Dissolved Gas in Oil Analysis – Part 1

The oil in an oil-filled transformer serves the dual purpose of providing dielectric insulation, and a means to transfer heat from the windings and core to cooling radiators.  Analysis of this transformer “blood” will provide information on the condition of the oil itself, and also of the transformer windings and core. (See also Transformer Talk Newsletter #2 – Standard Oil Tests)

Dissolved gas in oil analysis is a reliable test to assess Transformer Health and can effectively provide an early warning of overheating and other fault conditions that may lead to failure.  There are 3 steps:

  1. Take oil samples in a safe and effective manner.
  2. Obtain Laboratory analysis quantifying the dissolved gas in oil content for critical gases.
  3. Interpret the quantified results to help determine the relative health of the transformer, offer clues to the origin of potential problems and develop a strategy to avoid catastrophic failure.

Taking an Oil Sample

Caution:  When taking an oil sample from a sealed tank transformer, ensure that the transformer is not under vacuum by checking the vacuum/pressure gauge.  A sample drawn while a transformer is under vacuum has the potential of pulling air into the transformer, possibly leading to failure.  It is important to get a representative sample of the oil inside the transformer tank. Use a clean glass syringe (provided by the laboratory) and follow the proper sampling procedure.

Laboratory Analysis Performed

The laboratory will provide the parts per million, gas in oil content for the following gases:

Oxygen – O2   Hydrogen – H2   Nitrogen – N2

Carbon Monoxide – CO  Methane – CH4   Carbon Dioxide – CO2

Ethylene – C2H4  Ethane – C2H6     Acetylene – C2H2

A typical example of a healthy transformer could yield the following results.

Oxygen – 14000 ppm  Hydrogen – 23 ppm

Nitrogen – 76000 ppm  Methane – 7 ppm

Carbon Monoxide – 780 ppm Ethylene – 12 ppm

Carbon Dioxide – 3500 ppm  Ethane – 14 ppm

Acetylene – 1 ppm

Total Gas Content – 9.43 %

Interpretation of Results

To effectively interpret DGA results requires insights in the characteristics of dissolved gas in oil evolution, an understanding of transformer design, and knowledge of materials used in transformer manufacture.  There has been a number of ratio and other comparative methods developed to analyze dissolved gases. These methods must be applied with care and knowledge in regard for threshold levels, transformer operating conditions and design heritage.

Below see Figure 1, which indicates relative levels of carbon monoxide and carbon dioxide evolved, as a function of temperature.  Carbon monoxide and carbon dioxide gas find their origin in the transformer insulation – paper, pressboard and wood.  Figure 2 indicates the relative levels of the combustible gases that evolve from the transformer oil as a function of temperature.