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Appendix
12.9.2 Methods of Analysis
One of the most important and proven methods is Dissolved Gas Analysis
(DGA). The hydrocarbon gases dissolved in the oil allow conclusions to be made
about the condition of the transformer and specific faults by investigating
changes of the gas composition [4], [5], [6], [7]. The measured quantities of the
gases (expressed as a quotient) as they are shown in the last line of the table
(Table 12-1) suggest high temperatures.
One possible reason for the observation of high temperatures could be the
existence of high contact resistances at the tap selector. However, there are
many other possible reasons for increased gas values in the oil. An example
might be decomposed oil leaking from the diverter switch into the transformer oil
through a defective sealing between the diverter switch vessel and the
transformer tank. Exceptionally high current densities causing high
temperatures can also occur as a result of partial breaks in conductors
connected in parallel. Further interesting information about this can be found in
[4], [5], [6] and [7].
Table 12-1 Dissolved Gas Analysis
Type of fault
PD Partial discharge < 0.01 < 0.1 < 0.2
D1 Discharge with low energy > 1 0.1 - 0.5 > 1
D2 Discharge with high energy 0.6 - 2.5 0.1 - 1 > 2
T1 Thermal fault T < 300°C < 0.01 > 1 < 1
T2 Thermal fault T < 700°C < 0.1 > 1 1 - 4
T3 Thermal fault T > 700°C < 0.2 > 1 > 4
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