Carbon Dioxide / Carbon Monoxide Ratio

This ratio is not included in the Rogers Ratio Method of analysis. However, it is useful to determine if a fault is affecting the cellulose insulation. This ratio is included in transformer oil analyzing software programs, such as Delta X Research Transformer Oil Analyst. This analysis is available from the TSC at D-8440 and D-8450 in Denver.

Formation of CO2 and CO from the degradation of oil impregnated paper increases rapidly with temperature. Calculate a normal operating CO2/CO ratio at each DGA, based on the total accumulated amount of both gases. Look at several DGAs concentrating on CO2 and CO. Experience has shown that, with normal loading and temperatures, the CO2 generation rates runs 7 to 20 times higher than CO. With a CO2/CO ratio above 7, there is little concern. With some transformers, ratios down to 5 times more CO2 than CO might be considered normal. However, be careful with a ratio below 7. If H2, CH4, and C2H6 are increasing significantly as well as CO and the ratio is 5 or less, there is probably a problem. Take time to know the particular transformer by carefully checking all prior DGAs and establishing a normal operating CO2 to CO ratio.

CAUTION: After a suspected problem (a substantial increase in the amount of CO), the ratio should be based on the gas generation of both CO2 and CO between successive DGAs and not on accumulated total CO2 and CO gas levels.

If a problem is suspected, immediately take another DGA sample to confirm the problem. Take the amount of CO2 generated between the DGAs and divide it by the amount of CO generated at that same time to establish the ratio. An excellent indication of abnormally high temperatures and rapidly deteriorating cellulose insulation is a CO2/CO under 5. If the ratio is 3 or under, severe and rapid deterioration of cellulose is certainly occurring. In addition to DGAs, perform the Furans test. Extreme overheating from loss of cooling or plugged oil passages will produce a CO2/CO ratio around 2 or 3 along with increasing Furans. If this is found, de-energization and internal inspection is recommended, the transformer is in imminent danger of failure.

Table shown on Picture 1 is adapted from IEC 60599. Some of the wording has been changed to reflect American language usage rather than European.

Picture 1: Typical Faults in Power Transformers


Notes:

1. X wax formation comes from Paraffinic oils (paraffin based). These are not used in transformers at present in the United States but are predominate in Europe.
2. The last overheating problem in the table says “over 700 °C.” Recent laboratory discoveries have found that acetylene can be produced in trace amounts at 500 °C, which is not reflected in this table. We have several transformers that show trace amounts of acetylene that probably are not active arcing but are the result of high-temperature thermal faults, as in the example. It may also be the result of one arc, due to a nearby lightning strike or voltage surge.
3. A bad connection at the bottom of a bushing can be confirmed by comparing infrared scans of the top of the bushing with a sister bushing. When loaded, heat from a poor connection at the bottom will migrate to the top of the bushing, which will display a markedly higher temperature. If the top connection is checked and found tight, the problem is probably a bad connection at the bottom of the bushing.