LTC Dissolved Gas Analysis Ratio Interpretation
Dissolved Gas Analysis (DGA) can be a useful tool in detecting faults and abnormalities in LTCs. Unlike transformers, general threshold limits cannot be used because of the wide variation in LTC design, breathing configurations, and operating conditions. IEEE C57.139 is a guide to develop thresholds for specific models and operating conditions which can used.
For general interpretation of DGA, LTC ratio analysis provides a way to evaluate conditions occurring with a unit. During operation, the insulating fluid quenches the arc that occurs during the switching producing predominantly Acetylene at the event site. Like a ripple effect the energy is dissipated and gases of Ethylene, Ethane, Methane, Hydrogen are produced with decreasing energy in the oil.
Hydrogen is produced throughout this process but being such a light gas it diffuses rapidly out of the oil into the atmosphere via breathing and is not reliable for ratios. We can look at the ratios of the gases to get a picture of possible faults occurring. Since Acetylene is the predominant gas of normal operation, we look at the ratio of Acetylene to the other Hot Metal Gases. The ratios used are:
The Heating Gases to Arcing Gas can be key in evaluating overheating conditions that might be occurring in the LTC. For it to be statistically significant, an acetylene value of 500 ppm is used as a threshold to before calculating. A value of <0.5 is indicative of no abnormal overheating occurring while a ratio of >5.0 can be indicative of excessive overheating and possible dangerous situations.
The Heating Gas ratios can be used to further pinpoint the temperature that the overheating is occurring because of the different energy to create each gas.
