Dga Testing Of Transformers | Part 1

February 11, 2014

The topic of dissolved gas analysis (DGA) and testing for oil-filled transformers is one that’s inextricably linked to any routine maintenance program for those who employ these types of transformers in the field-including electric utilities, electrical contractors, and industries that own their own transformers. The reason behind DGA is simple: Combustible gasses are generated as the oil-filled transformer experiences thermal and electrical stresses. When you have internal thermal conditions, such as hot spots inside an oil-filled transformer, the extreme temperatures can produce certain types of combustible gasses. A different type of combustible gas is generated from electrical stresses. The thermal and electrical stresses may not be apparent until the resulting damage to insulation or internal components results in the failure of the oil-filled transformer. Obviously, a technician can’t look inside the transformer to determine what damage has been done because it’s a sealed box, but the different combustible gasses present in the oil-filled transformer can be analyzed with a small sample of the internal oil (roughly 40 milliliters) taken by a special syringe. These gasses include the following:

 

  • Hydrogen (H2)
  • Oxygen (O2)
  • Nitrogen (N2)
  • Methane (CH4)
  • Carbon Monoxide (CO)
  • Ethane (C2H6)
  • Carbon Dioxide (CO2)
  • Ethylene (C2H4)
  • Acetylene (C2H2)

 

After drawing, testing, and analyzing the oil (a process called “gas chromatography” which separates and analyzes compounds), the presence of these nine gasses can be used to determine if thermal or electrical stresses are present within the transformer. The gas chromatography method is extremely sensitive and measures representative fault gasses down to the parts-per-million range.

The various gasses found during a routine DGA can be in varying amounts. The presence of each, and the amount, give us a clear picture of what may be going wrong with the transformer. A build-up of hydrogen, acetylene, methane and ethylene, for example, are signs of extremely high internal temperatures and internal arcing (often caused by high voltage faults to ground or shorted turns within the transformer’s windings) and this requires immediate corrective action.

As stated earlier, DGA is part of a routine maintenance program for oil-filled transformers. It is typically recommended that the oil inside transformers be sampled on an annual basis. If the sampling and testing shows signs of an internal problem, there may be a need to resample oil from the transformer in question to monitor the rate of increases of the representative combustible gasses. The DGA sample and analysis can detect hot spots inside the transformer above 110° where cellulose insulation will start to break down. The rate and ratio of combustible gasses being generated will vary as with the severity of the electrical stresses or as the temperature of the hot spot increases. However, just analyzing the type of combustible gasses that are being generated doesn’t always provide a definitive cause for the internal problem. Further testing and inspection of the oil-filled transformer is often required to determine the cause and a plan for corrective action.