Evolution of the Three-Dimensional Wound Core
The three-dimensional wound core transformer was first invented in 1883 by Swedish engineer Jonas Wenström. That same year, it was installed at a paper mill and is now exhibited at the Swedish Royal Institute of Technology Museum. The transformer was originally manufactured and sold by Elektriska, a company founded in 1883, which later merged with Granström in 1890 to form ASEA.
In China, patents related to three-dimensional wound core transformers began to be filed and approved starting in 1995. By 1999, domestic manufacturers had begun research and development on this technology, and commercial sales started in 2002. However, widespread adoption was still limited at the time.
In 2004, Guangdong Haihong Transformer Co., Ltd. began developing the fundamental principles, production processes, and specialized tooling for three-dimensional wound core transformers, enabling the transition to mass production. They successfully developed the S13 oil-immersed 3D wound core transformer in 2006, followed by the launch of the open-type dry-type version in 2008 and the resin-insulated model in 2009. In 2012, they introduced a version utilizing amorphous metal. By 2015, the world’s first 110kV oil-immersed 3D wound core power transformer was developed and connected to the grid. In 2021, a 35kV 3300kVA amorphous metal open-type dry transformer was deployed in an urban rail transit system.
The first one is 31500kVA 110kV
Amorphous alloy three-dimensional wound core dry-type transformer

The first one is 3300kVA 35kV
Amorphous alloy three-dimensional wound core dry-type transformer
By 2007, over 100 manufacturers had partnered with Haihong to promote 3D wound core technology, significantly expanding the market. Today, the technology is well-established in China with major producers including Haihong Electric, Hebei Gaojing, Jiangxi Dazhu, TBEA Xinjiang, Jiangsu Guangte, Yantai Dongfang Electronics, and Shijiazhuang Tiantai Electric Power. China's production technology in this field is considered world-leading, and countries such as the U.S., Germany, Turkey, and several Southeast Asian nations are now attempting to license and adopt this Chinese innovation. Haihong has licensed the technology to manufacturers in the U.S., Germany, Turkey, and India for local production and distribution.
In recent years, 3D wound core transformers have been highlighted in China’s “14th Five-Year Plan” for industrial energy efficiency and have been included in several national recommendation lists, such as the Ministry of Industry and Information Technology’s "Energy Efficiency Star Product Directory" and the National Development and Reform Commission’s "Key Products and Services for Strategic Emerging Industries."
Recognized for their energy-saving, material-saving, environmentally friendly, and reliable performance, three-dimensional wound cores represent a major trend in the evolution of transformer technology. They have seen rapid growth across sectors such as power grids, renewable energy, rectifiers, rail transit, and furnace reactors.

12,500 KVA open-type three-dimensional wound core dry-type transformer Applied in the field of wind power generation

The 110kV three-dimensional wound core oil immersion power transformer is applied in the field of waste incineration power generation

The 30,000 KVA three-dimensional wound core oil immersion rectifier transformer is applied in the papermaking field
What Is a Three-Dimensional Wound Core?
A three-dimensional wound core is constructed by winding electrical steel strips into three identical rectangular frames with approximately semi-circular cross-sections. These frames are then joined side-by-side to form a triangular structure when viewed from above, creating a spatial (three-dimensional) magnetic core.
Manufacturing Process of 3D Wound Cores
1) Process Flow
Unlike conventional laminated cores, the 3D wound core uses a winding process. The diagrams below illustrate the process flow of oil-immersed transformers with 3D wound cores versus traditional laminated cores:
A. 3D Wound Core Oil-Immersed Transformer Process Flow
The process flow of three-dimensional wound core oil-immersed transformer
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B. Laminated Core Oil-Immersed Transformer Process Flow
Process flow of laminated core oil-immersed transformer
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C. Comparison of the Two Processes
Comparison of the production processes of three-dimensional wound cores and laminated cores ▼
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While the core manufacturing steps differ significantly, overall labor time and manpower costs are quite similar. For new investments, the 3D wound core process offers clear advantages in automation, ease of manufacturing, and process consistency.
2) Key Process Analysis
The most significant difference lies in the core fabrication: winding frames versus stacking laminations. As electrical steel undergoes different treatments in each method, the entire process flow differs substantially.
A. Curved Slitting
Purpose:
To ensure the cross-section of the wound frame is as close to circular as possible.
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If the frame’s cross-section is circular, the strip width must follow a curved profile.
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If the frame forms a polygonal approximation, the strip width follows a slanted line.
Most industry players favor the nearly circular option, as it balances material efficiency and ease of manufacturing.
This process enhances the core’s space utilization rate (filling factor) to 97.5–98.5%, reducing overall diameter and thus minimizing material usage for both the core and the winding.
B. Winding Process
Illustration of the iron core frame rolling
Purpose:
To wind the slitted strips into three identical core frames that can be assembled into a triangular 3D core structure. This is the most critical step and the main differentiator from the laminated core process.
Advantages of this process include:
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No Transverse Air Gaps Perpendicular to the Magnetic Path
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Greatly reduced magnetizing current:
In laminated cores, air gaps at the yoke-to-leg transition cause high magnetic reluctance. Even with multi-step slanted joints, the magnetizing current is still heavily consumed here.
In contrast, 3D wound cores eliminate such transverse gaps, leading to significantly lower excitation current. -
Lower noise levels:
Air gaps distort magnetic flux, leading to local saturation and magnetostriction. The mechanical discontinuity also causes additional vibration and friction noise—issues avoided in 3D cores. -
Reduced no-load losses:
Again, distortion and local saturation increase iron loss. The 3D core avoids this due to its continuous magnetic path.
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Magnetic Flux Follows Steel’s Grain-Oriented (100) Direction
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Lower no-load losses and current:
Grain-oriented electrical steel has optimal permeability along the rolling direction. 3D cores are slit and wound following this grain, so magnetic lines flow entirely along the material’s easy-magnetization axis.
In laminated cores, magnetic flux has to bend at corners, misaligning with the steel grain by up to 45°, which increases losses and requires higher magnetizing force. -
Reduced noise:
Better alignment also minimizes magnetostriction, reducing vibration-induced noise.
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Significant Stress at Core Corners
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In laminated cores, stresses are generally low if the process is well-controlled, resulting in 2–5% additional losses.
However, the corners of wound cores experience intense deformation during winding, introducing stress-related losses of over 60%, sometimes even exceeding 100%.
Therefore, stress-relief annealing becomes a critical step after winding.
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C. Annealing Process
Purpose:
To eliminate the stress introduced during winding and restore the magnetic properties of the electrical steel.
How These Key Steps Affect Performance and Material Usage
The influence of key processes of three-dimensional wound core on the product ▼
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(To be continued in Part 2)