4.4 Transformer Oils 

4.4.1 Transformer Oil Functions. Transformer oils perform at least four functions for the transformer. Oil provides insulation, provides cooling, and helps extinguish arcs. Oil also dissolves gases generated by oil degradation, moisture and gas from cellulose insulation, deterioration, and gases and moisture from whatever atmosphere the oil is exposed to. Close observation of dissolved gases in the oil, and other oil properties, provides the most valuable information about transformer health. Looking for trends by comparing information provided in several DGAs, and understanding its meaning, is the most important transformer diagnostic tool. 

4.4.2 Dissolved Gas Analysis. After 1 month of service and once each year, and more often if a problem is encountered, do a DGA. This is by far the most important tool for determining the health of a transformer. 

Caution: DGA is unreliable if the transformer is de-energized and has cooled, if the transformer is new, or if it has had less than 1 to 2 weeks of continuous service after oil processing. 

The purpose of this section is to provide guidance in interpreting DGA and to suggest actions based on the analysis. There are no “quick and sure” easy answers when dealing with transformers. Transformers are very complex, very expensive, and very important to Reclamation; and each one is different. Decisions must be based on experienced judgment founded on all available data and consultation with experienced people. Along with thorough periodic inspections covered earlier, the most important key to transformer life is periodic DGA and proper interpretation. Each DGA must be compared to prior DGAs so that trends can be recognized and rates of gas generation established. 

Although examples will be presented later, there is no universally accepted means for interpreting DGA [15]. Transformers are very complex. Aging, chemical actions and reactions, electric fields, magnetic fields, thermal contraction and expansion, load variations, gravity, and other forces all interact inside the tank. Externally, through-faults, voltage surges, wide ambient temperature changes, and other forces such as the earth’s magnetic field and gravity affect the transformer. There are few if any “cut and dried” DGA interpretations; even experts disagree. Consultation with others, experience, study, comparing earlier DGA’s, keeping accurate records of a transformer’s history, and noting information found when a transformer is disassembled will increase expertise and provide life extension to this critical equipment. 

Keeping accurate records of each individual transformer is paramount. If a prior through-fault, overload, cooling problem, or nearby lightning strike has occurred, this information is extremely valuable when trying to determine what is going on inside the transformer. Baseline transformer test information should be established when the transformer is new or as soon as possible thereafter. This must include DGA, Doble, and other test results, discussed in the testing section, “4.7 Transformer Testing.” 

4.4.3 Key Gas Method of interpreting DGA is set forth in IEEE [11]. Key gases formed by degradation of oil and paper insulation are hydrogen (H2), methane (CH4), ethane (C2 H6), ethylene (C2 H4), acetylene (C2 H2), 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 a 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 4.) 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, which produces hydrogen and trace amounts of methane and ethane, to very high energy sustained arcing, capable of generating all the gases including acetylene, which requires the most energy. 

4.4.4 Transformer Diagnosis Using Individual and Total Dissolved Key Gas Concentrations. A four-condition, DGA guide to classify risks to transformers with no previous problems has been developed by the IEEE [11]. The guide uses combinations of individual gases and total combustible gas concentration. This guide is not universally accepted and is only one of the tools used to evaluate transformers. The four conditions are defined below: 

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 4 should have additional investigation. 

Table 4.—Dissolved Key Gas Concentration Limits in Parts Per Million (ppm) 

Status H2 CH4 C2 H2 C2H4 C2H6 CO CO2 1 TDCG 

Condition 1 100 120 35 50 65 350 2,500 720 

Condition 2 101-700 121-400 36-50 51-100 66-100 351-570 2,500-4,000 721-1,920 

Condition 3 701-1,800 401-1,000 51-80 101-200 101-150 571-1,400 4,001-10,000 1,921-4,630 

Condition 4 >1,800 >1,000 >80 >200 >150 >1,400 >10,000 >4,630 

1 CO2 is not included in adding the numbers for TDCG because it is not a combustible gas. 

Condition 2: TDCG within this range indicates greater than normal combustible gas level. Any individual combustible gas exceeding specified levels in table 4 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 5 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 4 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 5.) 

Condition 4: TDCG within this range indicates excessive decomposition of cellulose insulation and/or oil. Continued operation could result in failure of the transformer (table 5). 

Condition numbers for dissolved gases given in IEEE C-57-104-1991 (table 4) are extremely conservative. We have transformers that have operated safely with individual gases in Condition 4 with no problems; however, they are stable and gases are not increasing, or are increasing very slowly. If TDCG and individual gases are increasing significantly (more than 30 ppm/day), the fault is active and the transformer should be de-energized when Condition 4 levels are reached. 

A sudden increase in key gases and the rate of gas production is more important in evaluating a transformer than the amount of gas. One exception is acetylene (C2H2). The 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 °C). A one-time arc caused by a nearby lightning strike or a high-voltage surge can generate acetylene. If C2H2 is found in the DGA, oil samples should be taken weekly to determine if additional acetylene 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 

Table 5.—Actions Based on Dissolved Combustible Gas 

Conditions 

TDCG Level  or Highest Individual Gas (See Table 4)  TDCG Generation Rates (PPM/Day) 

Sampling Intervals and Operating Actions for Gas Generation Rates 

Sampling Interval Operating Procedures 

#720 ppm of TDCG or highest condition based on individual 

<10 Annually: 6mo for EHV trans 

Continue normal operation. 

Condition 1 gas from table 4 

10-30 Quarterly 

>30 Monthly Exercise caution.  Analyze individual gases to find cause. Determine load dependence. 

Condition 2 

721-1,920 ppm of TDCG or highest condition based on individual 

<10 Quarterly 

Exercise caution.  Analyze individual gases to find cause. Determine load dependence. 

10-30 Monthly gas from table 4 

>30 Monthly 

1,941-2,630 ppm of <10 Monthly Exercise extreme caution. TDCG or Analyze individual gases to find cause. Plan outage. Call manufacturer and other consultants for advice. Condition 3 highest condition 10-30 Weekly gas from table 4 based on individual >30 Weekly 

>4,630 ppm of <10 Weekly Exercise extreme caution. TDCG or Analyze individual gases to find cause. Plan outage. Call manufacturer and other consultants for advice. 

Condition 4 based on individual gas from table 4 highest condition 

10-30 Daily 

>30 Daily Consider removal from service. Call manufacturer and other consultants for advice. 

NOTES: 1.  Either the Highest Condition Based on Individual Gas or Total Dissolved Combustible Gas can determine the condition (1,2,3, or 4) of the transformer [11]. 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 4.. 2. When the table says “determine load dependence,” this means, if possible, find out if the gas generation rate in ppm/day goes up and down with load. Perhaps the transformer is overloaded.  Take oil samples every time the load changes; if load changes are too frequent, this may not be possible. 3. To get 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 ppm/day is determined by the same method. 

internal arc and should be taken out of service. Further operation is extremely hazardous and may result in catastrophic failure. Operating a transformer with an active high energy arc is extremely hazardous. 

Table 4 assumes that no previous DGA tests have been made 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). Deciding whether gases are increasing significantly depends on your particular transformer. 

Compare the current DGA to older DGAs. If the production rate (ppm/day) of any one of the key gases and/or TDCG (ppm) has suddenly gone up, gases are probably increasing significantly. Refer to table 5, which gives suggested actions based on total amount of gas in ppm and rate of gas production in ppm/day. Before going to table 5, determine transformer status from table 4; that is, 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 [11]. 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. Sampling intervals and recommended actions. When sudden increases occur in dissolved gases, the procedures recommended in table 5 should be followed. Table 5 is paraphrased from table 3 in IEEE C57.104-1991. To make it easier to read, the order has been reversed with Condition 1 (lowest risk transformer) at the top and Condition 4 (highest risk) at the bottom. The table indicates the recommended sampling intervals and actions for various levels of TDCG in ppm. An increasing gas generation rate indicates a problem of increasing severity; therefore, as the generation rate (ppm/day) increases, a shorter sampling interval is recommended. (See table 5.) Some information has been added to the table from IEEE C57-104-1991; that is, inferred from the text. To see the exact table, refer to the IEEE Standard. If it can be determined what is causing gassing and the risk can be assessed, the sampling interval may be extended. For example, if the core is tested with a  megohmmeter and an additional core ground is found, even though table 5 may  recommend a monthly sampling interval, an operator may choose to lengthen the  sampling interval, because the source of the gassing and generation rate is known. A decision should never be made on the basis of just one DGA. It is very easy to contaminate the sample by accidentally exposing it to air. Mislabeling a sample is also a common cause of error. Mislabeling could occur when the sample is taken, or it could be accidentally contaminated or mishandled at the laboratory. Mishandling may allow some gases to escape to the atmosphere and other gases, such as oxygen, nitrogen, and carbon dioxide, to migrate from the atmosphere into the sample. If you notice a transformer problem from the DGA, the first thing to do is take another sample for comparison. In the gas generation chart (figure 18) [13, 16] and discussion below, please  note that temperatures at which gases form are only approximate. The figure is  not drawn to scale and is only for purposes of illustrating temperature relationships, gas types, and quantities. These relationships represent what generally has been proven in controlled laboratory conditions using a mass