What is the difference between a dry-type and an oil-type transformer? A transformer is an essential piece of equipment for the transmission and distribution of electricity in construction engineering. Transformers play a crucial role in changing voltage levels, reducing electrical energy losses during transmission, and ensuring safe power consumption. Transformers can be categorized into two types based on the cooling medium: dry-type and oil-immersed transformers.
Introduction to Oil-Immersed Transformers and Dry-Type Transformers
Mineral oil is utilized as both a heat dissipation and insulation medium in oil transformers. These transformers find widespread use in electrical equipment worldwide and are commonly employed in power transmission systems. Oil transformers come in two varieties: open-type (Conservator-type) transformers and sealed-type (hermetically-sealed type) transformers.
During operation, the heat generated by the coil and magnetic core is absorbed by the oil and dissipated through the metal shell of the transformer.
Dry-type transformers, on the other hand, use air for cooling. Dry transformers are easily distinguishable by their lack of a tank enclosure, which is a characteristic feature of oil transformers. There are four types of dry transformers available: open wound transformers, cast coil resin transformers (abbreviated as CRT), vacuum pressure impregnation transformers (abbreviated as VPI), and vacuum pressure encapsulated transformers (abbreviated as VPE). The CRT is the most commonly used type of dry transformer, followed by the VPI. Two types of VPE and dry open transformers are less common.
Cast Coil Resin Transformer – CRT
Cast resin transformers have epoxy resin-coated primary and secondary coils, making them suitable for high humidity environments. These transformers have a capacity ranging from 25 kVA up to 12,500 KVA.
The coil insulation reduces the risk of fire or explosion. CRT transformers are resistant to objects larger than 1.0mm and can withstand water sprays from all directions, providing a protection level equivalent to IP45 (actual protection levels may vary by manufacturer).
Vacuum Pressure Impregnation Transformer – VPI
Coils in VPI transformers are impregnated with polyester in a vacuum environment. The windings of a VPI transformer are designed in a disc shape and can be configured in series with higher power or in parallel depending on voltage. VPI transformers typically have a capacity ranging from 5 kVA to 30 kVA. VPI transformers offer a higher level of protection compared to CRT, typically rated at IP56 instead of IP45 (protection levels may vary by manufacturer). This protection is designed to shield the transformer from dust and prevent the ingress of large water currents, such as waves, making VPI transformers suitable for seagoing vessels, among other applications.
VPI transformers offer numerous advantages, including high mechanical strength, unlimited insulation life, low fire risks, easy maintenance, and convenient servicing. Oil transformers remain indispensable in transmission systems.
Advantages of Oil-Immersed Transformers over Dry-Type Transformers
- Oil transformers are typically 1.5-2 times cheaper than dry transformers of the same capacity.
- The heat dissipation capability of dry transformer epoxy resin is lower than that of mineral oil.
- Dry transformers are less efficient at dissipating heat than oil transformers, making them generally suitable for power distribution between intermediate stations and final consumption sites. Oil transformers, including those for transmission and distribution, offer greater versatility in grid systems.
- Oil-immersed transformers can be used for all voltage levels, including those below 35 kV.
- Temperature in dry transformers is monitored using a temperature sensor located within the machine body, providing a specific localized temperature reading. In contrast, the insulation oil in oil transformers distributes heat evenly, allowing the monitor’s temperature reading to be considered as the average temperature.
- Over time, insulating materials in oil transformers may dry out and age, accumulating defects. In such cases, oil transformers can be repaired, refurbished, and reused. Conversely, dry transformers are typically discarded if they fail, as components like copper (or aluminum) and magnetic cores are almost non-recyclable.
Comparison of Oil-Immersed Transformers and Dry-Type Transformers
- Despite their heat-dissipating properties, the cooling oil in oil transformers can pose a fire or explosion risk. Overpressure incidents can result in environmental pollution from oil spills.
- When installing oil transformers:
– Strict spatial arrangement is required, with distances between the wall/fence and the transformer meeting technical standards. Oil transformers are usually taller than dry transformers in indoor stations, often featuring external oil tanks.
– The building housing an oil transformer must adhere to fire resistance requirements.
– Fire prevention measures must be in place to prevent the spread of fires.
- Used oil transformers can sometimes command a higher price than their dry counterparts.
- Dry transformers exhibit greater fire resistance than oil transformers and are suitable for environments like hospitals and shopping malls. They can also be installed underground or in buildings with stringent fire safety requirements.
In transformer stations, which contain intermediate equipment for power regulation and control, dry transformers allow for more efficient space utilization. They are more compact and offer better fire resistance than oil transformers. Additionally, dry transformers lead to reduced maintenance costs, as there is no need to monitor oil levels, oil age, change oil, or address oil leaks.
This post was written by Justin Tidd, Director at https://beckersmcusa.com/. For nearly a half a century, Becker Mining has been at the forefront of mining transformers Becker/SMC is the industry’s leader in increasingly more sophisticated electrical control systems. Most of the major innovations, design features and specialized electrical components have been developed by Becker/SMC.