Key Gas Method

The key gas method of interpreting DGA is set forth in IEEE. Key gases formed by degradation of oil and paper insulation are hydrogen (H2), methane (CH4), ethane (C2H6), ethylene (C2H4), acetylene (C2H2), carbon monoxide (CO), and oxygen (O2). Except for carbon monoxide and oxygen, all these gases are formed from the degradation of the oil itself. Carbon monoxide, carbon dioxide (CO2), and oxygen are formed from degradation of cellulose (paper) insulation. Carbon dioxide, oxygen, nitrogen (N2), and moisture can also be absorbed from the air, if there is an oil/air interface or if there is a leak in the tank. Some of our transformers have a pressurized nitrogen blanket above the oil and, in these cases, nitrogen may be near saturation (see table on Picture 1). Gas type and amounts are determined by where the fault occurs in the transformer and the severity and energy of the event. Events range from low energy events, such as partial discharge, producing hydrogen and trace amounts of methane and ethane, to very high energy sustained arcing, capable of generating all the gases, including acetylene, requiring the most energy.

Picture 1: Dissolved Key Gas Concentration Limits (ppm)

CAUTION: Transformers generate some combustible gases from normal operation, and condition numbers for dissolved gases given in IEEE C57-104-1991™ (table on Picture 1 above) are extremely conservative. Transformers can operate safely with individual gases in Condition 4 with no problems, provided they are stable and gases are not increasing or are increasing very slowly. If TDCG and individual gases are significantly increasing (more than 30 ppm per day [ppm/day]), an active fault is in progress. The transformer should be de-energized when Condition 4 levels are reached.

Four-Condition DGA Guide (IEEE C57-104)

A four-condition DGA guide to classify risks to transformers with no previous problems has been published in the Standard IEEE Standard C57-104™. The guide uses combinations of individual gases and total combustible gas concentration as indicators. It is not universally accepted and is only one of the tools used to evaluate dissolved gas in transformers. The four IEEE® conditions are defined below, and gas levels are in table on Picture 1.

Condition 1: Total dissolved combustible gas (TDCG) below this level indicates the transformer is operating satisfactorily. Any individual combustible gas exceeding specified levels in table on Picture 1 should have additional investigation.

Condition 2: TDCG within this range indicates greater than normal combustible gas level. Any individual combustible gas exceeding specified levels in table on Picture 1 should have additional investigation. A fault may be present. Take DGA samples at least often enough to calculate the amount of gas generation per day for each gas. (See table on Picture 2 for recommended sampling frequency and actions.)

Condition 3: TDCG within this range indicates a high level of decomposition of cellulose insulation and/or oil. Any individual combustible gas exceeding specified levels in table on Picture 1 in Transformer Oils should have additional investigation. A fault or faults are probably present. Take DGA samples at least often enough to calculate the amount of gas generation per day for each gas (see table on Picture 2).

Condition 4: TDCG within this range indicates excessive decomposition of cellulose insulation and/or oil. Continued operation could result in failure of the transformer (see table on Picture 2).

A sudden increase in key gases and the rate of gas production is more important in evaluating a transformer than the accumulated amount of gas. One very important consideration is acetylene (C2H2). Generation of any amount of this gas above a few ppm indicates high-energy arcing. Trace amounts (a few ppm) can be generated by a very hot thermal fault (500 EC or higher). A one-time arc, caused by a nearby lightning strike or a high-voltage surge, can also generate a small amount of C2H2. If C2H2 is found in the DGA, oil samples should be taken weekly, or even daily, to determine if additional C2H2 is being generated. If no additional acetylene is found and the level is below the IEEE® Condition 4, the transformer may continue in service. However, if acetylene continues to increase, the transformer has an active high-energy internal arc and should be taken out of service immediately. Further operation is extremely hazardous and may result in explosive catastrophic failure of the tank, spreading flaming oil over a large area.

Picture 2: Actions Based on Dissolved Combustible Gas


NOTES:

1. Either the highest condition based on individual combustible gas or TDCG can determine the condition (1, 2, 3, or 4) of the transformer. For example, if the TDCG is between 1,941 ppm and 2,630 ppm, this indicates Condition 3. However, if hydrogen is greater than 1,800 ppm, the transformer is in Condition 4, as shown in table on Picture 1.
2. When the table says Adetermine load dependence,” this means to try to find out if the gas generation rate in ppm/day goes up and down with the load. The transformer may be overloaded or have a cooling problem. Take oil samples every time the load changes; if load changes are too frequent, this may not be possible.
3. To get the TDCG generation rate, divide the change in TDCG by the number of days between samples that the transformer has been loaded. Down days should not be included. The individual gas generation rate in ppm/day is determined by the same method.

Table on Picture 2 assumes that no previous DGA tests were performed on the transformer or that no recent history exists. If a previous DGA exists, it should be reviewed to determine if the situation is stable (gases are not increasing significantly) or unstable (gases are increasing significantly).
Look at the DGA and see if the transformer is in Condition 1, 2, 3, or 4. The condition for a particular transformer is determined by finding the highest level for any individual gas or by using the TDCG. Either the individual gas or the TDCG can give the transformer a higher condition number, which means it is at greater risk. If the TDCG number shows the transformer in Condition 3 and an individual gas shows the transformer in Condition 4, the transformer is in Condition 4. Always be conservative and assume the worst until proven otherwise.