The Duval Triangle diagnostic method for oil-insulated high-voltage equipment (mainly transformers), developed by Michel Duval of IREQ, is described in Appendix B of IEC 60599:1999.
Concentrations (ppm) of methane (CH4), ethylene (C2H4), and acetylene (C2H2) are expressed as percentages of the total (CH4 + C2H4 + C2H2) and plotted as a point (%CH4, %C2H4, %C2H2) in a triangular coordinate system on a triangular chart which has been subdivided into fault zones. The fault zone in which the point is located designates the likely fault type which produced that combination of gas concentrations.
The Duval Triangle method, like any other DGA diagnostic method, should be applied only when there is some suspicion of a fault, based on an increase in combustible gas or some other suspicious symptom. The diagnostic method itself is not a means of fault detection.
Because of the relative inaccuracy of gas-in-oil concentration measurements at low concentrations, DGA diagnostic methods, including the Duval Triangle, should not be applied unless the gas concentrations are well above the detection limit.
If reasonably stable concentrations of the gases were present before the onset of the suspected fault, it is advisable to subtract out the background concentrations, provided that the differences are large enough for interpretation. The diagnosis should be based on recently-formed gas if possible, and including pre-fault gas in the diagnostic calculations can lead to misleading results.
Suppose that a transformer is found to have recent increases of 45 ppm of CH4, 90 ppm of C2H4, and 15 ppm of C2H2. Such a significant increase in C2H4 and C2H2 is sufficient justification to suspect a fault and to apply the Duval Triangle method.
Calculate T = CH4 + C2H4 + C2H2 = 45 + 90 + 15 = 150.
Express the amount of each gas as a percentage of T.
CH4: 100*CH4/T = 100*45/150 = 30%
C2H4: 100*C2H4/T = 100*90/150 = 60%
C2H2: 100*C2H2/T = 100*15/150 = 10%
Plot a point on the triangle which is 30% of the way up the CH4 axis, 60% of the way down the C2H4 axis, and 10% of the way across the C2H2 axis. The arrows next to each side of the triangle show the direction of each axis.
The point representing our gas data falls in the T3 fault zone, indicating a high-temperature thermal fault (T3), which is typically a “hot metal” fault.
|T1||Low-range thermal fault (below 300 C)|
|T2||Medium-range thermal fault (300-700 C)|
|T3||High-range thermal fault (above 700 C)|
|D1||Low-energy electrical discharge|
|D2||High-energy electrical discharge|
|DT||Indeterminate – thermal fault or electrical discharge.|