How Power Transformers Are Made Full Production Workflow Explained

 

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How Power Transformers Impact Our Daily Lives

Have you ever wondered how electricity from distant power plants reaches your home safely? Or how massive factories, hospitals, and data centers stay powered 24/7 without interruption? The answer lies in power transformers.

Although power transformers are not visible in our daily routines, they are essential to nearly every modern activity that relies on electricity. Installed in transmission and distribution systems, power transformers ensure that electricity generated at power plants reaches homes, businesses, and factories safely and efficiently.

What is a Power Transformer?

A power transformer is a high-voltage electrical device designed to transfer electrical energy between two or more circuits through electromagnetic induction. It is mainly used in transmission networks to step up (increase) or step down (decrease) voltage levels, ensuring efficient long-distance power delivery with minimal loss.

Power transformers play a vital role in the electrical power grid, connecting generation stations, substations, and distribution networks.

Inside the Power Transformer Manufacturing Process

A power transformer consists of four key components: the coils, core, insulation, and tank. In a well-organized factory, these parts can be produced simultaneously when time permits. After the individual components are manufactured, they are transported to the assembly area using overhead cranes for final assembly. Before assembly, it’s crucial to understand how each component is made and the equipment required for its production. In this article, transformer supplier Scotech will take you inside the power transformer manufacturing process-exploring how the core components are produced and assembled to meet international performance standards.

Power Transformers winding

The winding is the core functional component of a power transformer – it’s where voltage is transferred, stepped up, or stepped down through electromagnetic induction. The quality of the winding, its geometry, insulation, and assembly precision directly affect the transformer’s Electrical performance, Thermal performance, Dielectric strength, and Mechanical strength.

There are three important requirements for making windings: the windings must be wound tightly, the windings must be tightly sleeved, and the windings must be pressed tightly.

This is to prevent the windings from being easily deformed, damaged, punctured, or burned when there is an external short circuit and the transformer is subjected to a strong mechanical impact caused by the short circuit current.

Coil winding

Although many aspects of the transformer are automated, manual winding remains essential for power transformers due to complex designs, customized insulation needs, production flexibility, and real-time quality control – all areas where human skills outperform machines. For some complex winding processes, especially vertical winding, we arrange experienced employees with nearly 10 years of winding technology to make them.

Horizontal winding

In this stage, copper or aluminum conductors are precisely wound onto a mandrel in horizontal orientation. This method allows better tension control, layer alignment, and is ideal for high-voltage windings. Skilled operators ensure proper insulation placement and winding geometry to meet exact design specifications.

Vertical winding

It is mainly used for high-voltage or large-capacity transformer coils, especially above 35kV. It provides better insulation, mechanical strength, and cooling, making it ideal for disc-type coil structures in power transformers

Equipment： high-low voltage rolling machine, welding machine, tensioner

Coil Press

After winding, the coils are placed in a hydraulic press to compact the structure. This step ensures tight bonding, reduces air gaps, and improves mechanical strength to withstand short-circuit forces. Uniform compression also supports better dielectric performance.

Equipment： Coil Pressing Machine

Coil Drying

The pressed coils are then transferred to a vacuum or hot air oven for drying. This process removes moisture from insulation materials and conductors, ensuring high insulation resistance and long-term reliability. Proper drying is essential before moving to the final assembly

Equipment： vacuum drying furnace

Power Transformers Core

Silicon Steel Sheet Shearing

The burrs on the core sheets will affect the no-load performance. When the burrs are larger than 0.03mm, they will cause overlapping short circuits between the core sheets, increasing eddy current losses. Large burrs can also reduce the lamination coefficient, resulting in a decrease in the net cross-sectional area of ​​the core within the effective area, an increase in magnetic flux density, increased losses, and increased noise. Burrs can also damage insulation and form eddy currents between sheets. When the local eddy current loss density at the short-circuit point is too large, it may cause local overheating of the core.

By precisely adjusting the shearing process parameters, using deburring equipment, and controlling material quality, the burrs generated by the automatic shearing line when shearing the core can be effectively reduced, thereby improving the performance and production efficiency of the transformer.

Equipment: 400-line semi-automatic shear line,400 line.automatic shearing line,600 line automatic shearing line

Manual vs. Automated Iron core lamination

The iron core stacking process requires the participation of many workers.

A small transformer can be stacked with only two workers. But in large power transformers – typically those above 63MVA or with voltages over 220kV – the iron core becomes extremely large and heavy. As a result, core stacking and assembly often require a team of up to 10 skilled workers to manually align, lift, and position each laminated sheet with high precision.

This teamwork ensures proper magnetic performance, mechanical stability, and loss control, which are critical for high-voltage, high-capacity operation.

However, with the advancement of automation technology, automated core stacking machines are increasingly being used in the production of medium-sized transformers. These machines provide higher precision and efficiency, ensuring accurate alignment of silicon steel laminations, reducing human errors, and significantly speeding up production. Despite the efficiency of automated systems, manual stacking is still necessary for very large cores due to their size and complexity.

Equipment： Core Lamination Table, Automatic Core Stacking Machine

Power Transformers Oil Tank

The oil tank is a critical component of a power transformer. It encases and protects the core and windings, and holds the insulating and cooling transformer oil, which dissipates heat and enhances dielectric strength. A well-designed tank ensures mechanical protection, sealing integrity, and thermal performance, directly impacting the transformer’s lifespan and reliability.

Oil Tank Materials

Typically made from mild steel plates or corrugated steel, the tank must be durable, corrosion-resistant, and capable of withstanding both internal pressure and harsh environmental conditions. High-voltage transformers may require reinforced tank structures and specialized coatings.

Oil Tank Manufacturing Process

	Steel Plate Cutting High-precision plasma or laser cutting machines are used to cut steel sheets into the required dimensions for the tank body and tank accessories.
	Bending (Folding) Hydraulic bending machines shape the plates into side walls, base plates, and reinforcing parts. This ensures tight fits and clean angles.
	Welding Manual or automatic welding (such as MIG/TIG welding) assembles the tank structure. Skilled welders ensure leak-free joints and high mechanical strength.
	Polishing & Surface Finishing The welded surfaces are polished to remove burrs, slag, and uneven joints, preparing the surface for further treatment and coating.
	Painting & Coating The tank is shot-blasted, primed, and then coated with anti-corrosive paint using spray painting booths. This improves resistance to harsh outdoor environments.

Equipment: CNC Plasma/Laser Cutting Machine, Hydraulic Bending Machine, MIG/TIG Welding Equipment, Surface Grinder/Polisher, Spray Painting Booth, Industrial Drying Oven.

Power Transformers Insulation- Solid Insulating Materials

Insulation components are crucial to the safety and performance of power transformers. They electrically isolate high-voltage parts, prevent short circuits, and help ensure dielectric strength, thermal stability, and long-term reliability. Without high-quality insulation, even a perfectly wound coil or well-constructed core can fail prematurely.

The insulation parts of transformers include core insulation parts, winding insulation parts and body insulation parts. Although the manufacturing of various forms of insulation parts has its own characteristics, their processes are similar. They are mainly made of electrical insulation paper and electrical paperboard through punching, bonding, winding, hot pressing, and mechanical processing (drilling, milling, grinding). The method is also applicable to other insulation parts. The support bars, pads, and lead wood parts of the winding and core column produced with wood materials can be processed according to the drawings, except that they must be dried.

Solid insulation materials include: cardboard molding, oil duct stay curtain, molded parts, cardboard brace, corrugated paper, rhombus dot adhesive tape, corrugated paper tube…

Equipment: Hydraulic press, punching machine, shearing machine, circular shearing machine, band saw machine, cardboard strip beveling machine…

Power Transformers Active-part assembly

 

Coil Insertion Requirements

1. Coils must be inserted tightly and evenly to ensure mechanical stability and performance.

2. When the coil is halfway inserted, cardboard spacers and supporting sticks should be adjusted. Apply adhesive to the sticks for firm positioning.

3. A certain amount of insertion friction is necessary to maintain tightness.

4. Coils must remain concentric during assembly. If cardboard adjustments are needed, they should be made symmetrically.

5. Inner and outer coil spacers and oil duct spacers must be properly aligned, with no obvious skewing.

6. Permissible deviation: typically within 4–6 mm, not exceeding 8 mm.

Power Transformer Assembly Process: Oil-Immersed Type

1. Insulation Assembly of the Active Part

The first stage focuses on the insulation setup of the transformer’s active part, ensuring electrical safety and structural integrity.

2. Winding Assembly of the Active Part

During this phase, windings are installed precisely to maintain alignment and spacing. Special care is taken to achieve tight-fitting windings.

3. Drying Process of the Active Part

The assembled active part undergoes vacuum drying to remove internal moisture, ensuring long-term insulation reliability.

4. Final Assembly into the Tank

After drying, the active part is placed into the transformer tank and sealed for final assembly.

Power Transformers Final Assembly 

Drying Process of the Active Part
The active part is vacuum-dried in a drying chamber to remove internal moisture and improve insulation performance.
After drying, insulation resistance and inter-turn dielectric tests are performed.

Tank Assembly & Accessory Installation
The dried active part is placed into the transformer tank and secured with positioning fixtures.
Accessories such as radiators, oil conservators, bushings, pressure relief valves, temperature controllers, and tap changers are installed.

Oil Filling & Sealing
Degassed and filtered transformer oil is filled under vacuum to ensure full saturation and eliminate air bubbles.
After filling, the tank is sealed and pressure-tested to confirm airtightness.

Final Testing & Factory Acceptance
Electrical tests include a series of conventional tests such as turns ratio, winding resistance, insulation resistance, voltage ratio, and vector group test. There are also full-wave lightning impulse withstand tests, chopped-wave lightning impulse withstand tests, partial discharge tests, and type tests to ensure compliance with standards.

This article only briefly introduces the production process of transformers. In the actual production process of transformers, it is much more complicated than described in the article. The production processes of different types of transformers and transformers of different capacities are different. It requires the cooperation of engineers, workers, quality inspectors and managers to produce a transformer that meets international standards.

At SCOTECH, as one of the experienced power transformer manufacturers, we bring over 25 years of expertise in transformer manufacturing, metallurgical solutions, and turnkey power substation projects. Backed by a proven track record and certifications including ISO9001, ISO14001, and OHSAS18001, SCOTECH is proud to be a trusted partner in the global power industry.

Our large power transformers have successfully passed KEMA and CESI type tests, reflecting our commitment to international performance and safety standards. From raw material inspection to final testing, every step is backed by advanced systems and rigorous quality control.

We warmly welcome you to get in touch-whether you’re exploring sourcing options, seeking technical advice, or simply curious to learn more about what we do. You’re also invited to visit our facilities and see firsthand how we deliver quality and reliability to clients worldwide.

Let’s connect and explore how SCOTECH can support your next power project.