Oct 2020

Transformer Oil Testing

Transformers can be either dry type or fluid filled. In this article, we will talk about fluid filled  transformers and, in particular, the types of testing that should be conducted on the fluid or oil  inside the transformer. If you own the transformers in your facility, it is important to include  them in the preventive maintenance programs for your plant. Even if you do not own the  transformers, it is still wise to make sure they are being properly tested and maintained. If a  transformer fails, it can take several days or longer to replace resulting in a costly interruption to  business operations. Sometimes, a transformer can fail catastrophically resulting in an explosion  and fire that can damage adjacent buildings and equipment.

Transformer Oil Testing is a relatively simple and effective tool for determining the condition of  the transformer fluid. Results from such tests help guide planning for future maintenance on a  transformer and help determine when replacement of a transformer may become necessary.

Transformer oil is used to insulate, stop arcing and corona discharge, and to dissipate the heat of  the transformer (i.e. act as a coolant). Transformer oil helps preserve the transformer’s core and  windings as these are fully immersed in the oil. It also helps prevent oxidation of the cellulose  paper insulation in the windings. Transformer fluids can be Mineral Oil (Paraffin and Naphtha  based), Silicone Oil, and Natural or Synthetic Esters. The recommended testing for these  different types of fluids can vary, but all should be tested in some form or other on a regular  basis.

The most common approach to testing is to have a trained electrician or contractor pull a sample of oil from the transformer and send it to a testing laboratory that specializes in transformer oil  testing. An important aspect of the testing process is interpretation of the results, so be sure lab  or electrician can assist with this. Interpretation is often based more on trends rather than absolute results of the tests. The following is a common list of common tests for transformer oil.  The number next to each test is the ASTM (American Society for Testing and Materials)  reference. 

Color (D1500): Used to observe darkening of the oil by comparing it to previous samples of oil  from the same transformer. 

Dielectric breakdown voltage (D877, D1816): Measures the voltage at which the oil fails  electrically, which is indicative of the amount of contaminant (usually moisture) in the oil. 

Interfacial tension (D971): Measures the presence of soluble contaminants and oxidation  products in transformer oil. A decreasing value indicates an increase in contaminants and/or  oxidation products within the oil. 

Karl Fischer (moisture) (D1533): Measured in parts per million (ppm) using the weight of  moisture divided by the weight of oil. Moisture content in oil lowers the insulating dielectric  strength.

Liquid power factor (Dissipation factor) (D924): Indicates the dielectric losses of the oil, or  energy that is dissipated as heat. Useful for measuring changes within the insulating oil resulting  from contamination or deterioration.  

Neutralization number (D974): New transformer oils contain practically no acids. The acidity  test measures the content of acids formed by oxidation and contaminates. 

Oxidation inhibitor content (D2668): Added to some oils to help protect the paper insulation  from oxidation. 

Resistivity (D1169): Measures the electrical insulating properties of transformer oil.  

Dissolved gas analysis (D3612): Identifies various gas levels that are present in transformer  insulating oil. Different gasses dissolved in the oil indicate various types of thermal and  electrical stress occurring within the transformer. For Example: 

  • Corona discharge – Low energy discharges create methane and hydrogen and smaller  quantities of ethylene and ethane.
  • Arcing – Large amounts of hydrogen or acetylene or minor quantities of ethylene and  methane can be produced.
  • Overheated Cellulose – If cellulose is overheated, then it will produce carbon monoxide
  • Overheated Oil – Overheating oil will produce methane and ethylene (300ºF) or methane  and hydrogen (1,112ºF). Traces of acetylene might be created if the unit has electrical  contacts or if the problem is severe.

IEEE C57.104: IEEE Guide for the Interpretation of Gases Generated in Oil-Immersed  Transformers, provides information on gas levels for oil-filled transformers. Sometimes, the rate  of change in the gas levels detected is a more important indicator that the absolute level of gas  detected in the oil. It is important to maintain a history and compare the results from test to test.

Other tests that can sometimes be relevant include: 

  • Relative density (specific gravity) (D 1298, D1524) 
  • Pour Point (D97)
  • Flash Point (D92) 
  • Furanic Compounds (D5837) 
  • Dissolved Metals (D7151) 
  • Corrosive sulfur (D1275) 
  • Acid number (D664) 
  • Visual examination (D1524) 
  • Polychlorinated biphenyls (PCB) content (D4059) 

These additional tests might be recommended depending on the results of initial tests; or based  on the operating conditions or history of the transformer. For example, testing for PCBs can be  important if the transformer is older, especially if it contained PCB contaminated oil in the past.  PCBs can continue to leach out of cellulose insulation over time.

Testing frequency can be influenced by a number of different factors. Factors that might point to  more frequent testing would include:

  • Transformers in operating environments with high temperatures, high humidity or  substantial contaminants.
  • Transformers loaded near their maximum capacity. 
  • Transformers subject to high stresses such as an arc furnace transformer, or a transformer  that drives large motors which start and stop frequently. 

A starting point for testing frequency might be twice per year for transformers subject to one or  more of the factors listed above, and every two years for other transformers. Testing frequency  can be adjusted based on results of the previous tests. Other factors to consider are how critical  the transformer is to your operations, and how easy it would be to replace. 

Transformer Oil Testing is a key part of any maintenance program. Contact Risk Logic with any  questions you may have regarding Transform Oil Testing for your facility.