The power transformer is the most important component of the power system. The design of the grid transformer varies depending on the cooling and electrical insulation system used. There are dry-type transformers and oil-immersed transformers. Most transformers in use are oil-immersed. Oil-immersed transformers are also called oil-filled transformers.
They use oil as the insulation and cooling medium. Oil-immersed transformers have the characteristics of reasonable structure, excellent performance, economy, and low noise. They are more suitable for urban and rural, industrial and mining enterprise power grid transformation and combined transformers, and prefabricated transformers for substations. Compared with dry-type transformers, the use of liquids can provide better heat dissipation, thereby achieving higher-rated power and higher efficiency.
This article will introduce the structure and working principle of oil-immersed transformers, as well as common types and precautions for use.
1. What is an oil-immersed transformer?
Oil-immersed transformers are a common type of power transformer, which uses insulating oil to cool and insulate the internal components of the transformer.
The oil-immersed transformer consists of two or more coils (primary winding and secondary winding) and an iron core. Its main body is enclosed in a welded steel tank filled with insulating oil. During operation, the heat generated by the coils and the core is first transferred to the insulating oil and then to the cooling medium. These coils and the core are immersed in insulating oil to maintain their cooling and insulation properties.
2. Working principle of oil-immersed transformer
The working principle of oil immersed transformer is based on electromagnetic induction and transformer principle. Its basic components include core, winding and bushing. The core facilitates the flow of magnetic flux while the winding consists of a conductor coil wound on the core, generating a magnetic field. The winding is insulated with a cardboard barrier and shielding layer, the thickness of which increases with the increase of voltage. The bushing is used to connect the transformer winding to the substation.
These transformers are used in power distribution and substations and have cores and coils immersed in oil for cooling and insulation. The oil circulates through tubing in and around the coil and core assembly by convection. The cooling method varies, with transformers with smaller power ratings being cooled externally by the oil tank and transformers with larger power ratings being cooled by air-cooled radiators.
Oil-immersed transformers require insulating oil to function properly. First, the insulating oil cools the internal components of the transformer to keep its operating temperature within a safe range. Second, the insulating oil has good insulation properties and can prevent arcing and discharge.
3. Oil-immersed transformers and dry-type transformers
An oil-immersed transformer is one in which the coil and core of the transformer are immersed in special transformer oil. The advantage of this is that the cooling coil can be isolated from the air, preventing moisture in the air from corroding the core of the transformer. It can also extinguish the arc. Therefore, in the early days in my country, power transformers and power switches were all immersed in oil.
The core and coil of a dry-type transformer are integrated and fixed. They can be directly exposed to the air or placed indoors, but they have high requirements for ambient temperature and humidity. Its characteristic is that it can be unattended and only needs to be checked regularly.
The cooling medium and insulation material of its windings and other components are all dry-type transformers. Its main purpose and general use are the same as those of oil-immersed transformers, which is to adjust the transformer’s primary voltage to the voltage level commonly used in industry, households, and enterprises.
4. Advantages of oil-immersed transformers
Oil-immersed transformers are cheaper, and the price of an oil-immersed transformer of the same capacity is usually only half of that of a dry-type transformer.
Oil-immersed transformers tend to be more successful at cooling the transformer than dry-type transformers. Oil is a better cooling medium. They also have higher voltage capacity. Dry-type transformers operate up to 35 kV, while oil-immersed transformers have no limit. Oil-immersed transformers are more versatile.
Dry-type transformers are usually connected to the power consumption source from an intermediate location. Oil-immersed transformers are used as power transformers and distribution transformers.
Oil-immersed transformers are better for the environment because they are easier to recycle than dry-type transformers. They are also smaller and work best in outdoor environments in case of oil leaks or other accidents.
5. Types of oil-immersed transformers
There are many types of oil-immersed transformers. The following are a few types that are commonly found.
- Single phase transformer uses one pair of windings. It is used in low load situations like rural areas.
- Windings make up three-phase transformers. The windings are usually wound on an iron core that is divided into three parts. Three-phase transformers are used in high-load areas and can supply power to three circuits.
- Power transformers are designed to handle higher loads. They step up or down voltage and transfer electricity from one place to another.
- Distribution transformers transfer lower voltages from the grid to homes and businesses. They are much smaller than power transformers.
- Pole transformers are connected to utility poles.
- A pedestal-mounted transformer is mounted on a concrete foundation in the ground.
6. Insulating oil used in oil-immersed transformers
Oil-immersed transformers require a special oil, called transformer oil or insulating oil. The quality of the oil will greatly affect the life and safety of the transformer.
Transformer oil is different from other oils in many ways, including:
1). Dielectric power
Since transformer oil has a high dielectric value, it acts as an insulator and prevents leakage. This is necessary for the equipment to function properly, as oil-immersed transformers need to withstand huge voltages.
2). Quality
Since transformer oil is chemically and heat resistant, it does not deteriorate over time.
3). Viscosity
Transformer oil consistency is closely monitored because low viscosity oils result in poor heat transfer, while high viscosity oils increase power losses.
4). Purification
In order to prevent damage to the oil-immersed transformer and maintain the insulating properties of the oil, the oil-immersed transformer must be thoroughly cleaned.
When an oil-immersed transformer in operation needs to be supplemented with insulating oil, the type of insulating oil currently used in the oil-immersed transformer should be determined first, and then the same grade of oil-immersed transformer oil should be injected. Various types of oil-immersed transformer oils cannot be mixed interchangeably.
7. Selection of oil-immersed transformers
Oil-immersed transformers are available in a wide variety of types and applications. Try to consider the following factors before choosing the right type.
1). Load type and load power
A transformer’s rated capacity should be determined by the type and power of the load. If the load is a peak load such as a motor or inverter, you need to select a transformer with a larger overload capacity.
2). Input voltage and output voltage
Select the corresponding transformer model according to the requirements of input voltage and output voltage.
3). Environmental conditions
The transformer should be placed in a ventilated, dry, dust-free, non-corrosive gas environment to avoid being affected by water, moisture, dust, etc. Choose a transformer with special requirements such as anti-corrosion, moisture-proof, and explosion-proof according to actual conditions.
4). Service life and maintenance cost
Considering the transformer’s service life and maintenance costs, choose the right product. Generally speaking, products with high brand awareness, mature technology and easy maintenance are more reliable.
5). Safety performance
Choose a transformer with good safety performance such as overload and short-circuit protection to ensure the safety of equipment and personnel.
6). Price
Choose the right product according to your budget. Take the quality and reliability of the product into consideration as well as the price.
8. Maintenance and precautions for oil-immersed transformers
First, the building where the transformer is placed must meet the fire resistance requirements specified in the building fire protection regulations;
When a fire occurs, measures must be taken to prevent oil from leaking out and causing the fire to spread; at the same time, fire-fighting measures must be in place to extinguish oil fires.
In order to ensure that the transformer system is operating properly, it must be inspected and maintained on a regular basis. Check the transformer to ensure that all parts are in good condition and there is no threat of leakage. Clean the outside and inside of the transformer to remove dust, dirt and other debris to ensure that the machine operates normally and safely, otherwise it is easy to fail, bringing extremely dangerous risks.
If the transformer leaks oil, the transformer must be shut down to eliminate the fault, which usually causes certain economic losses. At the same time, oil leakage will seriously interfere with the monitoring and judgment of the alarm status in the transformer storage cabinet and the correctness of the oil level indication. The full-sealed transformer loses its sealing state, which is easy to cause the invasion of outside air and moisture, thereby reducing the insulation performance, causing insulation aging, and affecting the safe and reliable operation and service life of the transformer. Once a transformer begins to leak oil, it must be checked and maintained as soon as possible in order to prevent further damage.
In addition, it is also important to note that the waste generated by oil can easily pollute the environment and affect the lives of people around it. Therefore, it is also very important to gradually dispose of the waste.
9. Conclusion
Oil-immersed transformers offer many advantages and are generally a better choice than dry-type transformers. They are efficient and affordable and can operate in almost any capacity. While they do require maintenance and pose some safety risks, with proper care and use, you can get the most out of your equipment.
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