Application of Grain-Oriented Electrical Steel in Power Transformers
In power generation and transmission networks, power transformers are the core equipment for voltage transformation and energy delivery. From step-up transformers at large power plants feeding extra-high-voltage (EHV) and ultra-high-voltage (UHV) long-distance transmission lines, to voltage transformation at regional hub substations, a transformer’s efficiency, reliability, and service life directly affect the energy consumption level and safety of the entire grid. The fundamental material that determines all of this performance is grain-oriented electrical steel for power transformers (cold-rolled grain-oriented silicon steel, CRGO). Selecting the correct grade of grain-oriented electrical steel for different voltage classes and capacity ratings is the critical first step in controlling losses from the source, meeting international energy-efficiency standards, and optimizing total life-cycle cost.
1. What Is Grain-Oriented Electrical Steel?
Grain-oriented electrical steel is an iron-silicon alloy containing approximately 3% silicon. Through specific cold-rolling and high-temperature annealing processes, its internal crystal grains are aligned in a highly consistent preferred orientation along the rolling direction — the so-called Goss texture. This directionality gives the material exceptionally superior magnetic properties along the rolling direction: very high permeability, very low hysteresis loss, and high saturation flux density.
By contrast, non-oriented electrical steel has randomly oriented grains and is used in rotating machines such as motors. In a power transformer core, the magnetic flux is alternating but unidirectional, so grain-oriented electrical steel must be used to achieve high magnetic induction and low loss.
2. Why Power Transformers Must Use Grain-Oriented Electrical Steel
Power transformers operate continuously under alternating magnetic fields at 50 Hz or 60 Hz, with the core constantly undergoing repeated magnetization. Even a very small core loss is dramatically magnified by the enormous capacity and decades of service life. Core loss consists of two components:
Hysteresis loss – the energy consumed by domain rotation and wall movement during each magnetization cycle.
Eddy-current loss – the heat generated by localized currents induced within the steel sheets by the changing flux.
Grain-oriented electrical steel attacks both types of loss simultaneously: the preferentially oriented grain structure drastically reduces hysteresis loss; the high silicon content raises the electrical resistivity of the material, effectively suppressing eddy currents; and further reducing the strip thickness and applying an insulating coating pushes eddy-current loss even lower.
In addition, the high permeability of grain-oriented electrical steel allows the transformer to achieve the required flux density with a smaller magnetizing current, improving voltage regulation and reducing reactive power demand. The excellent low magnetostriction property directly translates into lower core noise, which is particularly important for large power transformers located in load centers or environmentally sensitive areas.
[GDC_row]
[GDC_column size=”half”]
Our 220kV outdoor power transformer successfully commissioned and operating stably in a utility substation project.
[/GDC_column]
[GDC_column size=”half”]
Engineered for sustainability: This outdoor power transformer features an optimized magnetic core for ultra-low no-load loss.
[/GDC_column]
[/GDC_row]
3. Main Types of Grain-Oriented Electrical Steel
Grain-oriented electrical steel （GOES Steel）for power transformers is generally divided into two major families:
1. Conventional Grain-Oriented (CGO)
The traditional product, offering good magnetic properties at a competitive cost. It is still used in some power transformer applications where loss requirements are not the most stringent.
2. High-Permeability Grain-Oriented (Hi-B)
Features more precise grain alignment, higher magnetic induction, and lower core loss than CGO at the same thickness. Hi-B steel has become the mainstream material for high-efficiency power transformers and EHV equipment. On a Hi-B base, domain-refinement techniques such as laser scribing or mechanical scribing can further subdivide the magnetic domains and reduce anomalous eddy-current loss, yielding domain-refined high-permeability steel with the lowest loss levels currently available.
4. How to Read a Grain-Oriented Electrical Steel Grade
Understanding the grade designation is essential before selection. Taking the Chinese national standard (GB) grade23QG090 as an example:
23 – nominal thickness of 0.23 mm (expressed in hundredths of a millimeter)
Q – grain-oriented (from the pinyin “Qu Xiang”)
G (or H) – indicates high permeability (Hi-B); the absence of this letter denotes conventional grain-oriented steel (CGO)
090 – guaranteed maximum specific total loss of 0.90 W/kg at 1.7 T, 50 Hz (loss value × 100)
Some mills use their own prefixes, such as Baosteel’s “B” series (B23R080, B27G120, etc.), and domain-refined products may carry additional designations such as “R”, “RK”, or “P”. Internationally common grades include M-3, M-4, etc. (AISI standard), as well as the ZDKH series of high-permeability steels. Regardless of the naming system, the core logic is always thickness + permeability grade + maximum specific loss value.
5. Recommended Grain-Oriented Electrical Steel Grades for Different Power Transformer Types
Power transformers require different grades and thicknesses of grain-oriented electrical steel depending on voltage class, rated capacity, and target efficiency standards. The table below provides recommended material selection guidelines based on typical power transformer categories found in power generation and transmission.
Table: Recommended Grain-Oriented Electrical Steel Grades for Different Power Transformer Types
Energy-Saving Core Retrofit for Aging Power Transformers
As per original design
35–500 kV
Upgraded to Hi-B or domain-refined grades
0.23 mm, 0.27 mm (replace original 0.30 mm)
23QG090, 27QG100, etc.
Upgrade design based on original loss targets
Note: The above grade examples use the GB designation system. Commercial grade names from different mills may vary, but the correspondence between thickness and loss level is consistent. Final material selection should be based on the transformer’s target loss values, core geometry, and manufacturing process.
6. Material Selection Logic and Total Life-Cycle Cost Optimization
Material selection for power transformers has shifted from simple lowest-price procurement to a Total Cost of Ownership (TCO) evaluation. No-load losses are converted into a capitalized loss value and incorporated into bid evaluation systems. Therefore, for large power transformers that remain energized year-round, even using more expensive 0.23 mm or even 0.20 mm domain-refined grades can recoup the incremental cost through enormous electricity savings within just a few years, and then generate net benefits for the remaining 20 to 40 years of service life.
For power transformers with relatively smaller capacity or lower load factors, Hi-B steel (e.g., 27QG110, 30QG120) can achieve a better balance between efficiency and cost. During grade selection, the punching and shearing processability of thinner-gauge material and the requirements for the surface insulation coating during core annealing must also be considered.
7. Key Application Practices for Grain-Oriented Electrical Steel in Power Transformer Cores
Power transformer cores are almost exclusively built from high-quality grain-oriented electrical steel laminations. The silicon steel coils are slit longitudinally, cut to length, and then stacked in a step-lapped configuration to form the core legs and yokes. This laminated structure, together with precision joint design, keeps the magnetic flux path aligned with the rolling direction as much as possible, maximizing the use of the steel’s superior magnetic properties.
During manufacturing, the following points directly affect the final transformer performance:
Insulation coating – ensures interlaminar resistance, suppresses eddy currents, and remains intact during annealing.
Step-lap joints – reduce flux distortion at corners, lowering both no-load loss and noise.
Stress control – cutting and clamping must be controlled, and large cores require stress-relief annealing to restore the material’s magnetic properties.
8. Energy-Efficiency Standards Driving Material Upgrades
Increasingly stringent global transformer efficiency standards (such as the IEC 60076 series, China’s GB 20052, etc.) are continuously lowering allowable no-load loss limits, forcing manufacturers to upgrade to higher grades of grain-oriented electrical steel. Each step up the efficiency class typically requires a higher permeability, lower loss grade. Combined with the rapid growth in UHV transmission, renewable energy integration, and grid interconnection, market demand for high-performance grain-oriented electrical steel remains robust.
For transformer manufacturers, material selection is no longer merely a cost decision — it is a strategic issue concerning regulatory compliance and market competitiveness.
9. Quick Checklist for Selecting Grain-Oriented Electrical Steel
Define the loss target – work backward from the transformer’s efficiency class and no-load loss limit to determine the required core material loss level.
Choose CGO or Hi-B – high-voltage, large-capacity, and high-efficiency requirements generally necessitate Hi-B or domain-refined products.
Determine thickness and grade – refer to the table above, combine with the loss budget and cost constraints, and lock in the appropriate thickness and specific loss level.
Confirm dimensions, coating, and delivery form – coil width, insulation coating type, full coil, slit coil, or cut laminations must match the core manufacturing process.
Verify consistency and certification – stable magnetic properties and complete material documentation are the basis for ensuring batch production consistency and passing type test certification.
10. Wuxi Zhongxin Special Steel – Your Professional Supplier of Grain-Oriented Electrical Steel for Power Transformers
Wuxi Zhongxin Special Steel Co., Ltd. is deeply rooted in the electrical steel field, specializing in providing a full range of high-quality grain-oriented electrical steel materials for the power transformer industry. Our supply scope covers multiple grades and thicknesses, from conventional grain-oriented steel to Hi-B and domain-refined steel, including:
Domain-refined high-permeability steel: 0.20 mm, 0.23 mm, 0.27 mm ultra-low-loss grades
Hi-B high-permeability steel: 0.23 mm, 0.27 mm, 0.30 mm high-induction grades
Conventional grain-oriented silicon steel: 0.27 mm, 0.30 mm economical grades
All products undergo rigorous testing, with magnetic properties conforming to relevant international standards. We also offer professional slitting and precision shearing services, helping customers reduce material waste and shorten core manufacturing lead times. Leveraging our location in Wuxi, a core area of the steel supply chain, we can provide power transformer manufacturers worldwide with a comprehensive solution featuring consistent quality, timely delivery, and competitive cost.
Whether you are developing a new UHV transformer or need a stable supply of high-permeability steel for high-voltage power transformers, Wuxi Zhongxin Special Steel is your reliable partner.
11. Outlook: Grain-Oriented Electrical Steel Supporting a Greener Grid Future
With the global push for carbon neutrality and the construction of next-generation power systems, UHV and cross-regional interconnection projects are driving increasing demand for ultra-low-loss power transformers. Grain-oriented electrical steel technology is continuously evolving towards thinner gauges (0.18 mm), lower specific losses (P1.7/50 < 0.70 W/kg), and lower magnetostriction. Domain-refinement processes such as laser scribing are also undergoing constant iteration. It is certain that high-performance grain-oriented electrical steel will remain the cornerstone material supporting the enormous power transformer infrastructure of the future.
Selecting the right grain-oriented electrical steel grade for power transformers is the first step in building an efficient and reliable grid. Wuxi Zhongxin Special Steel is ready to support your every power transformer achieve its best performance with professional service and stable quality.
Frequently Asked Questions (FAQ)
Which grades of grain-oriented electrical steel are typically used for power transformers?
It depends entirely on the transformer’s voltage class, capacity, and efficiency targets. UHV and EHV large power transformers use the lowest-loss domain-refined Hi-B steel (e.g., 23QG080, 23QH075); 220 kV and 110 kV classes often use high-permeability Hi-B steel (e.g., 23QG090, 27QG100); station service transformers can use Hi-B grades such as 27QG110, 30QG120. Refer to the selection table in the article.
What is the difference between CGO and Hi-B grain-oriented electrical steel?
CGO (conventional grain-oriented) offers good magnetic properties at lower cost; Hi-B (high-permeability grain-oriented) provides higher magnetic induction and lower core loss, making it the mainstream choice for large-capacity, high-voltage power transformers. On a Hi-B base, domain refinement can further reduce core loss.
How do you read a grade like 23QG090?
23 = 0.23 mm thickness, Q = grain-oriented, G = high permeability, 090 = maximum specific loss of 0.90 W/kg at 1.7 T / 50 Hz. The lower the loss number, the higher the steel’s efficiency.
What is the thinnest grain-oriented electrical steel used for power transformers?
Currently, grain-oriented electrical steel thickness for power transformers ranges from about 0.30 mm down to 0.18 mm. 0.20 mm and 0.23 mm are the mainstream choices for UHV and very large transformers. Thinner gauges mean lower eddy-current loss and higher efficiency, but also higher demands on the manufacturing process.
For detailed grade data, samples, or partnership inquiries, please contact Wuxi Zhongxin Special Steel Co., Ltd. We are ready to provide professional material selection support and reliable supply assurance for your power transformer projects.
