
Unit of Measurement
|
Violation Limits
|
1.
|
Methane
(CH4)
|
Local Overheating
|
ppm
|
120 ppm
|
2.
|
Ethylene
(C2H4)
|
Thermal Degradation of oil
|
ppm
|
30 ppm
|
3.
|
Acetylene (C2H2)
|
Overheating
|
ppm
|
65 ppm
|
4.
|
Ethane
(C2H6 )
|
Arcing & sparking
|
ppm
|
35 ppm
|
5.
|
Hydrogen (H2)
|
Electrical / Thermal Fault
|
ppm
|
100 ppm
|
6.
|
Carbon Mono Oxide (CO)
|
Thermal Aging of Paper
|
ppm
|
350 ppm
|
7.
|
Carbon Dioxide (CO2)
|
Cellulose Decomposition
|
ppm
|
N/A
|
Furan Analysis of Transformer Oil
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