What are liquid-immersed transformers?
Liquid-immersed transformers have been in use for many years and are the preferred transformer type for many applications. The key benefits of liquid-immersed transformers are their high efficiency and low losses. In addition, liquid-immersed transformers provide excellent reliability and low maintenance. There are two main types of liquid-immersed transformers:
- Hermetic
- Conservator
This article discusses hermetic transformers in detail, please see our conservator transformer article to learn about this type of unit.
Mounted Electrical Transformers
Hermetically Sealed Transformers
A hermetic transformer is a transformer that is hermetically sealed and filled with a dielectric fluid such as mineral oil, silicone oil, or synthetic oil. They are used in a variety of applications where it is necessary to prevent moisture, oxygen, or other contaminants from entering the transformer tank and contaminating the insulating oil (dielectric liquid); this results in a longer transformer life and improved reliability. Hermetic transformers can be either air-cooled or liquid-cooled, and are often used in applications where the transformer must be operated in a hazardous environment. Unlike conservator type transformers, hermetic transformers have no expansion tank and they may or may not be fitted with a pressure relief device.
Hermetic transformer designs offer excellent cooling performance. The insulation oil provides efficient cooling of the transformer windings and core, which helps to reduce thermal losses and extend the life of the transformer. Over 95% of liquid immersed transformers use mineral oil as the dielectric insulating fluid.
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Hermetic Design
In electrical engineering, a hermetic design is the process of designing devices and systems that are resistant to outside elements such as dust, water vapour, and other airborne contaminants. The term "hermetic" comes from the Greek word for "sealed." Hermetic devices are typically used in critical applications where reliability is paramount, such as in aerospace and medical equipment. The hermetic design involves creating a barrier between the device and its environment, typically by using a sealant or gasket. This barrier can take many forms, but it must be strong enough to withstand the forces of nature and prevent any unwanted elements from entering the device. Hermetic design is an essential part of many transformer designs, as it helps ensure that the transformer will continue to function properly even in harsh conditions.
Different types of oils used in hermetically sealed transformers
The type of oil used can have a significant impact on the transformer's performance, lifespan, and reliability. There are four main types of oils used with hermetically sealed transformers:
- Mineral oil - the most common type of transformer oil. It is a highly effective dielectric and has excellent thermal properties.
- Silicone oil - also a popular choice for hermetically sealed transformers. It has very good dielectric properties and can withstand extreme temperatures. It is comparatively expensive.
- Natural esters - another option for hermetically sealed transformers. They have excellent thermal stability and are environmentally friendly.
- Synthetic esters - the most expensive type of transformer oil, but offer the best performance in terms of dielectric strength and thermal stability.
The type of oil used in a hermetically sealed transformer is important to consider when selecting a transformer for power distribution applications as it will effect its electrical characteristics. Each type of oil has its own advantages and disadvantages and these should be considered.
How Liquid Immersed Transformers Work
Liquid immersion transformers use a liquid medium, typically oil, for cooling and insulation. The windings and core are immersed in the liquid, which provides both mechanical support and electrical insulation. The liquid also helps to dissipate heat, making immersion transformers ideal for use in high-power applications. Transformers typically have two or more windings, which are conductors that inductively coupled to each other (electromagnetic induction). This enables the transformer to transfer energy from one circuit to another without physically connecting the two circuits. When used in an immersion transformer, the windings are wrapped around a central core, which is made of a material with high magnetic permeability (steel laminations); this increases the transformer's ability to store energy in its magnetic field, making it more efficient. Transformer windings are typically manufactured from copper or aluminium (aluminium is cheaper, but requires more space because its conductivity rating is lower than that of copper).
Advantages and disadvantages of hermetic liquid immersed transformers
Hermetic transformers are almost maintenance-free because there are no opportunities for moisture or other contaminants to enter the unit and cause corrosion, or oil contamination. Maintenance costs are low, whilst the service life of a hermetic transformer can be quite long (>20 years). Hermetic transformers are efficient, compact, and very reliable.
While hermetic transformers offer many advantages, they also come with a few disadvantages. Hermetic units are not as easily repaired as conservator type units because they are sealed. Breaking the seal of a hermetic transformer is unwise, as the units can never really be sealed again without significant effort. Oil analysis of a hermetic transformer is not possible unless the seal is broken (to obtain an oil sample), which means there are less condition monitoring techniques available for use with this type of unit.
Factors to consider when specifying hermetically sealed transformers
When hermetically sealed transformers are required, there are several factors to consider in order to ensure the proper transformer is selected for the correct application. The hermetic seal prevents oxygen, nitrogen, and other gasses from entering or exiting the transformer casing, and is maintained by welding, brazing, or soldering of the transformer tank and components. The following factors should be considered when specifying a hermetically sealed transformer:
- Environment - hermetically sealed transformers can be used in either indoor or outdoor applications. Considerations for temperature extremes, humidity, altitude, and pollution levels should be taken into account when selecting a hermetically sealed transformer.
- Temperature - the minimum and maximum operating temperatures must fall within the specified range of the transformer.
- Load - the load must be within the specified capacity of the transformer.
- Voltage - the input and output voltages must fall within the specified range of the transformer.
- Frequency - the input and output frequencies must fall within the specified range of the transformer.
- Power - the input and output power rating must fall within the specified range of the transformer. Electrical power is expressed in VA, kVA, or MVA.
- Efficiency - the minimum acceptable efficiency must meet or exceed the required efficiency of the application.
Considerations when installing a hermetically sealed transformer
Hermetically sealed transformers are designed to withstand extreme temperatures and humidity, making them ideal for use in harsh environments. However, hermetic transformers require maintenance in order to keep them functioning properly. The most important maintenance task is to regularly check the seals and gaskets for any leaks or damage. These seals are what keep the transformer hermetic, so it is vital that they remain intact. In addition, the electrical connections should be checked for any signs of corrosion or loose connections. Infrared thermographic inspections are useful for identifying loose electrical connections, and these can be performed when the unit is in service.
Another essential maintenance task is to regularly check the transformer oil level. Hermetically sealed transformers rely on an insulating liquid for cooling and insulation. If the oil level gets too low, it could lead to overheating and/or insulation breakdown with resulting electrical faults, all of which will damage the transformer. A visual check for oil leaks is usually sufficient to ensure the oil level is not reducing.
It’s important to keep hermetically sealed transformers clean. Over time, dust and dirt can build up on the exterior of the transformer parts and tank. These contaminants can eventually work their way into seals and cause sealing problems. Electrical bushings must be kept clean to reduce the likelihood of tracking or electrical creep.
Hermetic Transformer Manufacturers
Hermetic transformers are manufactured by many companies, and each has its own strengths and weaknesses. Some of the largest manufacturers include General Electric, ABB, Trafo Union, and Siemens.
Additional Resources
https://en.wikipedia.org/wiki/Transformer_oil
https://faradypowertransformer.wordpress.com/tag/oil-immersed-transformer