DGA or Dissolved Gas Analysis of Transformer oil

DGA (Dissolved Gas Analysis) is an important tool for checking the healthiness of oil-filled power transformers. Power Transformer is important and expensive equipment in Power Transmission and Distribution systems. Periodic checking or testing of the transformer is done to maintain the healthiness of the transformer.

Transformer oil is used for insulation and cooling of the Transformer. Due to any fault condition or chemical reaction inside the transformer, the oil properties will change and it will generate various gases which are dissolved in the oil.

DGA of the oil sample is checked to identify the generated gases inside the oil. Generally, DGA of Transformer oil checks in the testing lab. Portables kits are also available to test DGA at the field.

Transformer oil is a molecule of Hydro carbon, it contains Hydrogen and carbon atoms which are linked by chemical bonds. Due to the decomposition of Transformer Oil and paper insulation of Transformer, it generates the following gases, which are also called DGA gases. The DGA gases are Hydroge(H2), Methene(CH4), Ethane(C2H6), Ethylene(C2H4), Acetylene(C2H2), Carbon Monoxide(CO), Carbon Dioxide(CO2), Oxygen(O2), Nitrogen(N2).

GasFormulaTemperature at which gas formsSource of Gas
HydrogenH2<150°C for Cold Plasma ionization or corona in oil, >250°C for thermal & delectrical faultPartial Discharge(PD, Thermal Fault, Power Discharge, sunlight, Rusting
MetheneCH4150°C to 300°CLow and medium temperature thermal faults, Corona partial discharge
EthaneC2H6200°C to 400°CLow and medium temperature thermal fault
EthyleneC2H4300°C to 700°CHigh Temperature thermal fault
AcetyleneC2H2>700°CHigh energy discharge or Arc, Very hot spot
Carbon MonoxideCO105°C to 300°CThermal fault involving cellulose (paper, pressboard, Wood blocks) slowly from oil oxidation
Carbon DioxideCO2105°C to 300°CThermal fault involving cellulose (paper, pressboard, Wood blocks) slowly from oil oxidation Normal aging
OxygenO2Vacuum in oil or drop in oil temperatureLeakage, Expose of oil to atmosphere
NitrogenN2Vacuum in oil or drop in oil temperatureLeakage, Expose of oil to atmosphere
DGA in transformer oil

Why is DGA important?

  • Early detection of faults: By identifying the presence and concentration of specific gases, DGA can help detect developing faults within the transformer at an early stage, before they become critical and lead to costly failures.
  • Diagnosis of fault type: Different types of faults within the transformer produce different gas signatures. Analyzing the gas composition allows for accurate diagnosis of the specific fault, enabling targeted maintenance and repairs.
  • Predictive maintenance: Regular DGA monitoring can help predict potential transformer failures, allowing for preventive maintenance and extending the lifespan of the equipment.

How is DGA performed?

  1. Oil sampling: A small oil sample is drawn from the transformer using specialized equipment.
  2. Gas extraction: The dissolved gases are extracted from the oil sample using various techniques such as vacuum extraction or headspace analysis.
  3. Gas analysis: The extracted gases are identified and their concentrations are measured using a gas chromatograph.
  4. Interpretation of results: The gas concentrations are compared with established standards and guidelines to determine the health of the transformer and identify any potential problems.

Key gases analyzed in DGA:

  • Hydrogen (H2): Indicates overheating, arcing, and corona discharge.
  • Methane (CH4): Indicates overheating and arcing.
  • Ethane (C2H6): Indicates overheating and low-energy arcing.
  • Ethylene (C2H4): Indicates overheating and low-energy arcing.
  • Carbon monoxide (CO): Indicates overheating and thermal decomposition.
  • Carbon dioxide (CO2): Indicates overheating and thermal decomposition.

Benefits of DGA:

  • Non-destructive: DGA does not require any physical intervention within the transformer, making it a non-intrusive and safe method for assessing its health.
  • Cost-effective: DGA is a relatively inexpensive test compared to other diagnostic methods.
  • Reliable: DGA provides reliable and accurate information about the internal condition of the transformer.
  • Widely applicable: DGA can be used for transformers of all sizes and types.

By routinely performing DGA, transformer owners can:

  • Enhance the reliability and availability of their electrical equipment.
  • Reduce the risk of unexpected failures and costly outages.
  • Optimize maintenance schedules and costs.
  • Extend the lifespan of their transformers.

Overall, DGA is an essential tool for ensuring the safe and reliable operation of oil-filled transformers. By understanding its principles and benefits, you can better evaluate the health of your transformers and make informed decisions regarding maintenance and operation.

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