Building a Transformer

Building a Transformer

Subcomponents for Key Element 1: Dissolved Gas-in-oil Analysis 

Gases dissolve in oil because of excessive electrical and/or thermal energy breaks off fragments of the molecules that make up the insulation system. Even under normal operating conditions the insulation system undergoes some electrical and heat stress. But, under fault conditions, the stress substantially increases. If the energy is concentrated in a small area, the temperature of that small area dramatically increases. The fault temperature and the gas formation rate are illustrated in Figure 1 below. Although this chart is an approximation, it can be seen that faults in a transformer are temperature related and can be categorized using empirical and statistical analysis. The category can give a clue to what corrective actions are needed or estimate the severity of the fault. It may suggest more frequent sampling, infrared scan, or de-energizing the transformer to perform a full battery of electrical test. These test results along with any follow-up investigations are aggerated and produce the first element for the Transformer.

At the utility, the process begins by taking a syringe sample from each transformer periodically. DGA samples are collected in 50cc clear glass syringes with matching barrels and cylinders. The matching barrels and cylinders minimize the loss of the gas molecules. Also, proper flushing of the sample port and syringe is vital to obtaining a representative sample. Immediately after taking the sample, the small air bubbles should be purged from the syringe. Next, return the syringe to its protected box to avoid physical damage and prevent the sun’s ultraviolet light from reaching the sample. Only one syringe is needed per transformer. Samples are sent to the lab where the gases are extracted and analyzed in a gas chromatograph (GC). The GC reports the various gases in ppm. The fault gases fall into three groups which are “arcing”, “hot metal” and “insulation breakdown”. The atmospheric gases of oxygen and nitrogen are also reported by the GC.

The arcing gas is acetylene(C2H2). Acetylene can be generated in the oil starting at 500oC because of extremely overheated metal in contact with the oil. But, at temperatures greater than 700oC, which are associated with an arc, there is a sharp increase in the production rate. Acetylene produced by an arc is the most serious condition that can be found inside a transformer and should be investigated immediately. The investigation starts with looking at the ppm value of hydrogen relative to the acetylene. An arc always produces hydrogen along with acetylene. In the case where the hydrogen level is low (less than, the fault occurred in the past and is not on going. This is known because hydrogen is the least soluble gas in the oil which is around 7% at 25oC. This allows hydrogen to escape from the oil very easily after an event that produced an arc. Acetylene is the most soluble gas which is 400% at 25oC. Acetylene will tend to stay in the oil long after a fault occurs. The solubilities of other fault gases fall in between these solubilities. It can be concluded that if hydrogen is present along with other fault gases and hydrogen is increasing over time, a fault is active and growing. The presence of arcing should be taken very seriously and swift action should follow. Actions should include a confirmation sample, infrared scan, visual inspection, review of the most recent operating events, and a review of the most recent electrical test data. Always use caution on all activities that involve being near a transformer that is suspect.

The “hot metal” gases are methane (CH4), ethane (C2H6), and ethylene (C2H4). If one of the gases exceeds the limit, then the data indicates that hot metal is in contact with the oil. Possible locations include the following areas: flux shield, core steal, tap changer contacts, and bolted/crimped connections. In some cases, the faults may be located in the auxiliary equipment inside the transformer, such as a current transformer or pump. The severity of the overheating can be determined by the amount, rate of increase over time and the ratio of the individual fault gases. 

The “insulation breakdown” gases are carbon monoxide (CO) and carbon dioxide (CO2). These gases are generated when the paper, pressboard or wood are broken down [5]. The insulation system relates to the transformer the same way the skeleton relates to the human body. If one of the gases is greater than the IEEE C57.104 – condition 1 limit and Carbon Dioxide to Carbon Monoxide ratio generally falls greater than 10 to 1 or less than 3 to 1, the data indicates the cellulose insulation is breaking down. The damage of the overheating can be determined by the amount, rate of increase, and the ratio of the two gases. Ratios over 10 to 1 occur when there is general overheating because of overloading or insufficient cooling. Ratios below 3 to 1 are typically when the cellulose is in contact with hot metal or associated with an area of arcing. Trending the ratio over time speaks volumes about the longevity of the transformer especially if the ratio is greater than 10 to 1 and constantly increasing. Damage to the insulation system is more serious because if it is left uncorrected, it becomes harder to correct. Thus, the life of the transformer will be shortened. It has been said by many that “the life of the insulation is the life of the transformer.”