Dissolved Gas Testing

What exactly is a “Dissolved Gas Test”?

 To explain what the test is, a basic knowledge of the fluids contained in a transformer is warranted.

 Most transformers are filled with mineral oil. Similar to diesel fuel, mineral oil is also refined from crude oil and is used because of it chemical stability and high natural dielectric strength. Being that it is a hydrocarbon material, the principal molecular makeup of transformer oil is Hydrogen and Carbon in their various forms. It is for this reason that Dissolved Gas Analysis Test (DGA) can be used to readily determine the electrical condition of the transformer.

 All transformer oils breakdown naturally over time. However, this is a very slow process. Barring outside influences and causes, the expected life of good transformer oil will exceed 20 years. The main enemies of transformer oil are Oxygen, Water and Excessive  Heat.

 Whenever transformer oil is exposed to thermal of electrical faults, the natural breakdown process is greatly speeded up. During breakdown on the oil, certain hydrocarbon based gasses are produced in small quantity. The magnitude of the fault and surrounding materials will determine which gasses are produced.

 What gasses are produced?

 Although other trace gasses may be present, only the following gasses are used in the DGA analysis. This list is ordered by the energy required to produce the gas in the oil:

 Hydrogen                     H₂

Methane                       CH₄

Ethane                          C₂H₆

Ethylene                       C₂H₄

Acetylene                     C₂H₂

 In addition, the following gasses are produced when a fault occurs near cellulose insulation:

 Nitrogen                      N₂

Oxygen                        O₂

Carbon Dioxide            CO₂

Carbon Monoxide         CO

How are the gasses read?

 A good example of this process is in the case of Corona. Corona inside the transformer is a very low energy discharge. The presence of Corona will ionize the oil surrounding the fault and release hydrogen in detectable amounts. Since Corona is a low energy condition and Hydrogen is the easiest gas to produce ( requires the least amount of energy ), A DGA test that reveals higher than normal Hydrogen in the oil, with no other elevated gasses, would point to a Corona condition occurring inside the transformer. Conversely,  a DGA test that revealed a detectable amount of Acetylene (the hardest gas to produce – requires the most energy) gas in addition to Ethylene and Ethane would signify a high energy arc had occurred. These are just two of the many different combinations of gasses and problems that can be found by performing a DGA test as part of your maintenance plan.

 It becomes a simple process of drawing an oil sample and testing for which gasses are present in the oil and in what quantities.

 How is the DGA test performed?

 First, a sample of the oil is carefully drawn from the bottom of the unit, using a specially prepared glass syringe. Extreme care is taken to insure that the oil is not exposed to the atmosphere. After drawing the oil, any air bubbles that remain in the syringe are purged and the syringe is sealed. Glass is used to prevent the gasses from being absorbed into the walls of a plastic sampling device. After sampling, complete nameplate data is recorded for use in identifying the sample and for diagnostic purposes.

After receipt of the sample at the laboratory, the oil is subjected to a very high vacuum that extracts the gas from the oil molecule. The amount of gas extracted per milliliter is carefully recorded. The extracted gasses are passed thru a highly accurate gas chromatograph. The amount of each gas is recorded in parts per million.

 A report is prepared showing the amount of each gas in proportion to the total amount of gas contained in the oil sample. The numbers are entered into a computer program that performs different ratio comparisons as described by ANSI and other accepted methods. The output of the program identifies any fault conditions that exist inside the transformer; and in some cases, even pinpoints the actual source area of the fault.

Is past history of a particular unit used?

 A copy of all tests results that are performed on a particular unit is kept in a computer database. Whenever a transformer is tested, this database is checked to see if any past history is on file. If found, the results from all previous tests are compared with the latest test to see if any trends in the production of gasses are present and/or changing. For example, if the past yearly test results for the gas Ethylene showed results of 8, 9,11,13 and 15 and the result of a test sample that was just performed showed an Ethylene content of 45, this would indicate that something was changing inside the transformer. In this particular case, it would suggest that some minor amount of core heating was present, and that the process was accelerating. It would be recommended that another DGA test be performed within 4-6 months to further support the evidence this problem.

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