Introduction
During the operation of a transformer, the main magnetic flux passing through the core produces core losses, while the current flowing through the windings generates copper losses. In addition to these primary losses, leakage flux produced by winding currents and stray flux escaping from the core under overexcitation conditions can induce additional losses in transformer structural components. These components include windings, core clamps, structural steel parts, and the transformer oil tank.
When leakage magnetic flux enters conductive steel structures, eddy currents are generated. These eddy currents create stray losses that may lead to localized overheating. Excessive local temperatures accelerate insulation aging, reduce the mechanical strength of insulation materials, and ultimately shorten transformer service life. Such cases of local overheating have been reported in transformer operations worldwide.
Although additional losses have little influence on small-capacity transformers, they become increasingly significant in large power transformers. When the leakage flux density exceeds a certain level, stray losses increase rapidly. According to industry data, additional losses in large transformers operating at rated load may account for approximately 30%–40% of total load losses. Therefore, minimizing stray losses caused by structural components and materials has become an important aspect of modern transformer design.
To improve efficiency and ensure reliable operation, medium- and large-capacity transformers, especially power transformers and reactors rated at 110 kV and above, commonly employ magnetic shielding or electromagnetic shielding inside the oil tank. These shielding systems effectively reduce additional losses by approximately 20%–30%, improve thermal performance, and extend transformer service life.
Today, transformer manufacturers around the world use a variety of shielding structures and manufacturing methods. Many modern shielding designs have evolved through the absorption and optimization of technologies originally developed by leading manufacturers such as Alstom and Hitachi. Their successful application has demonstrated significant benefits in reducing stray losses and preventing local overheating.
Why Transformer Oil Tank Shielding Is Necessary
Among all transformer structural components, the oil tank wall is one of the most vulnerable areas to leakage magnetic flux. Due to its large surface area and proximity to leakage flux paths, the tank wall often becomes a major source of stray losses in large-capacity transformers.
These losses not only reduce transformer efficiency but may also create localized hot spots on the tank wall. Excessive temperature rise can accelerate insulation deterioration, reduce operational reliability, and shorten transformer life expectancy.
To effectively control these problems, shielding structures are installed inside the oil tank to redirect or suppress leakage magnetic flux before it reaches the tank wall. Two shielding methods are commonly used in transformer manufacturing: magnetic shielding and electromagnetic shielding.
Transformer Leakage Magnetic Field
In order to effectively reduce additional losses in the oil tank wall and prevent local temperature rise, magnetic shielding made of silicon steel laminations or electromagnetic shielding made of copper or aluminum plates is commonly installed inside large-capacity transformer tanks.
These shielding materials guide, absorb, or redistribute leakage flux, thereby reducing its impact on the oil tank and surrounding structural components. As a result, transformer efficiency, thermal performance, and service life can be significantly improved.
Magnetic Shielding Structure
Strip-Type Magnetic Shield
The strip-type magnetic shield consists of silicon steel strips wound into ring-shaped structures. In this arrangement, leakage flux enters the magnetic shield through the thickness direction of the silicon steel sheet, resulting in relatively low eddy current losses within the shield itself.
Laminated Sheet-Type Magnetic Shield
The laminated sheet-type magnetic shield is constructed by cutting silicon steel sheets into specific dimensions and stacking them to form a shielding layer of a predetermined thickness. The assembled magnetic shield is then mounted on the inner wall of the transformer tank.
Compared with the strip-type design, this structure is easier to manufacture and install. However, because leakage flux enters the laminated sheets vertically, eddy current losses within the shield are generally higher.
In practical transformer design, the width and thickness of magnetic shields are selected to maintain the magnetic flux density within the shielding structure at approximately 1.7 T, ensuring acceptable temperature rise and minimizing shielding losses.
Leakage Flux Distribution in the Oil Tank Wall
a) Transformer leakage magnetic field
b) Leakage flux density entering the oil tank wall without magnetic shielding
c) Leakage flux distribution within magnetic shielding
The design of a magnetic shield is based on the distribution of leakage magnetic flux within the transformer. The magnetic flux entering the oil tank wall can be analyzed to determine the flux density distribution and the required shielding dimensions.
In Figure 3, Bm represents the peak leakage flux density entering both the oil tank wall and the magnetic shield. The magnetic flux within the shielding structure is obtained by integrating the flux density distribution over the shielding height. By dividing the total magnetic flux by the effective cross-sectional area of the shield, the magnetic flux density inside the shield can be calculated.
When magnetic shielding is applied, part of the leakage magnetic field may be redistributed, potentially increasing the radial leakage flux component in certain winding regions. Therefore, when calculating winding eddy current losses and hotspot temperatures, the influence of magnetic shielding on leakage field distribution must be considered.
Magnetic shielding can significantly reduce oil tank losses. After shielding is installed, total losses consist of losses in both the oil tank wall and the magnetic shield itself.
Proper installation and grounding are critical. Poor fixation may increase transformer vibration and noise, while inadequate grounding can result in floating potential and local discharge.
Principle of Magnetic Shielding
Magnetic shielding uses high-permeability materials such as silicon steel laminations to create a magnetic path with significantly lower reluctance than the transformer tank wall.
According to the principle of minimum magnetic reluctance, leakage magnetic flux naturally flows through the silicon steel shielding rather than entering the tank wall. This effectively reduces eddy current losses in the tank wall and minimizes the risk of local overheating.
At the same time, the magnetic shield confines leakage flux within a controlled path, reducing stray magnetic fields outside the tank and minimizing electromagnetic interference with nearby equipment.
Although the magnetic shield itself generates hysteresis and eddy current losses, these losses are generally much lower than the losses that would otherwise occur in the tank wall. As a result, overall transformer performance is improved.
Electromagnetic Shielding
Electromagnetic shielding is also widely used in large-capacity transformers, particularly near high-current leads where leakage magnetic fields are strongest.
Electromagnetic shielding uses conductive materials such as copper or aluminum plates mounted on the inner wall of the oil tank. Under alternating magnetic fields, eddy currents are induced within the conductive shield.
These induced currents generate magnetic fields that oppose the original leakage magnetic field. As a result, leakage flux entering the tank wall is reduced, decreasing eddy current losses and preventing local overheating.
Because eddy currents are generated within the shield itself, electromagnetic shielding also produces losses. Therefore, total tank losses consist of losses in both the tank wall and the shielding material.
Typical shielding thicknesses are approximately:
- Aluminum plate: 8 mm
- Copper plate: 4 mm
Compared with magnetic shielding, electromagnetic shielding can reduce the radial component of leakage flux, thereby reducing both tank losses and winding eddy current losses.
The relative effects of different shielding methods on transformer additional losses are shown below.
| Shielding Method | Total Additional Loss (%) | Oil Tank Loss (%) | Winding Loss (%) |
|---|---|---|---|
| Magnetic Shielding | 100 | 8 | 86 |
| Electromagnetic Shielding | 172 | 8 | 24 |
| No Shielding | 372 | 318 | 54 |
Principle of Electromagnetic Shielding
The operating principle of electromagnetic shielding is based on eddy current reaction.
When conductive materials such as copper or aluminum are exposed to an alternating magnetic field, induced eddy currents generate an opposing magnetic field according to Lenz’s Law. This opposing magnetic field partially cancels the original leakage magnetic field, reducing magnetic flux entering the oil tank wall.
After grounding, the conductive shield also acts as an electrical shield. Charges induced by electric fields are discharged through the grounding system, suppressing electric field coupling and preventing electromagnetic interference from propagating outside the transformer tank.
Comparison Between Magnetic and Electromagnetic Shielding
Although both shielding methods reduce stray losses, their operating principles differ significantly.
Magnetic shielding primarily redirects leakage magnetic flux through a low-reluctance path. It has little effect on the overall magnetic flux distribution of the transformer. Practical testing has shown that magnetic shielding does not significantly influence transformer short-circuit impedance.
Electromagnetic shielding, however, relies on eddy current reaction to oppose leakage flux. In areas with large current conductors, the induced eddy currents directly reduce leakage flux generated by the conductors. As a result, electromagnetic shielding can influence local leakage field distribution more significantly than magnetic shielding.
Manufacturing Methods of Transformer Tank Shielding
Welded Shield Structure
The welded shielding structure is commonly used in large-capacity, low-voltage transformers. One of the earliest applications was in Alstom’s 37,700 kVA / 10 kV rectifier transformer.
In this design, stainless steel shielding plates are installed on the low-voltage lead side of the tank wall to reduce eddy current losses and prevent local overheating. Similar designs have been used by Hitachi, where 4 mm copper plates are mounted inside the tank wall near lower clamping structures and welded using TIG welding techniques.
The welded structure provides reliable mechanical fixation and good shielding performance.
Wound Magnetic Shield Structure
The wound magnetic shield is manufactured by winding silicon steel strips on a special fixture and forming them into magnetic shielding units.
Because the laminated layers are aligned parallel to the leakage magnetic flux direction, magnetic flux can flow through the shield more effectively, reducing the amount entering the tank wall. This significantly reduces stray losses and prevents local overheating.
The number of shielding units depends on transformer size and the area requiring protection. In regions with high electric field intensity, additional insulation measures may be required to avoid local discharge caused by electric field distortion.
Bonded Magnetic Shield Structure
The bonded magnetic shield consists of silicon steel laminations coated with epoxy adhesive and bonded together to form a shielding assembly.
Typical dimensions include a width of approximately 300 mm and a thickness of about 10 mm, while the length depends on the required shielding height.
This design is simple to manufacture and does not require special equipment. The shielding assembly is bonded directly to the tank wall, making installation straightforward.
Compared with wound magnetic shields, the shielding effectiveness is somewhat lower because leakage flux enters the laminations vertically. Nevertheless, bonded magnetic shields are widely used in large power transformers due to their simplicity and cost-effectiveness.
A typical design used by Hitachi includes an additional insulating protective cover, which improves insulation performance and enhances shielding effectiveness.
Conclusion
Magnetic and electromagnetic shielding are essential technologies for reducing stray losses and preventing localized overheating in large-capacity transformers.
By controlling leakage magnetic flux inside the transformer tank, these shielding systems reduce oil tank losses, improve transformer efficiency, enhance thermal performance, and extend insulation life.
Whether implemented through magnetic shielding using silicon steel laminations or electromagnetic shielding using copper and aluminum conductors, proper shielding design plays a critical role in modern transformer manufacturing. As efficiency and reliability requirements continue to increase, advanced oil tank shielding technologies will remain an important area of development for power transformers worldwide.
