Analyzing the Color of Transformer Oil Using Spectrophotometric Instrumentation

color of transformer oil
The color and clarity of transformer oil can tell electrical engineers a great deal about the oil’s age and quality. Image Source: Shutterstock user ifong

An engineer working for a power utility company is asked to check whether the electrical transformers in the field are working properly. Rather than looking closely at the transformers’ hardware or performing a test run of the machine, the engineer first looks at the color and clarity of the transformer oil being used. Why? Because the color of the transformer oil can tell the engineer a great deal about how well the transformer is currently operating and detect even the smallest signs of trouble before the device is powered on. Oil tests are a fast, simple, and inexpensive method for detecting operational problems in a transformer; it’s similar to checking a car engine’s oil every month or so to ensure that the engine is working properly.

Many power utility companies frequently test the color and quality of the transformer oils they use because high-quality oils can protect their transformers from damage. In fact, transformer oil tests detect an estimated 70% of incipient failures in transformers.1 Using UV-Vis spectrophotometry, engineers and on-site operations managers can quickly test the color of transformer oil to ensure that it meets exacting standards. By performing these tests periodically, either on-site or under lab conditions, power utility companies and other industries that rely on transformers can ensure that all of their equipment is operating at peak efficiency.

petroleum factory
Certain types of transformer oils precipitate sediment in the bottom of the transformer as the oil ages. This is why some utility companies have on-site oil filters to remove impurities in the oil. Image Source: Pexels user Scott Webb

Color is a Sign of Oil Age and Purity

Transformer oil (or insulating oil) is used to keep an electrical power transformer cool during normal operations. Because the voltage used in electrical transformers is so high, the resulting heat can damage the transformer’s core and winding.2 The oil serves as a protective liquid barrier for the transformer’s core, allowing electricity to pass through without overheating the core. These oils also prevent oxidation of the winding’s cellulose insulation by limiting the exposure of oxygen in the system. However, although transformer oil plays a major role in the function of an electrical transformer, the oil can also damage equipment if it’s not filtered properly or changed frequently enough.

Darkened coloration is the first sign that transformer oil needs to be changed or filtered. As the oil ages, it slowly changes from a pale yellow, transparent color to a deep brown, nearly opaque shade. Most transformer oils need to be filtered or changed at least every six months in order to prevent damage to the transformer, and testing the color of the oil is one of the easiest methods to determine exactly how old the oil is and whether it will still work effectively. Generally, oil that is yellow, orange, or even somewhat red in color is fresher and will work as intended. As oil ages, it transforms to a brown or black color and its efficacy diminishes. If engineers or on-site managers fail to detect these important color changes, the transformer may be more prone to problems like arcing, corona discharge, overheating, or decreased insulation strength.3 This is why testing for color is an essential step to maintaining a fully-operational electrical transformer.

Without careful oil color analysis, a transformer may experience one of the many problems listed above, which in turn could disrupt normal operations and lead to greater maintenance costs. It is usually much less expensive to periodically test for oil color consistency than it is to repair a transformer after it has failed due to overheating or arcing. Moreover, it takes time to repair the equipment after it has already been damaged, which may lead to delays in service. By testing your transformer oil color and standardizing your color measurement protocol, you may be able to prevent these problems before they become more costly.

transparent oil color measurement
The darker and more opaque transformer oil is, the more likely it is to disrupt a transformer’s normal operations. Image Source: Pixabay user Angelorosa

Analyzing the Color of Transformer Oil Using Spectrophotometry

Spectrophotometric color measurement is already recognized as a highly accurate method for testing the color and purity of transformer oil.4 According to a 2014 study conducted by the National Institute of Technology in India, UV-Vis spectrophotometers are an effective tool for analyzing the deterioration rate of transformer oils due to aging or thermal stress. Researchers who conducted the study found that spectrophotometers were able to accurately detect the color of oil that had aged naturally in the field, the color of laboratory-aged oil, and the color of fresh oil. The team of researchers confirmed that as the color of the oil changed (becoming darker), the proportion of aromatic hydrocarbons in the oil increased, meaning that the oil wouldn’t be as effective as an insulator in the field. In other words, color was directly correlated with oil efficacy, allowing for important insight into transformer function if analyzed with the precision afforded by spectrophotometric instrumentation.

Because transformer oil is distilled from crude oil, you can use the Saybolt or ASTM color scales to accurately detect the color of the oil and determine whether it falls within a safe tolerance. Spectrophotometers are capable of quantifying variations in color and can match them to the numbers on both of these color scales, giving you a clear idea of the oil’s age and quality. However, the type of spectrophotometer you choose will depend on your industry’s needs. For example, spectrophotometers in transmittance mode can measure the color of transparent and translucent oils, but this may not be the most effective tool when you’re working with transformer oil, as it ranges from transparent to opaque, depending on the oil’s age. A spectrophotometer that can work in both transmittance and reflectance mode is a better choice measuring the color of transformer oils, as the instruments can measure the oil at every stage of its lifecycle, from the clear, pale yellow of fresh oil to the dark, opaque brown of aged oil.

With advances in spectrophotometric technology, accurate color measurement of transformer oils and other crude oil products is a simple, streamlined process that easily be performed by operators. Using these tools, you can consistently analyze the color of your oils to keep your transformers working at peak efficiency, potentially extending the life of the transformers, preventing costly repairs, and avoiding inconvenient downtime.

HunterLab Reliability

For more than 60 years, HunterLab has worked closely with oil and chemical industries to refine their color measurement protocols and detect color change with extraordinary precision. Using state-of-the-art technology, including advanced color measurement software that can compare data to well-established industry color scale standards, our spectrophotometers represent the highest level of color analysis available. Contact us today to find out more about our renowned instruments and let us help you select the right tools for your needs.
Perfect Your Summertime Clothing Colors Using Instruments From HunterLab

  1. “Insulating Oil Decaying Assessment”, October 20, 2013,
  2. “Transformer Insulating Oil and Types of Transformer Oil”,
  3. “Transformer Oil Dehydration and Process”, April 16, 2016,
  4. “Condition Assessment of Transformer Oil Using UV-Visible Spectroscopy”,
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