IEEE 485-2010 pdf download IEEE Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers
Attempts to assign greater significance to gas than justified by the natural variability of the generating andmeasuring events themselves can lead to gross errors in interpretation. However, in spite of this, these gas-generating mechanisms are the only existing basis for the analytical rules and procedures developed in thisguide. In fact, it is known that some transformers continue to operate for many years in spite of aboveaverage rates of gas generation
4.4 Establishing baseline data
Establishing a reference point for gas concentration in new or repaired transformers and following this witha routine monitoring program is a key element in the application of this guide. Monitoring the healthserviceability) of a transformer must be done on a routine basis and can start anytime-it is not just fornew units.
Generally, daily or weekly sampling is recommended after startup, followed by monthly or longer intervalsRoutine sampling intervals may vary depending on application and individual system requirements. Forexample, some utilities sample generator step-up (GSU) transformers four to six times a year, units ratedover 138 kV are sampled twice a year, and some 765 kV units are sampled monthly.
4.5 Recognition of a gassing problemEstablishing operating priorities
Much information has been acquired on diagnosing incipient fault conditions in transformer systems. Thisinformation is of a general nature but is often applied to very specific problems or situations. Oneconsistent finding with all schemes for interpreting gas analysis is that the more information availableconcerning the history of the transformer and test data, the greater the probability for a correct diagnosis ofthe health of the unit.
A number of simple schemes employing principal gases or programs using ratios of key gases have beenemployed for providing a tentative diagnosis when previous information is unavailable or indicated no faulcondition existed. Pincipal gas or ratio methods require detectable or minimum levels of gases to bepresent or norms to be exceeded, before they can provide a useful diagnosis.
5. Interpretation of gas analysis
5.1 Thermal faults
Referring to Figure 1, the decomposition of mineral oil from 150 °C to 500 °C produces relatively largequantities of the low molecular weight gases, such as hydrogen (H) and methane (CH). and tracequantities of the higher molecular weight gases ethylene (C,H,) and ethane (C,H). As the fault temperaturein mineral oil increases to modest temperatures. the hydrogen concentration exceeds that of methane, butnow the temperatures are accompanied by significant quantities of higher molecular weight gases–first ethane, and then ethylene. At the upper end of the thermal fault range, increasing quantities of hydrogen and ethylene and traces of acetylene (C 2 H 2 ) may be produced. In contrast with the thermal decomposition of oil, the thermal decomposition of cellulose and other solid insulation produces carbon monoxide (CO), carbon dioxide (CO 2 ), and water vapor at temperatures much lower than that for decomposition of oil and at rates exponentially proportional to the temperature. Because the paper begins to degrade at lower temperatures than the oil, its gaseous byproducts are found at normal operating temperatures in the transformer. A GSU transformer, for example, that operates at or near nameplate rating will normally generate several hundred microliters/liter (ppm) of CO and several thousand microliters/liter (ppm) of CO 2 without excessive hot spots.