Diagnostic DGS - Characteristics

Diagnostic DGS - Characteristics
Fault indicator gases

CO2/CO ratio should be in the range of 1:10, if this ratio decreases to 1:3 or less, overheating of cellulose is most likely to occur.
O2/N2 this ratio should be maintained at less than 1:10, in order to prolong the life of the fluid in the transformer.

Solubilité des gaz dans l'huile de transformateur (Ref. http://www.nttworldwide.com)

Distribution typique de gaz de défaut de condition différente (Corona, pyrolyse, Arcing)
(Ref. http://www.nttworldwide.com)

Normal values of dissolved gas in oil

Dissolved gas concentration limit (ppm)

Condition 1 - TDCG below this level indicates that the transformer is operating satisfactorily
Condition 2 - TDCG within this range indicates greater than normal combustible levels & should prompt additional investigation. (any gas exceeding specified levels)
Condition 3 - TDCG within this range indicates high level of decomposition (additional investigation required)
Condition 4 - TDCG within this range indicates excessive decomposition, continued operation could result in the failure of the transformer.

Diagnostic DGA - Methods
Method of gas ratios:

• IEC 60599
• Rogers
• Dornenburg

Duval Triangle
Gas patterns (EPRA Japan)

Dornenburg Chart (Ref: http://www.nttworldwide.com)

Duval Triangle

    

Key Ratios                                                                                                

• CH4/H2                Partial discharge
• C2H4/C2H6         Oil overheating
• C2H2/C2H4         Arcing
• C2H2/C2H6         Electrical discharges

Auxiliary Ratios

• C2H2/H2             Contamination OLTC
• C2H2/C2H4        Identification of thermal defects OLTC
• CO2/CO             Cellulose overheating
• N2/O2                 Oxygen consumption, bad sealing

Gas Ratios Method (IEC 60599)

Rogers Method (IEEE)

Gas patterns

Water Content (Ref: IEC 60422)
Depending on the amount of water, the temperature of the insulating system and the status of the oil, the water content of the insulating oil influences:

• The breakdown voltage of the oil.
• The solid insulation.
• The ageing tendency of the liquid and solid insulation. 

Two main sources of water increase in transformer insulation:

• Ingress of moisture from the atmosphere.
• Degradation of cellulose and oil.

The solubility of water in oil, given in mg/kg (ppm), depends on the condition of the oil, the temperature and type of oil. Wabs (the absolute water content) is independent of the temperature type and condition of the oil and the result is given in mg/kg. Wabs can be measured according to IEC 60814.

Water solubility should be determined at the same temperature as the oil has been taken.

Sampling temperature at or above 20 °C
For the proper interpretation of moisture content, the analytical result of water content of the oil at a given sampling temperature needs to be corrected to that at a defined temperature.
For practical reasons, the defined temperature is set at 20 °C, since below 20 °C the rate of diffusion of water is too slow to achieve equilibrium in operational equipment.