IEC 60599 Guide to the interpretation of dissolved and free gases analysis
IEC 60599 Mineral oil-impregnated electrical equipment in service - Guide to the interpretation of dissolved and free gases analysis
9 Recommended method of DGA interpretation (figure 1)
a) Reject or correct inconsistent DGA values (see 6.1). Calculate the rate of gas increase since the last analysis, taking into account the precision on DGA results.
If all gases are below typical values of gas concentrations and rates of gas increase, report as "Normal DGA/healthy equipment".
If at least one gas is above typical values of gas concentrations and rates of gas increase, calculate gas ratios and identify fault using table 2 (see 5.3). Check for eventual erroneous diagnosis (see 4.3).
If necessary subtract last values from present ones before calculating ratios, particularly in the case of CO, CO2 (see 6.1).
If DGA values are above typical values but below 10 x S (S = analytical detection limit), see 6.2.
b) Determine if gas concentrations and rates of gas increase are above alarm values. Verify if fault is evolving towards final stage (see 5.8). Determine if paper is involved (see 4.2 and 5.4).
c) Take proper action according to best engineering judgment and/or with the help of figure 1. It is recommended to
1) increase sampling frequency (quarterly, monthly or other) when the gas concentrations and their rates of increase exceed typical values,
2) consider immediate action when gas concentrations and rates of gas increase exceed alarm values.
10 Report of results
NOTE The report should be adapted to the specific type of equipment considered.
The DGA interpretation report should include the following, when available:
a) DGA analysis report, including S values, method of DGA analysis, and date of analysis.
NOTE Values of 0 µl/l on a DGA report or below the S values are replaced by "below the S value for this gas".
b) specific information on the equipment such as:
1) date of commissioning, voltage, general type (e.g. power or instrument transformer), rated power;
2) special features (e.g. sealed or air-breathing, type of OLTC (see A.1.6));
3) oil volume;
4) oil or gas sampling date;
5) oil or gas sampling location;
c) special operations or incidents just before and after the oil or gas sampling, such as tripping, gas alarm, degassing, repair, outage;
d) previous DGA on the equipment;
e) indication of typical values for this specific equipment, if known;
f) indication of "Typical DGA/healthy equipment" or "Fault";
g) in case of "Fault", identification of the fault using table 2 (see 5.3), with values of the calculated gas ratios indicated;
h) indication of paper involvement or not, with value of the CO2/CO ratio;
i) recommended actions:
1) new frequency of oil sampling,
2) furanic compound analysis if CO2/CO ratio is lower than 3,
3) other tests.
