Oil-immersed transformers are a critical component of power systems, yet many in the industry lack a thorough understanding of their types, functions, and key components. Without this knowledge, costly mistakes can be made in transformer selection, installation, and maintenance.
In this blog post, we dive deep into oil-immersed transformers, covering their cooling and insulation functions, the various types based on oil, cooling method, phase, and application, and the key components that make them work.
What is an Oil-Immersed transformer
An oil-immersed transformer is an electrical device that transfers energy between two or more circuits through electromagnetic induction. The transformer core and windings are submerged in insulating oil inside a sealed tank.
Functions of Oil-Immersed transformers
The oil in an oil-immersed transformer fulfills two primary purposes:
- Cooling: The oil acts as a cooling medium, dissipating the heat generated by the transformer coils and core. As the oil circulates and comes into contact with the tank walls, it transfers heat to the surrounding environment, maintaining safe operating temperatures.
- Insulation: The oil provides electrical insulation between the transformer components. It prevents electrical discharges and short circuits by maintaining a high dielectric strength. The oil also helps to suppress corona discharge and reduces the effects of moisture on the insulation of the windings.
Types of Oil-Immersed Transformers
Oil-immersed transformers can be classified based on various criteria such as the type of oil used, cooling method, phase, and application.
Oil Type
- Mineral Oil: Mineral oil is the most commonly used insulating oil in transformers. It is derived from refined crude oil and has excellent dielectric and cooling properties. Mineral oil is cost-effective and widely available.
- Vegetable Oil: Vegetable oil, such as soybean oil or sunflower oil, is an eco-friendly alternative to mineral oil. It is biodegradable and has a higher flash point, making it safer in case of fire. However, vegetable oil is more expensive and may require more frequent monitoring and maintenance.
Cooling Method
- Oil Natural Air Natural (ONAN): In this cooling method, the oil circulates naturally through convection, and the heat is dissipated to the surrounding air without the aid of fans or pumps. ONAN cooling is suitable for small to medium-sized transformers with low heat generation.
- Oil Natural Air Forced (ONAF): ONAF cooling uses fans to force cool air over the transformer tank’s external radiators or cooling fins. This method enhances the heat dissipation capacity and allows for higher power ratings compared to ONAN cooling.
Phase
- Single-phase transformers: Single-phase transformers have one primary winding and one secondary winding. They are commonly used in low-power applications, such as residential power distribution and small electronic devices.
- Three-phase transformers: Three-phase transformers have three sets of primary and secondary windings, which are phase-shifted by 120 degrees. They are used in high-power applications, such as industrial power systems and electric power transmission.
Application
- Power transformers: Power transformers are used in electrical power transmission and distribution systems. They step up the voltage for long-distance transmission and step it down for distribution to end-users. Power transformers handle high voltages and currents.
- Distribution transformers: Distribution transformers are used to step down the voltage from the distribution level to the utilization level, suitable for end-users. They are typically found in substations and on utility poles.
- Dual Voltage Transformers: Dual voltage transformers have two secondary windings with different voltage ratings. They allow for the simultaneous supply of power at two different voltage levels, such as 120V and 240V.
- Transformers with Hybrid Insulation Winding: These transformers combine different insulation materials in the winding insulation system. For example, they may use a combination of oil-impregnated paper and epoxy resin. Hybrid insulation can enhance the transformer’s performance and durability in specific applications.
Components of an Oil-Immersed Transformer
- Core: Typically made of laminated steel, the core provides a low-reluctance path for the magnetic flux, minimizing losses and improving efficiency.
- Coils: The primary and secondary windings, made of insulated copper or aluminum conductors, are wound around the core to facilitate voltage transformation.
- Insulating Materials: Various insulating materials, such as paper, pressboard, and plastic, are used to provide electrical insulation between the windings and the core.
- Transformer Oil: The insulating and cooling liquid, usually mineral or vegetable oil, in which the core and windings are submerged.
- Tank: A sealed container that houses the core, windings, and oil, protecting them from the environment and containing any oil leaks.
- Tap Changer: A mechanism that allows for the adjustment of the transformer’s voltage ratio, enabling voltage regulation and load balancing.
- Bushings: Insulated conductors that facilitate the connection of external electrical circuits to the transformer windings.
- Ventilation: Cooling fins, radiators, or fans that aid in heat dissipation from the transformer oil to the surrounding air.
- Conservator: An oil expansion tank that accommodates changes in oil volume due to temperature variations and helps maintain a constant oil level in the main tank.
Testing and Standards for Oil-Immersed Transformers
Dielectric Strength Test
This test measures the insulating oil’s ability to withstand electrical stress without breakdown. It involves applying a high voltage across a sample of the oil and monitoring for any discharge or failure.
Insulation Resistance Test
The insulation resistance test assesses the integrity of the transformer’s insulation system. It measures the resistance between the windings and the ground, as well as between different windings, to detect any deterioration or moisture ingress.
Temperature Rise Test
This test determines the transformer’s ability to dissipate heat under loaded conditions. It measures the temperature increase of the windings, oil, and other components to ensure they remain within acceptable limits.
Short-Circuit Test
The short-circuit test evaluates the mechanical strength and thermal stability of the transformer under fault conditions. It involves applying a short circuit to the secondary winding and measuring the primary current and voltage to calculate the transformer’s impedance and losses.
