Diagnostics of dissolved gas in insulating oil of transformers Pekka Pietarinen

Contents Condition monitoring of transformers Gas formation Gas detection Analysis Conclusion

CONDITION MONITORING METHODS Moisture content PD measurements DP factor – degree of insulation degradation Acidity Gas analysis Furfural analysis Dielectric strength test (AC/impulse) Thermal imaging

Condition monitoring of transformer oil Asses the internal condition of the transformer Assist in prediction of failures and length of lifespan Identify faults Detect misuse of transformers Essential for maintenance and replacement strategies Reduction of unplanned outages

Thermal fault, local hotspot Gas formation Two main causes of gas formation within a transformer Oil decomposition Mineral oil mainly composes of hydrocarbons, decomposition forms smaller hydrocarbon molecules (faults, thermal) Different fault types produce different gases Cellulosic degradation or decomposition Thermal decomposition of oil-impregnated paper (cellulose) insulation H2 PD, high temperature C2H2 Arcing CH4 C2H4 C2H6 Thermal fault, local hotspot CO CO2 C5 H4 O2 Paper decay

Detection of gases A sample of gas has to be obtained (sample of oil or blanket gas) Gas chromatography Most effective way to detect gases from sample Provides concentrations of gases from sample

DGA (Dissolved Gas Analysis) The ratios of dissolved gases are used to determine the condition of the transformer or fault types Different methods and guidelines (standards) for analysing the gas ratios are available Key gas IEC gas ratio Duval triangle Fuzzy logic Artificial Intelligence

DGA (Dissolved Gas Analysis) A chemical analysis of the contents of the gas Not affected by electrical interference Very inexpensive Basically a method of comparing measured gas concentrations to existing numbers to determine results TCG and blanket analysis

Diagnostics of dissolved gas in insulation oil of transformers Different faults yield different gases Arcing, PD, hotspots, degradation of cellulose Gas chromatography – the most effective way to analyse gas composition DGA More a chemical test than electrical Increase lifespan of transformers by redusing misuse, overload or to detect faults Reduce damage or destruction of property Reduce the amount of outages