DGA Testing

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Introduction

Dissolved Gas Analysis (DGA) is a test used to assess paper-oil insulation systems, and the most commonly used technique for oil-insulated power transformers. Over time (and due to electrical stresses and thermal decomposition), the cellulose paper and insulating (mineral) oil in paper-oil insulation systems break down and forms gases which are dissolved in the oil. In DGA testing, a sample of oil is collected and analysed for the types, volumes, concentrations and formation rates of dissolved gases. This can provide important information on the condition of the insulation system, as well as the nature of the electrical disturbances that the system has been subjected to.

Gas decomposition typically involves chemical reactions that break up the carbon and hydrogen bonds of the hydrocarbon chains in both the cellulose and oil. The following gases are most commonly formed and are considered the most significant in the context of DGA testing:

  • From insulating oil: hydrogen (H2), methane (CH4), acetylene (C2H2), ethylene (C2H4) and ethane (C2H6)
  • From cellulose: hydrogen (H2), methane (CH4, carbon monoxide (CO) and carbon dioxide (CO2)

Interpretation of Results

ANSI/IEEE Std C57.104 [1] provides guidance on concentration limits of the key gases:

Dissolved gas concentration limit (ppm) Interpretation
Hydrogen (H2) Methane (CH4) Acetylene (C2H2) Ethylene (C2H4) Ethane (C2H6) Carbon monoxide (CO) Carbon dioxide (CO2) TDCG (*)
100 120 1 50 65 350 2500 720 Satisfactory
101 - 700 121 - 400 2 - 9 51 - 100 66 - 100 351 - 570 2500 - 4000 721 - 1920 Fault may be present
701 - 1800 401 - 1000 10 - 35 101 - 200 101 - 150 571 - 1400 4001 - 10000 1921 - 4630 Fault probably present
>1800 >1000 >35 >200 >150 >1400 >10000 >4630 Continued operation could result in failure

(*) TDCG = Total dissolved combustible gas

Types of Faults

The relative proportions of gas types can suggest the type of fault that has occurred in the transformer to form the particular gases, as shown in the figure below (courtesy of IEEE C57.104 [1]):

DGA Fault Types.png

References