I. Introduction to Oil-Immersed Transformer Manufacturing
II. Raw Material Preparation and Quality Control
III. Core Manufacturing (Iron Core Processing)
IV. Coil Winding and Insulation Processing
V. Core and Coil Assembly (Active Part Assembly)
VI. Tank Fabrication and Component Installation
VII. Vacuum Drying—The Critical Step
VIII. Vacuum Oil Filling (Vacuum Impregnation)
IX. Final Assembly and Testing
X. Quality Assurance and International Standards
XI. Industry Trends: Energy Efficiency and Smart Transformers
XII. Why Choose Xinhong Electrical
Oil-immersed transformers are the backbone of modern power systems, widely used for voltage conversion and power transmission across industrial facilities, power grids, mining operations, and renewable energy projects. These transformers rely on insulating oil as both a dielectric medium and a cooling agent, offering exceptional heat dissipation, high reliability, and long service life.
The manufacturing process of an oil-immersed transformer requires precision engineering, strict quality control, and adherence to international standards. At Xinhong Electrical, we have perfected this process over years of dedicated research and development, producing oil-immersed transformers ranging from 30kVA to 10,000kVA, with voltage ratings of 6kV, 10kV, 20kV, and 35kV.
This article provides a detailed walkthrough of the oil-immersed transformer manufacturing process—from raw material preparation to final testing—offering valuable insights for engineers, procurement professionals, and anyone interested in transformer technology.
Before any manufacturing begins, all raw materials undergo rigorous quality inspection. The primary materials include:
Iron core materials: High-permeability cold-rolled grain-oriented silicon steel (CRGO)
Conductor materials: Oxygen-free copper wire or aluminum wire
Insulation materials: High-grade insulating paper, pressboard, and barrier materials
Transformer oil: High-quality mineral oil or natural ester biodegradable oil
Tank materials: High-strength steel plates
Accessories: Bushings, tap changers, cooling fins, valves, and gaskets
All incoming materials are tested for compliance with design specifications. For silicon steel, magnetic properties are verified; for conductors, conductivity and tensile strength are measured; for insulating oil, dielectric strength and moisture content are analyzed. Xinhong Electricalmaintains an ISO 9001-certified quality management system to ensure consistent material quality throughout the supply chain.
The transformer core is the magnetic circuit that channels the magnetic flux between primary and secondary windings. Core quality directly determines transformer efficiency, no-load losses, and noise levels.
Key steps in core manufacturing:
Cutting: High-quality cold-rolled grain-oriented silicon steel sheets are precisely cut using fully automated CNC slitting and shearing lines. Advanced equipment ensures minimal burr formation and accurate dimensional tolerances.
Stacking: Cut silicon steel sheets are stacked according to a predetermined pattern—typically a step-lap configuration that minimizes magnetic reluctance and reduces core losses.
Clamping: The stacked core is secured with clamping structures to maintain mechanical integrity and prevent vibration during operation.
Core Testing: The completed core undergoes magnetic property testing, including no-load loss measurement and excitation current analysis, to verify electrical performance.
Modern transformers, such as Xinhong Electrical’s S11, S13, S20, and S22 series, utilize high-permeability CRGO silicon steel with laser scribing technology to further reduce core losses. Advanced stacked core designs can achieve a 5% reduction in no-load losses compared to traditional manual stacking methods.
The windings (coils) are the electrical circuit of the transformer, responsible for voltage transformation. This stage is one of the most critical in oil-immersed transformer manufacturing.
Coil winding steps:
Winding: Conductors (copper or aluminum) are wound onto mandrels using precision winding machines or foil winding machines. Turn count is precisely controlled according to the transformer’s voltage ratio and capacity requirements.
Insulation application: Insulating paper or film is interleaved between layers of the winding to provide electrical isolation between adjacent turns.
Lead-out preparation: Terminals are prepared for connection to bushings, tap changers, and other external components.
Drying and impregnation: Freshly wound coils contain moisture that can compromise insulation performance. Coils undergo oven drying to remove moisture, followed by impregnation with insulating oil (or varnish for certain applications) to enhance dielectric strength and moisture resistance.
The coil winding configuration varies by transformer voltage level. For high-voltage applications (≥220kV), interleaved or shielded winding structures are employed to improve impulse voltage distribution. Xinhong Electrical uses oxygen-free copper electromagnetic wire for all critical winding applications, ensuring optimal conductivity and thermal performance.
Once the core and coils are manufactured separately, they are assembled into the transformer’s active part (also called the core-and-coil assembly or “active part”).
Assembly steps:
Core-coil fitting: The pre-wound coils are lowered onto the core legs, with insulating barriers placed between high-voltage and low-voltage windings to prevent electrical breakdown.
Clamping and pressing: The assembled core and coils are mechanically clamped to resist electromagnetic forces generated during short-circuit conditions. Precision-engineered insulating press plates and clamping structures ensure uniform pressure distribution.
Lead connection: Internal leads from the windings are connected to external bushing terminals, with careful attention to insulation clearance and creepage distances.
Component installation: Accessories such as the tap changer (for voltage regulation), temperature sensors, and oil-level indicators are installed.
Throughout assembly, Xinhong Electrical maintains cleanroom conditions to prevent contamination of insulation materials. Any dust or metal debris introduced at this stage can create conductive paths and lead to premature failure [6†L9-L11].
The transformer tank houses the active part and insulating oil, providing mechanical protection and environmental sealing.
Tank fabrication steps:
Cutting and forming: Steel plates are cut, bent, and welded into tank shells using automated fabrication equipment.
Component welding: Cooling fins, radiators, mounting brackets, lifting lugs, and grounding terminals are welded to the tank.
Leak testing: Completed tanks undergo both positive pressure and vacuum leak testing to ensure no oil leakage will occur during service. All gaskets and seals are made from high-quality acrylic rubber to prevent seepage.
Surface treatment: Tanks are shot-blasted, primed, and painted with corrosion-resistant coatings for outdoor service.
The tank design includes features such as a sealed conservator (oil expansion tank) to isolate transformer oil from atmospheric moisture, and pressure relief valves for overpressure protection.
After tank fabrication, the active part is carefully lowered into the tank. This operation requires precise alignment to avoid damaging windings or insulation.
The assembled active part (core and coils) absorbs moisture from the ambient air during handling. Even trace amounts of moisture drastically reduce the dielectric strength of insulation materials. Vacuum drying is the process that removes this moisture.
The vacuum drying process:
The assembled active part is placed inside a large vacuum drying oven. The oven temperature is raised to approximately 70–100°C while a vacuum is applied. Heating and vacuuming continue for 12 hours or longer, until the insulation has been fully dried and outgassed. The process produces only water vapor, which is exhausted from the oven.
For high-voltage transformers, drying cycles can last 24–48 hours to achieve extremely low residual moisture levels. Xinhong Electrical uses advanced vacuum drying systems that continuously monitor insulation resistance throughout the process, terminating the cycle only when predetermined dryness targets are met.
After drying, the transformer must be filled with insulating oil. Traditional oil filling methods introduce air bubbles and moisture, compromising insulation performance. Vacuum oil filling is the industry-standard method for high-quality oil-immersed transformers.
The vacuum oil filling process:
Oil pre-treatment: Raw transformer oil is passed through a vacuum oil purifier that removes dissolved gases, water, and solid particles. The oil is heated to reduce viscosity and improve filtration efficiency.
Tank evacuation: The assembled transformer tank is evacuated to a high vacuum level (typically below 0.1 mbar) using vacuum pumps connected to the tank’s oil filling valve.
Oil injection: While maintaining vacuum, pre-treated oil is admitted into the tank through the bottom filling valve. Oil flows upward, displacing the remaining gases without trapping air pockets.
Soaking: Once filled, the transformer is allowed to stand for approximately 10 hours. This soaking period allows oil to fully penetrate insulation materials and any trapped gas bubbles to rise to the surface.
Final vacuum: A final vacuum application removes gas bubbles that have collected at the oil surface.
Transformers designed for distribution applications (such as S11, S13, and S15 series sealed-type oil-immersed transformers) rely on this vacuum filling process to ensure optimal insulation performance and long-term reliability.
Following vacuum oil filling, the transformer undergoes final assembly and comprehensive testing.
Final assembly includes:
Installing bushings, tap changer controls, thermometers, oil-level gauges, and pressure relief devices
Mounting radiators or cooling fans (for forced-cooled designs)
Connecting external terminals and grounding points
Final cosmetic inspection and touch-up painting
Testing procedures (performed in a dedicated test bay):
| Test Type | What It Verifies |
|---|---|
| Winding resistance measurement | Continuity and proper termination of windings |
| Voltage ratio test (Turns ratio) | Correct voltage transformation ratio |
| Insulation resistance test (Megger) | Quality of insulation between windings and ground |
| Dielectric withstand test (Hi-Pot) | Ability to withstand overvoltage without breakdown |
| No-load loss and excitation current | Magnetic circuit efficiency, core loss performance |
| Load loss and impedance voltage | Copper losses and short-circuit impedance |
| Oil dielectric strength test | Quality and cleanliness of insulating oil |
| Pressure leak test | Integrity of seals and gaskets under pressure |
Transformers that pass all tests receive a test certificate and are prepared for packaging and shipment. Any unit failing a test is returned for rework and retesting before delivery.
Xinhong Electrical designs and manufactures all oil-immersed transformers in strict compliance with internationally recognized standards:
IEC 60076 series – International standard for power transformers, covering insulation, cooling, and testing
GB 1094 series – Chinese national standard equivalent to IEC 60076
GB/T 6451 – Technical specifications and requirements for three-phase oil-immersed power transformers
GB 20052 – Energy efficiency grades for power transformers
In addition to these design and testing standards, Xinhong Electrical operates an ISO 9001-certified quality management system and holds CCC certification for products intended for the Chinese market.
Routine audits by SGS and BV further validate our quality processes and product performance.
The oil-immersed transformer industry is evolving rapidly, driven by stricter energy efficiency regulations, environmental concerns, and digitalization.
Key trends include:
Energy efficiency upgrades: The revised GB 20052-2024 standard has raised the bar for transformer efficiency. Compared to older standards, new Class 3 energy-efficient 35kV oil-immersed transformers achieve a 30% reduction in no-load losses. Class 1 units reduce no-load losses by an additional 10%. Xinhong Electrical produces transformers in energy efficiency classes S11, S13, S18, S20, and S22, ensuring compliance with the latest global standards.
Eco-friendly insulating oils: Traditional mineral oil is being replaced by biodegradable alternatives. Natural ester oils such as FR3 offer 100% biodegradability and a flash point twice that of mineral oil, significantly improving fire safety. These are now mandated for sensitive installations such as hospitals and schools in several European countries, with pilot projects underway in China.
Smart transformers: Power utilities worldwide are demanding transformers equipped with online monitoring capabilities. Digital transformers integrated with dissolved gas analysis (DGA), temperature sensors, and edge computing functions enable remote diagnostics and predictive maintenance. Early adopters report a 70% reduction in fault response time.
Global market growth: The global oil-immersed transformer market is expected to grow from approximately $370 billion in 2024 at a compound annual growth rate exceeding 6.5% through 2032. Drivers include power infrastructure upgrades, renewable energy integration, and expanding industrial electrification.
Xinhong Electrical is actively investing in R&D for next-generation oil-immersed transformers that combine high efficiency, eco-friendly insulating media, and smart monitoring capabilities—ensuring our products remain at the forefront of the industry.
Xinhong Electrical (Jiangsu Xinhong Electrical Equipment Co., Ltd.) is a trusted manufacturer of high-quality oil-immersed transformers, with decades of experience in research, development, production, and international trade.
Our strengths include:
Comprehensive product range: Oil-immersed transformers from 30kVA to 10,000kVA, voltage ratings 6kV/10kV/20kV/35kV, meeting a wide variety of distribution and power transmission needs.
Customized solutions: Bespoke engineering to match client specifications—from special voltage ratios to unique enclosure configurations.
Global compliance: Products manufactured to IEC 60076, GB 1094, and GB/T 6451 standards, with SGS/BV certification.
Robust after-sales support: Dedicated technical support team and comprehensive after-sales service system ensuring timely assistance throughout the product lifecycle.
Commitment to quality: ISO 9001-certified quality management with strict process controls at every manufacturing stage.
Whether you are upgrading your power infrastructure, expanding industrial facilities, or developing renewable energy projects, Xinhong Electrical delivers oil-immersed transformers engineered for reliability, efficiency, and long-term value.
Contact Xinhong Electrical today to discuss your transformer requirements.
