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How to Choose a Transformer: Dry Type vs. Oil Cooled

Transformers are common and useful devices which take high voltage electricity directly from a power station and convert it to a lower voltage. This allows the energy to be safely and efficiently used by machinery and appliances that can only handle low voltage in spaces such as offices, transportation hubs, schools and factories.

Through this process transformers generate a lot of heat that must be dissipated to keep them running safely. There are two types of transformers being used in the industry currently: Dry-type transformers and oil-cooled transformers. Dry-type uses air as a cooling medium, and liquid cooled uses oil. Although both types have the same end result there are a number of differences between them worth noting, that will affect which type you choose.

Maintenance: Oil cooled transformers required more maintenance procedures, which must be performed more often than dry-type. The oil needs to be sampled to test for contamination, whereas dry type transformer is very resistant to chemical contaminants.

Costs (Initial and Operating): Compared to oil cooled, dry type has a significantly higher operating loss. Oil filled transformers have a higher standard energy efficiency, and as a result have a higher lifespan than dry type.

Noise: Oil cooled transformers have a lower operating sound level, thus less noise pollution than dry-type.

Recyclability: The end of life recycling for dry type is limited, while oil units boast an easier core/coil reclamation. Oil cooled have superior operating life and maintainability, producing less waste and requiring less replacements and labor.

Efficiency: Dry type transformers are larger units, limited in voltage and size, making them more prone to overheating if they experience overload. As a result, they have higher electrical losses, and it is more expensive to maintain dry type power supply over time. Oil cooled units are smaller and more efficient. They require less demand and create a smaller environmental footprint.

Voltage Capabilities: Dry type transformers are designed to handle small to medium MVA and voltage ratings, making them ideal for smaller applications. Oil cooled transformers can handle heavier loads, so applications that require higher voltages will require oil units.

Location: Location of the transformer will be the biggest determinant for which type you will need. Dry type is specified for use in buildings and near buildings, simply because they are environmentally safer. Dry type transformers are less flammable and pose less of a fire risk, making them ideal for shopping malls, hospitals, residential complexes and other commercial areas. Oil cooled transformers are used in outdoor installations due to the possibility of oil leakage and spills which pose a fire risk, but these units are more environmentally friendly.

Taking these variables into account, oil units appear to be the better option overall with higher energy efficiency, recyclability, low noise pollution, lower operational costs and a small environmental foot print. However, oil units simply cannot be used in any situation. Dry type is the best and many times, required option for commercial and indoor operations, because they are safer units to operate around people and areas where fire hazards may exist.

Types of transformer

Oil Immersed Transformer

As the name suggests the coils in this type of transformers are immersed in oil (mostly mineral oil) which helps in keeping the temperature of the transformer under control. This oil type transformer dissipates the through the radiators which are attached on the tank of the transformer and are referred to as ONAN type transformer. To further improve the cooling of the transformer the radiators are installed with fans which helps in bringing down the temperature and referred to as ONAF type transformer. This type of transformer can reach high voltage capacity, in some cases 1000kV.

Dry Type Transformer

In this type of transformer, air is used as the cooling medium. They are made using vacuum pressure impregnation in polyester or silicone varnish. Some of them are also made using VPI epoxy and cast resin for tougher environmental conditions. Since they are limited with regards to cooling aspect the maximum voltage is limited up to 35kV.


Switchgear is electrical distribution equipment: it accepts power from a source, routes it to a number of outputs and provides overcurrent protection and control functions. Of the types of distribution equipment described in the NFPA 70: National Electrical Code Article 408: Switchboards, Switchgear and Panelboards, switchgear is generally the most robustly constructed, the largest and the most expensive. It’s typically applied in high-reliability facilities, like hospitals or data centers, where continuity of power is critical to effective operation.

Switchgear is available in a wide range of voltage ratings, from below 1,000 volts to more than 200 kilovolts. Medium-voltage switchgear, rated above 1,000 volts, is manufactured by producers in a variety of configurations. Assemblies are available for exterior padmount installation, vault installation or installed in dedicated freestanding metal buildings, with air, gas, vacuum or oil as insulating media. This discussion will focus on interior low-voltage switchgear.

The alternative to switchgear is switchboard construction. Switchboards generally require less space and are less expensive. Both are typically constructed of a number of vertical sections. Each section is enclosed in sheet metal, with openings in front for overcurrent protection devices, monitoring equipment and control devices. A section may contain a main overcurrent protection device, metering devices, automatic control and monitoring systems, overcurrent protection devices for distribution feeders or a combination of these or other equipment specific to the installation. Overcurrent protection is typically accomplished with circuit breakers, with fused switches are less frequently.

Electric power substation

An assembly of equipment in an electric power system through which electrical energy is passed for transmission, distribution, interconnection, transformation, conversion, or switching. See Electric power systems

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