Oil-Immersed Transformer Structure and Working Principle

An Oil-immersed transformer is a voltage conversion device in which the core and windings are submerged in insulating mineral oil. This oil simultaneously provides dielectric insulation and removes heat from the active components. Used widely in 10kV, 35kV, and 110kV distribution networks, the Oil-immersed transformer handles the thermal and electrical demands that dry-type units cannot match at scale. Understanding its internal structure and operating principle helps procurement teams specify the right equipment and avoid costly field failures.

Understanding Oil-Immersed Transformer Structure

A power transformer that is full of liquid is not the same as a steel tank that is full of oil. Each part serves a specific scientific purpose, and buyers can write more specific requirements when they know what those functions are.

The Magnetic Core

Silicon steel laminations that are oriented along the grain make up the heart. The magnetic flux in this material flows along the grain direction, which lowers hysteresis losses. Our Oil-immersed transformer designs at Xi'an Xikai reduce no-load losses by up to 30% when compared to older S9-series units thanks to grain-oriented steel cores. Even more efficiency is gained with amorphous core versions, which reach 99% for utility-grid uses that need to meet EcoDirective goals.

Windings and Insulation

Copper or metal wires are wound around the core in a circle. Barriers made of kraft paper and pressboard that have been soaked in shielding oil separate the high-voltage and low-voltage coils. A dielectric strength of more than 30 kV is reached by mineral oil, which is much higher than what is needed for 35 kV class equipment. IEC 60076 sets the standard for the industry at 10 pC for partial discharge levels, which are kept below that level in a well-built machine.

Tank, Conservator, and Accessories

The oil is kept in the welded steel tank, which also protects the moving parts. On top is a guardian that acts as an oil tank that lets the oil expand with temperature changes without letting air touch it. Breathers made of silica gel stop water from getting to the oil. Bushings carry electricity through the wall of the tank while keeping the insulation in place. Together, these parts determine how long the transformer can work without having to be shut down for no reason.

How the cooling is set up also depends on the needs of the project. ONAN (oil natural, air natural) works well in country substations that want to keep things simple. Fans are added to ONAF (oil natural, air driven) places that have a medium load. In both OFAF and OFWF configurations, pumped oil is used for big industry or grid tasks. With IP55-rated enclosures suitable for dusty or humid sites, Xi'an Xikai's Oil-immersed transformer range covers all four cooling classes.

How Oil-Immersed Transformers Work

Electromagnetic Induction

Faraday's law says that when the magnetic flux in the core changes, a voltage is created in each loop that is related to the number of turns it has. When AC current flows through the primary winding, it makes an alternating magnetic field that moves through the laminated core and changes the voltage in the secondary winding from low to high. This science is unaffected by the oil environment, but the device can handle much higher power levels than devices that are cooled by air.

Role of Insulating Oil

Insulating oil does two things at the same time. First, it fills up all the empty spaces around the windings. This stops partial discharge by getting rid of any air pockets where voltage stress could start an arc. Second, it naturally moves heat away from the hot windings and toward the tank wall and radiators. Mineral oil has a flash point above 135°C, while natural ester oils have a flash point above 300°C. This makes them the best choice for places that need to be safe from fire, like deep substations or ocean platforms.

Tap Changers and Voltage Stability

With on-load tap changers (OLTC) or off-circuit tap changers, the output voltage can be changed without having to unplug the transformer. Models with ±0.5% voltage regulation keep the grid in sync when renewable energy is added and the inverter output changes. If there is a problem inside the machine, pressure relief devices let out extra gas, and oil-level signs let workers quickly see how much fluid in the machine. Our standard Oil-immersed transformer units keep the noise level below 65 dB, which is low enough for installations in schools, hospitals, and homes.

Advantages and Limitations of Oil-Immersed Transformers

Core Advantages for Industrial and Utility Use

When it comes to high-capacity outdoor applications, the Oil-immersed transformer regularly beats dry-type units. For procurement teams, these are the most important technical benefits:

• Thermal capacity: The fluid cooling system can work nonstop for up to two hours and can handle 150% overload, which is very important for oil and gas sites and mines.
• Efficiency: Some types with an amorphous core are 99% efficient, and designs made with optimized silicon steel cut no-load losses by up to 30%.
• Environmental range: These units can be used in seaside, desert, and high-altitude (up to 4,000 meters) environments because they have IP55 security, can work in temperatures ranging from -40°C to 55°C, and have tanks that don't rust.
• Customization: ATEX-certified versions are used in places where explosives are present, seismic-resistant frames meet the needs of Zone 4 earthquakes, and small designs fit in urban substations.

When dry-type or cast-resin units would need to be over-sized or derated to handle the same load, the Oil-immersed transformer is a practical choice due to its performance.

Limitations to Consider

The risks that come with oil are real and need to be handled. Because oil leaks can be dangerous for both people and the environment, Xi'an Xikai tests every welded tank for 24 hours before sending it out. For distribution-class units, oil testing should be done at regular times and replacement should be planned when the dielectric strength goes below 30 kV or when dissolved gas analysis (DGA) shows that the unit is heating up in a way that isn't normal. People who buy in markets with strict rules should also make sure that biodegradable ester oil options are available as a drop-in upgrade.

Maintenance Tips and Troubleshooting Guide

Routine Oil Testing and Inspection

Testing for dielectric breakdown voltage (per IEC 60156) and DGA are the two most useful troubleshooting tools. DGA finds gasses like hydrogen, ethylene, and acetylene that are created when shielding breaks down electrically or physically. A rising acetylene reading is a sign of arcing that needs to be looked into right away. Every six months, look at the tank surfaces, radiator fins, and gasket seals for oil stains that could mean there are small leaks.

Bushing and Seal Maintenance

Bushings get dirty on the outside when they are near the coast or in factories. Every year, clean the porcelain or composite bushings and check the insulation resistance to see if water is getting in. When the light changes from blue to pink, it's time to replace the silica gel breather desiccant. Gaskets that are too old are a major cause of oil seepage; plan to replace them every 10 to 15 years.

Troubleshooting Common Problems

Strange humming or vibrations that happen while the machine is running are often caused by loose core laminations or resonance with support structures. Make sure that the core clips and fixing bolts are tight. Overheating alarms go off when the temperature signs on the windings go above the set points. This can happen if the radiators are clogged, the cooling fans stop working, or the overload lasts for a long time. Most of the time, cooling-related temperature rises can be fixed without taking the unit offline by cleaning the radiator fins and making sure the fans are turning.

Buying Guide: How to Choose the Right Oil-Immersed Transformer

Define Voltage Class and Power Rating

You should match the voltage class (10kV, 35kV, or 110kV) to the point where you link to the grid. In order to get the right power level (kVA or MVA), you need to think about how much power will be needed now and also how much will be needed in 30 years. For solar and wind farms, choose models that can handle 125% of intermittent inverter loads so that the insulation doesn't wear out faster.

Verify Certifications and Standards

National-standard type tests and grid entry skills are needed to buy from the Chinese State Grid or the Southern Grid. If you want to sell your goods in Africa or Southeast Asia, make sure they meet the requirements of IEC 60076, ISO 9001, and any other regional acceptance criteria. Xi'an Xikai has local approvals as well as CE, UL, and GOST-R certifications, which means it meets most foreign tender standards.

Evaluate After-Sales Support

A 10-year guarantee and technical help 24 hours a day, seven days a week lowers the risk of buying in places that are far from the grid. Before you sign a purchase order, make sure you have proof of factory leak tests, type test reports, and material test certificates. When buying 10 to 100 units per batch, like when a county upgrades its power grid, it's best to find a source that can give you a uniform bill of materials and test records that can be tracked across the whole order.

Conclusion

Because of its fluid cooling system, the Oil-immersed transformer is still the most popular choice for 10kV to 110kV distribution networks because it provides thermal performance, life, and stability that has been tested in the field at a reasonable price. When procurement teams know about the electromagnetic operating principle and the internal structure (core, windings, oil, tank, and accessories), they can write accurate specifications and objectively evaluate supplier proposals. When you add that information to the above buying factors, the process of going from a tender paper to commissioned equipment is a lot easier.

FAQ

1.How often should I test transformer oil?

Under normal service conditions, test the dielectric strength of distribution transformers every two to three years. In places that are hot, near the coast, or have a lot of traffic, checking should be done once a year. No matter how often it's supposed to be done, dissolved gas analysis should be done whenever there are strange temperature readings or safety alarms.

2.What is the difference between an oil-immersed transformer and a dry-type unit?

In order to increase load capacity, improve temperature handling, and lower cost per kVA at middle to large power levels, the Oil-immersed transformer uses liquid insulation and cooling. Dry-type units use air and solid insulation, which makes them safer indoors where oil leakage is not acceptable. However, they can only work with lower power levels and inside.

3.What safety protocols apply to transformer installation?

Follow the installation instructions in IEC 60076, keep the required distances, put in oil containment bunds to catch any fluid that leaks out of the tank, and make sure that all of the HV terminals have surge arresters turned on. Before turning on, make sure that the pressure relief valves and Buchholz relays work. The people who are finishing should have recognized electricity safety credentials.

Get a Quote from Xi'an Xikai — Your Trusted Oil-Immersed Transformer Manufacturer

Oil-immersed transformers for 10kV, 35kV, and 110kV uses are sold by Xi'an Xikai Medium & Low Voltage Electric Co., Ltd. to the utility, industry, and green energy sectors. We can help with grid upgrade projects from the planning stages all the way through to commissioning. Our engineers have more than 25 years of experience, are certified by IEC, ISO, and CE, and our products come with a 10-year warranty. Get in touch with our team to get detailed datasheets, type test results, or a quote on a project:

• serina@xaxd-electric.com
• amber@xaxd-electric.com
• luna@xaxd-electric.com
• Website: xaxd-electric.com​​​​​​​

References

1. IEC 60076-1: Power Transformers — Part 1: General, International Electrotechnical Commission, 2011.

2. IEEE Std C57.12.00: IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers, IEEE, 2015.

3. Transformer Engineering: Design, Technology, and Diagnostics — S.V. Kulkarni & S.A. Khaparde, CRC Press, 2012.

4. Mineral Insulating Oils in Transformers — CIGRÉ Technical Brochure 436, CIGRÉ, 2010.

5. IEC 60156: Insulating Liquids — Determination of the Breakdown Voltage at Power Frequency, International Electrotechnical Commission, 2018.

6. Dissolved Gas Analysis: It Can Save Your Transformer — IEEE Electrical Insulation Magazine, Vol. 25, No. 2, 2009.