Liquid filled transformers – Everything you need to know at one place

Liquid filled transformers – Everything you need to know at one place

Liquid-filled transformers are essential components in modern electrical power systems, providing efficient and reliable voltage transformation. These transformers use various cooling liquids to manage the heat generated during operation, making them suitable for a wide range of applications from power generation plants to industrial facilities. In this article, we will take a closer look at these incredible inventions that are essential for the modern society.

Core components and structure of liquid filled transformers

Liquid filled transformers share many similarities with other transformers, however, there are also some differences in terms of structure. The main components are:

Core and Windings

Similar to other transformers, liquid-filled transformers consist of a magnetic core and two sets of windings (primary and secondary). The core is typically made of laminated silicon steel to minimize energy losses, and the windings are usually made of copper or aluminum, a standard in both oil filled and dry transformers. The core’s lamination reduces eddy currents, thereby decreasing energy dissipation, while the high conductivity of copper or aluminum ensures efficient electrical performance.

Cooling System

The liquid acts as a coolant and insulator. It circulates through the transformer, absorbing heat from the core and windings and transferring it to the transformer tank’s walls. Some transformers also have external radiators or cooling fins to enhance heat dissipation. This cooling mechanism not only prevents overheating but also prolongs the transformer’s operational life by maintaining optimal temperature conditions.

Conservator Tank

Many liquid-filled transformers, particularly those designed as oil-type transformers, have a conservator tank, which is a small, auxiliary tank connected to the main transformer tank. The conservator tank allows for the expansion and contraction of the cooling liquid due to temperature changes, preventing the main tank from being overfilled or underfilled. This system helps maintain a consistent level of insulation and cooling, crucial for the transformer’s reliability and safety.

Buchholz Relay

This is a gas-actuated protection device installed in oil-filled transformers with a conservator. It detects gas accumulation due to internal faults and issues an alarm or shuts down the transformer to prevent further damage. The Buchholz relay is a critical safety component, providing early fault detection and helping to avoid catastrophic failures by triggering timely maintenance actions.

Working mechanism of liquid-filled transformers

The working mechanism of liquid-filled transformers begins with the primary winding. When an alternating current (AC) is applied to the primary winding, it creates a varying magnetic field around the core. This varying magnetic field induces a magnetic flux in the core, which then travels through the laminated core to the secondary winding. According to Faraday’s Law of Electromagnetic Induction, this varying magnetic flux induces an electromotive force (EMF) in the secondary winding. The magnitude of this induced EMF depends on the turns ratio between the primary and secondary windings, determining the voltage transformation.

The transformer changes the voltage level based on the turns ratio; if the secondary winding has more turns than the primary, the transformer steps up the voltage, and if it has fewer turns, it steps down the voltage. During this process, heat is generated due to electrical losses in the core and windings. This heat is absorbed by the cooling liquid, which rises and is cooled down by external radiators or cooling fins before circulating back to the core and windings. This efficient cooling mechanism prevents overheating and ensures the transformer’s optimal performance and longevity.

Types of liquids used in liquid-filled transformers

Liquid-filled transformers can use various types of cooling mediums for cooling and insulation. The choice of liquid depends on the specific application, environmental considerations, and performance requirements. Here are the main types of liquids used in liquid-filled transformers:

Mineral Oil: Mineral oil is the most commonly used liquid in transformers. Derived from refined petroleum, it has excellent cooling and insulating properties. Its widespread availability and cost-effectiveness make it a popular choice. However, it is flammable and poses significant environmental risks if spilled, necessitating careful handling and containment measures.

Silicone Oil: Silicone oil is a synthetic compound known for its high thermal stability and good dielectric properties. It is non-flammable and has a high flashpoint, making it a safer option in terms of fire hazards. Although silicone oil is more expensive than mineral oil and less efficient at cooling, its stability at high temperatures makes it ideal for specialized applications where safety and reliability are paramount.

Synthetic Esters: Synthetic esters are man-made organic compounds designed for high-performance applications. They are biodegradable, have a high flashpoint, and offer good cooling performance. Despite their higher cost compared to mineral oil and silicone oil, synthetic esters are favored in applications where environmental impact and high safety standards are critical. Their use can significantly reduce the ecological footprint of transformer operations.

Natural Esters: Natural esters, derived from vegetable oils such as soybean or rapeseed oil, are another environmentally friendly option. These oils are biodegradable, renewable, and have a lower environmental impact compared to mineral oils. However, their higher viscosity can affect cooling efficiency, and they are more prone to oxidation than synthetic esters, which may require more frequent maintenance.

What are the advantages and disadvantages?


Efficient Cooling: Liquid-filled transformers have superior cooling capabilities compared to dry type transformers. The liquid coolant efficiently transfers heat away from the core and windings, allowing for higher power ratings and more efficient operation. This effective heat dissipation ensures that the transformer can operate continuously without overheating, even under heavy load conditions.

Higher Overload Capacity: Thanks to better cooling, liquid-filled transformers can handle higher overloads for short periods without significant risk of overheating. This capability is particularly beneficial during peak demand times or unexpected load surges, ensuring reliable power supply without immediate risk of failure.

Extended Lifespan: The effective cooling and insulation provided by the liquid can extend the operational life of the transformer by protecting the core and windings from thermal stress and degradation. By maintaining a stable internal temperature, the risk of insulation breakdown and other thermal-related damages is minimized.

Compact Design: Liquid-filled transformers can be more compact than dry-type transformers of the same rating, making them suitable for installations where space is limited. This smaller footprint can lead to cost savings in terms of space utilization and installation expenses.


Environmental Risks: The use of mineral oil or other synthetic coolants poses a risk of environmental contamination in case of leaks or spills. Special precautions and containment measures are often required to mitigate this risk, such as oil containment systems and regular monitoring for leaks.

Fire Hazard: The insulating oil used in these transformers is flammable, presenting a potential fire hazard. Fire prevention and protection measures, such as fire barriers and suppression systems, are essential in installations to minimize the risk of fire spreading in case of an accident.

Maintenance Requirements: Liquid-filled transformers generally require more maintenance than dry-type transformers. The condition of the insulating liquid must be regularly monitored, and the liquid may need to be replaced or purified over time. Routine maintenance ensures the transformer operates efficiently and safely, but it can also incur additional operational costs.

Applications of Liquid-Filled Transformers

Liquid-filled transformers are versatile and play a critical role in various sectors due to their superior cooling and insulating properties. Here are some of the key applications:

Power Generation Plants: Liquid-filled transformers are extensively used in power generation plants to step up the voltage for efficient transmission over long distances. They are essential in converting the generated low-voltage electricity to high-voltage levels, which minimizes energy loss during transmission.

Substations: In substations, these transformers step down the high transmission voltage to levels suitable for distribution. This makes the electricity safe and usable for industrial, commercial, and residential consumers. Their efficient cooling systems ensure reliability and continuous operation even under high load conditions.

Industrial Facilities: Industrial operations often require high power levels for heavy machinery and equipment. Liquid-filled transformers are used to ensure stable power supply, handling high loads and frequent voltage fluctuations. Their ability to manage higher overload capacities without overheating makes them ideal for industrial applications.

Renewable Energy Installations: With the growing emphasis on renewable energy, liquid-filled transformers are critical in wind farms and solar power plants. They facilitate the integration of renewable energy into the grid by transforming the voltage levels generated by renewable sources to the appropriate levels required for grid distribution.

Railways and Traction Systems: Liquid-filled transformers are used in railway and traction systems to provide the necessary power for trains. They convert the high-voltage supply from the grid to lower voltages suitable for traction motors, ensuring efficient and reliable train operations.

HVDC Systems: High Voltage Direct Current (HVDC) systems use liquid-filled transformers in the conversion process between AC and DC. These transformers are crucial in HVDC transmission, which is used for long-distance and underwater power transmission due to its lower energy losses and efficient handling of large power loads.

Commercial Buildings and Complexes: In large commercial buildings and complexes, liquid-filled transformers are used to manage the electrical load efficiently. They ensure a stable power supply for HVAC systems, elevators, lighting, and other electrical equipment, contributing to the smooth operation of modern infrastructures.


liquid-filled transformers play a pivotal role in ensuring the stability and efficiency of electrical power systems across various sectors. Their superior cooling capabilities, extended lifespan, and compact design make them invaluable in power generation, industrial applications, and renewable energy installations. However, it is essential to consider the environmental risks, fire hazards, and maintenance requirements associated with these transformers. By carefully selecting the appropriate cooling liquid and implementing robust safety measures, the advantages of liquid-filled transformers can be maximized. As the demand for reliable and efficient power solutions continues to grow, liquid-filled transformers will remain a cornerstone of modern electrical infrastructure.

At TTES, we have over 100 combined years of experience manufacturing, maintaining, and repairing liquid-filled transformers. If you’re looking for industry-leading lead times of just 20 weeks on average, don’t hesitate and reach out to us for a free quote

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