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What is an oil immersed transformer?

An oil-immersed transformer is an electrical transformer whose windings and magnetic core are submerged in a pool of insulating mineral oil (or alternative insulating fluids such as synthetic esters, natural esters, or silicone fluids). The oil in the power transformer performs two very important functions: electrical insulation and heat transfer. By surrounding the conductor windings and magnetic core, the oil increases the dielectric strength between components and removes heat generated during operation via convection and conduction to the transformer tank and external cooling surfaces。The core function of an oil transformer is to enable efficient long‑distance transmission via step‑up voltage conversion, and to satisfy distribution and end‑user needs via step‑down conversion. Owing to its large capacity, high reliability, and cost‑effectiveness, it is widely employed in power plants, transmission and distribution networks, and large industrial and mining facilities.

The technical parameters of oil immersed transformer

Operation conditions

Normal Service Conditions

1. Altitude: not exceeding 1,000 m above sea level

2. Maximum ambient temperature: +40°C

3. Maximum monthly average temperature: +30°C

4. Maximum annual average temperature: +20°C

5. Minimum ambient temperature: –25°C (standard); –45°C for special designs

6. Maximum daily temperature variation: 25 K

7. Relative humidity: ≤ 90% (at 25°C ambient)

Special Service Conditions

Upon request, transformers can be engineered to accommodate special environments, including:

1. High‑altitude sites (above 1,000 m)

2. Extreme temperature ranges (–30°C to +45°C)

3. High humidity, salt‑fog, or corrosive atmospheres

4. Seismic zones

If the actual operating conditions fall outside the above normal limits, proper derating measures shall be applied in compliance with recognised standards (e.g., GB 6450).

What are the core components of an oil immersed transformer？

The Oil immersed transformers  main components including bellow:

1. Core (Magnetic Circuit)

The core provides the low-reluctance magnetic path for flux linkage between primary and secondary windings. It is typically constructed from stacked grain-oriented electrical steel laminations (less than 0.35 mm thick) to minimize eddy current and hysteresis losses. Core has two main categories: core-type (where windings surround the core) and shell-type (where the core surrounds the windings).

2. Windings (Electrical Circuit)

The oil immersed power transformer windings are  coils made of copper or aluminum wires arranged in the required turns ratio. They usually have two types:

Primary winding: Receives the input voltage

Secondary winding: Delivers the transformed output voltage

Small capacity low-voltage windings often use copper wires, while large capacity ones use cylindrical copper foil. The high-voltage winding is made into multiple layers of cylindrical shape, which can ensure uniform distribution of current between each turn, with small magnetic leakage, high mechanical strength, and strong short-circuit resistance.

3. Tank and Enclosure

The tank is a leak-tight steel vessel that houses the core and windings and contains the insulating oil. Tanks may be plain or fitted with corrugations and radiators to increase external surface area for cooling.

4. Insulating Oil (Transformer Oil)

Traditionally, refined mineral oil serves as the insulating and cooling medium. The oil must possess high dielectric strength, low viscosity, high flash point, and low pour point, and must be free from acids, alkalis, dust, and moisture. Alternative fluids (synthetic esters, natural esters, silicone fluids) are sometimes used for improved fire safety or biodegradability.

5. Conservator (Oil Conservator)

A conservator tank mounted above the main tank provides space for thermal expansion and contraction of the oil. It also simplifies operation by allowing monitoring of the oil level.

6. Breather

There is a silicone desiccant inside the breather to prevent water vapor from entering the transformer. Because transformer oil will expand and contract with increasing or decreasing temperature, silicone desiccant can effectively block moisture, prevent oil contamination, and also make the oil last longer.

7. Radiators

External radiator units collect heat from the oil and dissipate it to the surrounding air.

8. Bushings

Insulated fittings that provide safe connections between the high- and low-voltage windings and the external circuit. For voltages below 1 kV, porcelain bushings are used; for 10–35 kV, gas-filled or oil-filled bushings are employed; for voltages above 110 kV, capacitive bushings are required.

9. Buchholz Relay (Gas Relay)

A safety device installed in the pipe between the tank and conservator that automatically detects gas accumulation resulting from insulation failures and alerts operators to potential faults.

10. Tap Changer

A device that changes the number of turns in the winding to regulate output voltage. Tap changers are classified as on-load tap changers (OLTC) or off-load (no-load) tap changers.

11. Pressure Relief Device (Explosion Vent)

The pressure relief device is installed on the tank cover, and this safety device can release excess pressure in case of internal failure to prevent tank rupture.

Product Features

Oil-immersed transformers offer a range of distinctive features that make them the preferred choice for many applications.

Main Applications Scope

Oil-immersed transformers are widely deployed verious of areas:

1. Power Transmission Networks: We often see step-up transformers in power plants, as well as step-down transformers (primary voltage above 52 kV) in high/ultra-high voltage, ultra-high/high voltage, and high/medium voltage substations.

2. Power Distribution Systems: In the power distribution system, especially in various substations, distribution transformers are mainly used to reduce medium voltage (10 kV, 20 kV, 35 kV) to low voltage (400 V), providing low voltage domestic power supply for end users.

3. Industrial Facilities: Large industrial parks, mines, metallurgical bases, and manufacturing factories require stable power supply, which needs to be distributed to various applications through distribution transformers.

4. Commercial and Residential: Residential areas, commercial districts, and urban infrastructure require the use of power transformers to convert the 10KV medium voltage in the transmission system to 0.4KV for electricity and lighting.

5. Renewable Energy: In wind and solar power generation systems,need the power tansformers upper and down the voltage.

6. Railway and Transportation: Traction power supply systems also need the power distribution transformers to step-down the voltage.

The classification for the power transformers

Oil-immersed transformers can be classified according to several criteria:

1. By Number of Phases

Three-phase transformers: Most commonly used in three-phase power systems

Single-phase transformers: Used when capacity is very large and transportation constraints apply; three single-phase units may form a transformer bank

2. By Number of Windings

Two-winding transformers: The most common type, with one primary and one secondary winding

Three-winding transformers: Used for larger capacities (above 5,600 kVA) to connect three different voltage levels

3. By Core Structure

Core-type transformers: Windings surround the core; simple construction; used in most power transformers

Shell-type transformers: Core surrounds the windings; more complex construction; typically used in small dry-type transformers

4. By Enclosure Type

Non-sealed (conservator) type: Equipped with an oil conservator to accommodate oil expansion

Fully sealed type: Corrugated tank walls absorb oil expansion, eliminating the need for a conservator and preventing oil contamination from atmospheric exposure

5. By Cooling Method(as shown in the table)

What are the difference between a ring main unit and a metal-clad switchgear?

Understanding the distinctions between oil-immersed and dry-type transformers is essential for proper selection.

FAQS:

FAQ 1: What is service life of an oil-immersed transformer, and what factors affect it?

Usually a well-maintained oil-immersed transformer typically  service life is designed to exceed 30 years. However, actual service life is influenced by several critical factors such as operating temperature (excessive heat accelerates insulation aging), moisture ingress (which degrades dielectric strength), oxygen exposure (which causes oil oxidation), loading patterns (frequent overloading shortens life), and the quality of routine maintenance including periodic oil testing and filtration. As long as the transformer operates within the rated thermal range and receives regular maintenance, it can usually achieve reliable operation for 40 to 50 years.

FAQ 2: What are the different cooling methods for oil transformation and how to choose the appropriate model?

Oil immersed transformers mainly use four cooling methods according to IEC and IEEE standards:

1. ONAN (Oil Natural, Air Natural): Relies on natural oil convection and radiator heat dissipation. Suitable for small to medium capacity transformers with stable loads.

2. ONAF (Oil Natural, Air Forced): Equipped with fans for forced air cooling, increasing the transformer's capacity. Suitable for areas with significant load fluctuations.

3. OFAF (Oil Forced, Air Forced): Uses oil pumps to circulate oil combined with forced air cooling. Suitable for large capacity transformers in high-load hub substations.

4. ODAF (Oil Directed, Air Forced): Directs oil flow through specific winding paths for optimal cooling. Used for very large capacity transformers where maximum cooling efficiency is required.

Selection depends on transformer capacity, load profile, ambient conditions, and economic considerations

FAQ 3: What are the most common faults in oil-immersed transformers, and how can they be prevented?

Common faults include:

1. Overheating: Caused by overloading, poor ventilation, or malfunctioning cooling systems. Prevention: Monitor load levels, ensure adequate ventilation, and regularly inspect cooling equipment.

2. Oil Leaks: Result from degraded gaskets, corrosion, or mechanical damage. Prevention: Regular visual inspections and timely gasket replacement.

3. Moisture Ingress: Reduces dielectric strength and accelerates oil degradation. Prevention: Maintain breather desiccant, ensure proper sealing, and perform oil testing regularly.

4. Dissolved Gas Accumulation: Indicates internal arcing, overheating, or insulation breakdown. Prevention: Install Buchholz relays for early detection and perform dissolved gas analysis (DGA) regularly.

5. Bushing Failures: Caused by contamination, cracking, or electrical stress. Prevention: Regular cleaning and inspection of bushing surfaces.

Routine maintenance including oil sampling, DGA, insulation resistance testing, and thermographic inspection is essential for early fault detection and prevention.

FAQ 4: Which standards govern the design, testing, and application of oil‑immersed transformers?

Oil‑immersed transformers must comply with a comprehensive set of international and national standards that define their electrical, mechanical, thermal, and dimensional requirements, as well as the corresponding test procedures. The principal standards applicable to these transformers include:

IEEE Standards (North America):

• IEEE C57.12.00 – Establishes general requirements for liquid‑immersed distribution, power, and regulating transformers, covering fundamental performance and construction criteria.
• IEEE C57.12.10 – Provides specific standard requirements for liquid‑immersed power transformers, addressing rating, insulation, and temperature rise limits.
• IEEE C57.129 – Sets out general requirements and test codes for oil‑immersed HVDC converter transformers, which are used in high‑voltage direct‑current transmission systems.

IEC Standards (International):

• IEC 60076 (series) – The primary international standard series for power transformers, covering general specifications, temperature rise, insulation levels, short‑circuit withstand capability, and other essential technical aspects. It serves as the global benchmark equivalent to many national standards.

Chinese National Standards (GB/T):

• GB/T 1094 (series) – The Chinese equivalent to IEC 60076, comprising multiple parts that address general requirements, temperature rise, insulation coordination, and other performance indicators.
• GB/T 6451 – Specifies the technical parameters and requirements specifically for oil‑immersed power transformers used in power systems, including rating, losses, and dimensional constraints.

These standards collectively encompass routine tests (performed on every unit), type tests (conducted on prototype designs), and special tests (such as short‑circuit or sound‑level measurements). Strict adherence to the applicable standards is mandatory to ensure transformer reliability, operational safety, and seamless interoperability within the broader power grid infrastructure.