What Are the Best Transformer Oils?

Transformer oil serves as both insulators and coolants. However, like many other fluids, they degrade over time through a machine’s normal use. That’s why any preventive maintenance program should include checking the oil for sludge or signs of contamination. However, selecting the correct transformer fluid is also crucial for ensuring a machine operates as expected.

General Electric gets the credit for initially using petroleum-based oils as insulating liquids in transformers. That practice began in 1892, but commercial production occurred seven years later. That fluid was paraffinic-based mineral oil.

By 1930, polychlorinated biphenyl (PCB) came on the market as a transformer oil. That option had the desired heat-resistant and insulative properties. However, further investigations showed it was environmentally toxic and would build up in a person’s body over time. That meant a person could get poisoned by the substance simply by being exposed to it over prolonged periods.

Since then, significant progress has occurred to formulate transformer oils that perform as expected without preventable adverse effects to a machine, people, or the environment. Market research forecasts a $3 billion value for the global transformer oil sector by 2025.

The optimal kind depends on a user’s needs and the specific application, but these are some of the most widely used and newly available options.

Naphthenic Oil

Naphthenic transformer fluid comes from crude oil with very low levels of naturally occurring waxes or none. Naphthenic oil also has a low pour point. A liquid’s pour point is the temperature below which it loses the ability to flow freely when tipped out of a container.

Since naphthenic transformer fluid has a lower wax content than some other oils, it demonstrates a low pour point without requiring people to use additives to reduce it artificially. Other advantages are that the oil has a low viscosity and longer life expectancy. However, regarding the life expectancy, technicians should still inspect the oil at least annually.

Sludge is an ongoing problem with some transformer oils. However, naphthenic fluid has a moderately soluble sludge. That characteristic means it will not create deposits on windings and have a negative effect on the transformer’s cooling capabilities. Moreover, this benefit prevents temperature-rise problems in the transformer that could shorten the equipment’s lifespan.

Paraffinic Oil

Paraffinic transformer fluid also comes from crude oil, but it contains more wax compared to the naphthenic type. The increased amount of wax also raises the liquid’s pour point, meaning it may be necessary to reduce it with additives.

When an application calls for operating a transformer in frigid temperatures, water will naturally form as ice melts. However, if that water flows into a transformer’s electrode area, the liquid will cause a significant reduction in the oil’s dielectric strength or its breakdown voltage. A low breakdown voltage value is a sign of moisture or other conducting substances in the oil.

Paraffinic transformer oil has a lower density than naphthenic oil at cold temperatures. Relatedly, there is a significant difference in density between floating ice and paraffinic oil.  Thus, people often choose it for low-temperature applications where floating ice could interfere with a transformer’s operations.

This oil has high antioxidation stability, which is a characteristic reflecting its resistance to oxidation. This property results in the fluid offering a long service life during prolonged usage.

Ester-Based Transformer Oil

Despite progress made over the years, a downside associated with some of the most widely used transformer oils is that they harm the environment. Thus, people who use them must contain the liquid with troughs or similar methods. However, there’s a growing trend of using ester-based oils, which are often biodegradable.

Another advantage of esters is that they play a vital role in fire mitigation. Statistics indicate that more than 2.5 million transformers worldwide use natural ester oil instead of mineral oil. One of the reasons for that shift is that esters can operate at temperatures more than 30 degrees Celsius higher than mineral-based oils.

Some analysts believe the combination of warmer climates and the increasing tendency of transformers to become overloaded will cause more decision-makers to gravitate towards ester oils. A liquid’s fire point is the temperature at which it will create a sustained flame once ignited by an external ignition source. Natural ester has a fire point of 360 degrees Celsius, which is reportedly the highest of the alternative liquids used in transformers. Thus, choosing an ester-based oil could be a smart decision for preventing fires that quickly spread to the surrounding wilderness.

Another benefit of using ester fluids in transformers is that the decision allows for making substantially smaller equipment, such as which could provide backup power in emergencies. For example, these transformers may be up to one-third smaller than those using other types of oil.

Here are some of the ester-based oils worth considering:

Shell’s New Biodegradable Option

Shell recently introduced a sulfur-free biodegradable option. It prevents the corrosion that oils containing sulfur can cause. The new option, known as Shell Diala S5 BD, is a smart choice for projects with stringent environmental requirements that necessitate people being exceptionally careful to avoid and thoroughly clean up spills.

The fluid also has a low pour point of -51 degrees Celsius. That means it’s a good option for some of Earth’s coldest climates. While speaking about the new product, Ed van Schaik, product application specialist for Shell Diala Transformer Oils, said, “Shell believes that improving sustainability credentials does not mean sacrificing performance.”

He continued, “Shell Diala S5 BD supports transformer operators in overcoming the biggest challenges currently facing the industry by enabling their equipment to handle harsh cold climates and new environmental needs. By diminishing downtime and extending effective transformer life, Shell Diala S5 BD helps businesses lower total cost of ownership of equipment.”

Vegetable Oil

Vegetable oil is another transformer option that’s gaining popularity as a replacement for petroleum-based mineral oil. In one recent example of a large-scale project, Cargill Bioindustrial South Africa won the right to become the sole producer of an ester-based transformer fluid used in the transformers associated with the country’s electrical grid.

Besides biodegrading in less than a month, this vegetable oil is non-toxic and does not harm water or soil.

Renewable Ester-Based Oils

Companies are also developing renewable options for renewable ester-based transformer oils. These options are particularly attractive because they support organizations’ environmental sustainability efforts.

Tata Power Transmission, which handles about 70% of the power requirements in Mumbai, India, recently announced it would use a natural ester oil for its transformers. News about the development did not name any specific oil brands. However, it mentioned the company’s ongoing commitment to using renewable energy when possible. This innovation aligns with that aim.

Bio-Based Hydrocarbons

Swedish oil brand Nynas recently brought its 100% renewable oil to the market. However, this new product, branded as NYTRO BIO 300X, is neither a mineral oil nor an ester. Instead, it’s a bio-based hydrocarbon.

One of the product’s main advantages outside of the environmental benefits relates to its ability to minimize hot spots associated with a transformer’s windings. Controlling hot spots is crucial because failing to do so can degrade a transformer’s insulative lining.

Early tests showed that winding hot spots were 10 degrees cooler with this product compared to average mineral oils. Moreover, those areas 20 were degrees cooler than when operated with ester-based oils. When this product arrived on the market in 2019, it was the first of its kind. However, as people become more concerned about environmental sustainability, items in this category may become more widely available.

Sampling Transformer Oil

Ester-based oils are rapidly gaining momentum in the marketplace. Even so, more traditional options still hold considerable market share. Regardless of the specific oil type chosen for a transformer, following the recommended maintenance schedule goes a long way in helping the transformer and oil serve their purposes.

Transformer oil maintenance centers on taking periodic samples — usually every 6-12 months — to confirm it still has sufficient dielectric properties. Tests can also reveal water contamination and particulate matter.

Take the sample from the bottom of the transformer when the specific gravity of the fluid is less than 1.0. That’ll be the case when using mineral-based oils and synthetic products. Sourcing the fluid from the bottom is crucial for detecting water because that liquid will drop.

People should also avoid taking samples when environmental conditions are more than 70% relative humidity. Otherwise, they could mistakenly introduce moisture, causing an inaccurate sample. Similarly, it’s best to gather samples during non-windy conditions so blown particles don’t affect the results.

No Single Ideal Type of Transformer Fluid

This overview will get people familiar with some of the most common categories of transformer oil on the market today. The main takeaway is that people should not expect to come across one fluid that fits all of their needs for every application. Instead, they should consider their priorities and which oils match all or most of those.

For example, biodegradable oils may be the best option in an area where avoiding environmental harm is crucial. However, naphthenic possibilities may be more suitable when sludge is a significant concern. Taking care to weigh all the influential factors before choosing a type of oil will help people make the most appropriate decisions.