When selecting a transformer for an indoor application, there are two basic categories to evaluate: liquid-filled (cooled by oil) and dry-type (cooled by air).
In the past, liquid-filled transformers used mineral oil as the cooling element, which made them flammable and more hazardous than dry-type transformers. Containment also was a safety concern in the case of an oil spill.
These hazards are no longer an issue because of advancements in liquid-filled transformers. Vegetable-based oil, which has a high flash point, is used instead of mineral oil, therefore reducing the risk of flammability. New liquid-filled transformers also can be specified with built-in containment pans, mitigating the risk of leakage.
The Benefits of Liquid-Filled Transformers
Liquid-filled transformers use vegetable-based oil to keep the coil cool. Because oil is a more efficient coolant than air, liquid-filled transformers operate at a lower temperature. This means they produce less heat, take up less space and have a longer overall life span than dry-type transformers.
“The liquid-filled transformers are more efficient, so for every kilowatt the transformer is on, you’re saving money,” explained RTM Principal Tim Larson, P.E.
Liquid-filled transformers also allow for better maintenance over time. Preventive maintenance for this type of unit involves health and longevity analyses through samples of the vegetable oil. With a dry-type transformer, there is no way to easily test its health, and invasive procedures often can lead to omitting maintenance altogether. These undesirable characteristics result in decreased efficiency or unforeseen failures.
The Best Application for Each Transformer
The size of the application is the main factor in deciding which type of transformer is necessary. Dry-type transformers are useful for smaller applications—for use in facilities requiring step down transformation of loads 750 kVA or less. Liquid-filled transformers are generally manufactured for loads around 1000 kVA and higher.
For example, a dry-type transformer would be a good solution for an office floor with a small panel board system. It would be too small of a load to realize the efficiencies obtained from a liquid-filled transformer.
“I still see a lot of dry-type transformers being specified, and I think it’s because the advancements made in liquid-filled transformers are being overlooked,” said Larson. “For us, typically for applications of 750 kVA and less, I’m looking at dry-type transformers. Anything larger than that, my first thought is liquid.”
RTM was the electrical engineer on a project at Elmbrook Memorial Hospital in Brookfield, Wisconsin, in which the goal was to proactively upgrade an existing dry-type unit substation that was nearing the end of its useful life. RTM recommended replacing the existing unit substation with a long-term solution of a liquid-filled transformer.
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