July 2017, by Elisa Zambrano Gómez, Product Technology Leader
Just as blood test analysis are used to identify health issues, dissolved gas analysis (DGA) in the transformer’s oil can be useful for the early detection of possible failures before they appear.
The main cause the combustible gases are produced and confined inside the tank is because of the transformer’s insulation materials degrade, a trace of these gases is dissolved in the oil and can be related to specific dangerous operating conditions that may cause a major failure in the transformer. Thus, the continuous study of this gas content can help the user prevent any equipment to go out of order.
The method to perform a DGA test is extracting an oil sample during the routine maintenance, following the standard procedure to avoid altering results. Therefore, gases are extracted from the sample and a gas chromatography is performed, where the volume of each gas is detected in parts per million (ppm).
Each gas can be related to a specific internal issue, this tool helps identify each one of them without the need of having to de-energize and open the transformer.
The 9 main gases that could be found in this test are the following:
· Atmospheric gases: Nitrogen and Oxygen
· Carbon Oxides: Monoxide and Carbon Dioxide
· Hydrocarbons: Acetylene, Ethylene, Methane, Ethane
· Other: Hydrogen
Each gas concentration has a direct correlation to the temperature reached, as well as the oil volume at that temperature. Therefore, each gas concentration is closely related to the failure’s severity.
The following failures can be detected in transformers using mineral oil.
· Cellulose insulations degraded by overheating generate high levels of carbon monoxide and carbon dioxide.
· Overheating in oil results in the production of hydrocarbons.
· Partial discharges can be detected when high levels of hydrogen are present.
· Arcing between energized components can be detected when acetylene gas is found.
· Issues with the airtight sealing of the tank can be evidenced by the presence of nitrogen and oxygen.
However, to further understand each failure, several models are used such as Duval Triangle (Figure 1), Rogers Ratios and Doernenberg Ratios. These tools correlate hydrocarbon concentrations to detect the overtemperature range the oil has been exposed to and the intensity of the discharges or partial discharges.
PD= Partial Discharge
T1= Overtemperature under 300°C
T2= Overtemperature between 300 y 700°C
T3=Overtemperature over 700°C
D1= Low energy discharge (spark)
D2= High Energy discharge (arc)
DT= Combination of electric and thermal failures.
It is important to know low gas concentrations can be normal and may had been introduced during the transformer’s manufacturing process such as welding of the tank. Daily regular operations of loadbreak switches and fuses or the activation of other protective equipment may also introduce low levels of gases. Therefore, a single test does not provide sufficient evidence of an issue. The test needs to be performed in several occasions during a time lapse to confirm any problem. The rate of gas change is directly related to its severity.
It is recommended to practice this test once a year, especially in transformers that supply energy for a critical application. If high levels of a gas are found, it is recommended to repeat this test every 3 o 6 months according to its severity.
This method cannot identify the specific location of the failure, and cannot be used if the oil in the unit has been changed or processed. Nonetheless, it is a preventive maintenance test with great use in preventing a transformer from ceasing operation in an inconvenient manner.
The complete guide for a correct diagnose based on the gas volume present, the rate of change and correlations among gases are found in the standards: IEC 60599 and ANSI/IEEE C57.104.