Differences between Oil Type and Dry Type Transformers.
- Grupo Edmar
- Sep 15
- 5 min read
September 15, 2025 - Differences between Oil-Type and Dry-Type Transformers
Differences between Oil Type and Dry Type Transformers.
In any electrical installation, the choice of transformer is one of the most important decisions. It's not just a technical issue; it also influences personal safety, budget, available space, and even the project's environmental impact. The two most common types are oil-filled transformers and dry-filled transformers , each with specific characteristics that make them more or less suitable depending on the case.
In this article, we'll detail the differences between them, how they perform in key aspects such as efficiency, maintenance, and safety, and in what situations it's best to choose one over the other. I'll also provide practical examples and advice based on the experience of Grupo Edmar , so you have clear and useful information before making a decision.
What is an oil-type transformer and what is a dry-type transformer?
An oil-filled transformer operates with an insulating liquid that serves a dual purpose: it helps cool the equipment and electrically insulate the windings. The most common oil is mineral oil, although more modern options exist, such as biodegradable vegetable oils. This type of transformer has been used for decades and remains very common in electrical substations and projects requiring large capacities.
In contrast, the dry-type transformer does not use any insulating liquid. Instead, it uses air or solid materials such as epoxy resins to insulate and protect its components. This makes it safer indoors and in locations where an oil spill would pose a risk.
For example, an oil-filled transformer may be seen in industrial parks or outdoor power stations, while a dry-type transformer is ideal in a hospital or shopping mall, where safety and cleanliness are a priority.
Performance and thermal efficiency.
The performance of a transformer is closely linked to how it handles heat.
Oil-filled transformers utilize fluid circulation to dissipate heat evenly. This allows them to operate more stably under overload or high-temperature conditions. Furthermore, by maintaining lower temperatures, their internal components typically last longer.
Dry-type transformers , lacking a coolant, rely on air circulation or forced ventilation systems. This means they can reach higher temperatures under heavy loads, which in some cases limits their use in high-power applications.
In high-demand applications—such as factories, mines, or power plants—oil-fired pumps are typically more efficient. In projects with more stable loads, such as an office building, dry-fired pumps work perfectly.
Safety and risks.
Safety is a critical point when choosing.
In oil-filled transformers , the main risk is fire due to leaks or overheating. Even small leaks can cause contamination or costly damage. Therefore, it is recommended to install containment trays, leak detection systems, and comply with environmental regulations.
In dry-type transformers , the risk of fire is drastically reduced because they do not contain flammable liquids. This makes them safer in locations where there are people, sensitive equipment, or enclosed spaces.
A clear example: in a hospital, a dry-type transformer is almost always chosen because it prioritizes the safety of patients and staff. In contrast, an open-air substation can operate with an oil-filled transformer without any problems.
Maintenance and Operating Costs.
There are major differences here that impact long-term costs.
Oil-filled transformers require periodic testing of the insulating fluid to verify its dielectric quality, moisture content, and the presence of dissolved gases. Leak monitoring, radiator cleaning, and, in some cases, oil replacement are also required. All of this requires specialized personnel and ongoing expenses.
Dry-type transformers , on the other hand, require less maintenance . Visual inspections, electrical tests, and cleaning are generally sufficient to prevent dust buildup. However, in humid environments, it is important to check the condition of the resin or insulation.
Although a dry-type transformer may be more expensive initially, over its lifetime it can be more economical by reducing maintenance costs and constant monitoring.
Installation, Environment and Place of use.
The environment largely determines which one is suitable.
Oil: These are ideal for outdoor spaces with natural ventilation and where they pose no risk to people. For example, industrial plants, generating stations, or rural areas. These devices also tend to withstand extreme climates better if properly maintained.
Dry: They excel in enclosed spaces such as shopping malls, airports, hospitals, universities, and corporate buildings. Furthermore, since they don't require oil containment systems, they take up less space and simplify installation.
In areas with high humidity, dust, or corrosive environments, epoxy-encapsulated dry-type transformers are a great option, as their solid insulation is resistant to these factors.
Initial Costs vs Total Cost of Ownership.
The price is not limited to the purchase price, but also to the operating cost throughout its useful life.
Oil-filled engines: These tend to be cheaper upfront, especially for high-power engines. However, their constant maintenance, oil handling, and additional safety infrastructure (trays, detectors, special fire extinguishers) increase the total cost.
Dry type: They require a higher initial investment, but generate savings in maintenance, safety, and the peace of mind of operating indoors without the risk of leaks.
For example, a dry-type transformer in a shopping center may be more expensive upfront, but it avoids the expense of containment trays and reduces the risk of emergency shutdowns.
At Grupo Edmar, we help you calculate the total cost of ownership for each option, so you can compare not only initial prices but also the real economic impact over time.
Environmental Impact.
The ecological aspect is decisive today.
Oil transformers: A spill can contaminate soil and water, and final disposal requires regulated processes. However, biodegradable oils have reduced this impact.
Dry transformers: By not using liquids, they reduce the risk of contamination. Although their manufacture with epoxy resins also has an environmental impact, they are generally considered more sustainable in operation.
More and more companies are seeking equipment that complies with environmental regulations, and in that sense, dry-type transformers are often more accepted in urban projects or those certified with ecological standards.
When to choose each type?
The choice depends on the context and not on which is “best at everything.”
Choose an oil-filled transformer if you need high power, the installation is outdoors, and your priority is to optimize initial costs.
Choose a dry-type transformer if you work indoors, seek security, and want to reduce long-term maintenance.
Quick summary of advantages and disadvantages
Oil type
✅ Excellent heat dissipation.
✅ Long service life with good maintenance.
❌ Risk of leaks and fires.
❌ Higher maintenance costs.
Dry type
✅ Indoor security.
✅ Low maintenance.
❌ Higher initial cost.
❌ Thermal limitations at very high powers.
In any case, it's best to consult with a specialized supplier. At Grupo Edmar, we offer both types of transformers, technical advice, and customized designs to ensure your investment is safe, efficient, and profitable.
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