Hermetic Transformers (Oil-Type Transformers)
Transformers, used for the safe and efficient transmission of electrical energy, are designed with different structural characteristics depending on the application area. These systems are generally divided into two main groups: oil-filled transformers and dry-type transformers. The most common and widely used solution in the field, especially suitable for outdoor applications, is the oil-filled transformer.
Thanks to their durable structure and high efficiency, oil-filled transformers can operate for many years in harsh environmental conditions. They are also more advantageous in terms of cost compared to dry-type models. However, in these systems, the insulation and cooling mechanism is provided by transformer oil, unlike dry-type transformers. Transformer oil performs a dual function: it reduces the temperature of the windings while also providing electrical insulation.
The Difference Between Hermetic and Oil-Filled Transformers with Expansion Tanks
Oil-filled transformers are divided into two types according to their design: hermetic and expansion tank type. Both models have an oil cooling system, but they differ in how they handle oil expansion.
Hermetic transformers are sealed to provide complete insulation between the interior and exterior atmosphere. The oil, filled under vacuum, circulates in a closed tank. When the oil heats up, its volume expands, and this expansion is absorbed by the corrugated wall structure of the transformer tank. Because there is no direct contact with the external environment in this type, contaminants such as moisture, dust, or liquids cannot enter the oil; this reduces the need for maintenance.
In transformers with an expansion tank, the oil is located in an open-system expansion tank along with the tank and is in contact with the external environment. This controls the increase in oil volume caused by heating. When the heat increases, the oil expands towards this tank, and when it cools, it returns. However, because there is contact with air in this design, silica gel is used to protect against internal moisture. Silica gel retains moisture from the air, preserving the insulation quality of the oil. It needs to be replaced as its color changes.
Internal Structure of Hermetically Sealed Oil-Type Transformers
The basic components of an oil-filled transformer are:
Aluminum or copper windings
Magnetic core made of silicon steel
Transformer oil
Tap changer
Hermetic tank
Radiator or corrugated walls
Oil temperature thermometer
Hermetic control relay (pressure, gas, temperature gauge)
Cable bushings and wheels
If the transformer has an expansion tank, a Buchholz relay and expansion tank are also included.
Transformer Winding Material: Aluminum or Copper?
Transformer windings can be made from aluminum or copper conductors depending on the application. In Türkiye, according to TEDAŞ specifications, aluminum is generally preferred for distribution transformers up to 2500 kVA. This is because aluminum is both cheaper and lighter. However, using aluminum in high-power transformers such as 10 MVA increases the tank volume, thus raising the total cost. Therefore, as the power increases, copper windings are preferred, resulting in more compact and economical solutions.
Oil-Type Transformer Cooling Methods
Heat generated in transformers generally comes from two sources:
Copper loss (loss at load)
Core loss (loss at no load)
To dissipate this heat, transformer oil absorbs the heat from the windings and transfers it to the external environment via radiators or corrugated walls. Cooling systems are classified as follows:
ONAN Transformer oil + natural air cooling
ONAF Transformer oil + fan-assisted air cooling
OFAF Fan-assisted oil + fan-assisted air cooling
OFWF Fan-assisted oil + water cooling
ONAN type cooling is generally preferred in distribution transformers, while ONAF type cooling is preferred in power transformers.
Oil-based Transformer and Insulation Oil
Transformer oil is not just a coolant; it is also a critical component providing electrical insulation. Its properties are even more important, especially in hermetically sealed transformer systems. The oil used must have high dielectric strength, conduct heat effectively, and be free from external factors such as moisture, air, and dust. The quality of the oil should be checked through periodic oil tests and sample analyses. Otherwise, the insulation may weaken, and a short circuit may occur between the windings.
Protection Relays in Oil-Filled Transformers
In hermetic transformers, digital control relays are used. These relays monitor parameters such as gas release, internal pressure, and oil temperature, and when limit values are exceeded, they trip the circuit breaker in the MV cell. Oil-filled transformers with expansion tanks use an analog type relay, called a Buchholz relay. It detects oil leaks, gas accumulation, or sudden fluctuations and generates a protection signal.
Oil-filled transformer prices and application areas.
There are many leading transformer companies and manufacturers in Türkiye in this field. Transformer prices vary according to the following factors:
Power capacity (e.g., 400 kVA, 1000 kVA, 1600 kVA, etc.)
Winding material (aluminum or copper)
Cooling type (ONAN, ONAF, etc.)
TEDAŞ (Turkish Electricity Distribution Company) or private project approval
Protection relay type and communication feature
Use and Maintenance of Hermetically Sealed Oil-Type Transformers
Hermetically sealed oil-filled transformers stand out with their long lifespan and reliable construction. Thanks to their high efficiency ranging from 98% to 99%, they minimize energy losses. They generally only require periodic maintenance, significantly reducing the need for intervention during operation. In TEDAŞ-approved projects, models up to 1600 kVA are mostly used in concrete substation applications. They have a wide range of uses, from industrial facilities and organized industrial zones to housing projects, power plants, and infrastructure applications. Especially in systems operating in open areas and challenging environmental conditions, hermetically sealed oil-filled transformers offer an ideal solution with their high level of safety and low operating costs.