In the transmission and transformation of electrical energy in power systems, transformers are core hub devices, and their selection directly determines the safety, stability, economy, and operation and maintenance costs of facility power supply. Dry-type transformers and oil-immersed transformers, as the two mainstream types in current industrial and civil fields, have fundamental differences in insulation medium, cooling methods, and performance characteristics, and each has different application scenarios. This article delves into the differences between the two in terms of core structure, key performance, and applicable scenarios from three dimensions and provides a scientific selection method to assist enterprises and facility managers in making the optimal decision that aligns with their specific needs.

 

I. Core Structural and Operational Principle Differences

The core difference between dry-type transformers and oil-immersed transformers lies in the insulation medium and cooling methods, which directly determine their structural design, operational characteristics, applicable scope, and are the primary considerations in selection.

A. Dry-Type Transformers
Dry-type transformers use air (or inert gas) as the insulation medium, where the windings are solidly insulated with epoxy resin casting, Nomex paper wrapping, among others. They do not require insulating oil for cooling and insulation but rely on the solid insulation processes. The core structure consists of iron cores, windings, insulation systems, cooling systems, and accessories. Their operation principle is based on the electromagnetic induction law: high-voltage windings connected to an AC power supply generate an alternating magnetic field, which is transferred to the low-voltage windings through the iron core. Heat dissipation is achieved through natural airflow or forced air cooling (with the addition of axial-flow fans), eliminating the need for additional cooling medium circulation systems.

 

Mainstream dry-type transformers are divided into epoxy resin cast and impregnated types. Epoxy resin cast transformers, known for high insulation strength, good mechanical properties, and dust and moisture resistance, are the most widely used type in the market adaptable to various complex environments. The impregnated type, with excellent heat dissipation and lightweight structure, is suitable for clean environments with high heat dissipation requirements.

 

B. Oil-Immersed Transformers
Oil-immersed transformers use mineral insulation oil (or synthetic insulation oil) as the core insulation and cooling medium. The iron core and windings are completely immersed in a sealed oil tank. In addition to the iron core and windings, the core structure includes components such as the oil tank, oil cushion, radiator, gas relay, pressure release valve, and other specialized accessories. While their operational principle is similar to dry-type transformers, heat transfer relies on natural convection or forced circulation of the insulation oil (driven by oil pumps), dissipating heat to the air through the oil tank walls and radiator. Insulation oil also functions in arc suppression, air isolation, and retarding insulation aging, ensuring long-term stable operation of the equipment.

Oil-immersed transformers have three cooling methods: oil-immersed self-cooling, oil-immersed air cooling, and forced oil circulation air/water cooling. They respectively cater to small-capacity, medium-capacity, and large-capacity, high-load scenarios. Notably, forced oil circulation significantly enhances heat dissipation efficiency and meets the operational requirements of ultra-large capacity equipment.

 

II. Comparative Analysis of Key Performance Parameters (Professional Dimension)

Starting from the core requirements of facility operation and combining with industry standards, the following professional comparisons of both types across four key dimensions — safety performance, operation and maintenance costs, environmental adaptability, and electrical performance — present a quantitative reference for selection:

A. Safety Performance
Dry-type transformers have a natural fire and explosion advantage due to the absence of combustible insulating oil. They do not produce toxic gases during operation and are unlikely to cause fires even in the event of a short-circuit fault. They reach fireproof levels of F and H (resistant to temperatures of 180°C), eliminating the need for additional fire or leakage prevention facilities, making them suitable for locations with high occupancy or high fire safety requirements.

The insulating oil of oil-immersed transformers is combustible. In the event of a damaged oil tank or seal failure causing oil leakage, exposure to high temperatures or ignition sources can lead to combustion and explosion, posing certain safety risks. Therefore, during installation, safety facilities such as oil reservoirs and fire extinguishing devices need to be equipped. They are unsuitable for installation in areas with high occupancies or in environments prone to combustion and explosions. Their insulation grades typically range from Class A (resistant to temperatures of 105°C), lower than that of dry-type transformers.

 

B. Operation and Maintenance Costs
The operational process of dry-type transformers is straightforward. Without the need for oil quality testing or oil changes, only regular dust removal, inspection of terminal connections, and winding insulation status are required. This leads to lower annual maintenance costs and extends maintenance intervals to 6-12 months, suitable for scenarios with limited professional maintenance capabilities.

Oil-immersed transformers have higher maintenance requirements, necessitating regular oil quality tests (analyzing parameters like dielectric losses, moisture content, and chromatography). Insulation oil needs replacement every 3-5 years, and along with that, inspections of sealing elements, breathing apparatus silicone, gas relays, and other accessories are vital. Maintenance demands are extensive, costs are high, and a professional maintenance team is required, making them suitable for enterprises or institutions with well-developed maintenance capabilities.

 

C. Environmental Adaptability
Dry-type transformers are compact and leak-free, exhibiting strong adaptability to environmental humidity and dust. They can be directly installed indoors, in basements, or in restricted spaces like equipment compartments, without necessitating separate machine rooms. Particularly suitable for indoor settings like urban commercial complexes, high-rise buildings, and data centers, they can reach protection levels up to IP54 and above, shielding against dust and moisture intrusion.

In contrast, oil-immersed transformers are voluminous and heavy, demanding separate machine rooms or installations on outdoor platforms or container substations. They require high installation foundation capabilities, are significantly impacted by environmental temperatures, and may require anti-freezing measures in low-temperature environments, with enhanced cooling in high-temperature settings. Additionally, insulation oil leaks may pollute soil and water sources, making them unsuitable for environments with high environmental protection standards.

 

D. Electrical Performance

1. Capacity and Voltage Levels: Dry-type transformers are more suitable for low to medium capacities (typically ≤35 kV, below 20 MVA), with a capacity ceiling often not exceeding 3150 kVA. They are ideal for decentralized load supply. Oil-immersed transformers can cater to super-large capacities and ultra-high voltages (hundreds of MVA, 500 kV and above), making them the preferred choice for large-capacity centralized loads and long-distance power transmission, such as in wind power and photovoltaic step-up stations and large substations.

2. Overload Capacity: Dry-type transformers have a stronger overload capacity, capable of withstanding short-term operation at 1.2-1.5 times the rated load. With a forced air cooling system, their overload performance can be further improved, making them suitable for scenarios with large fluctuations in power load. Oil-immersed transformers generally have a lower overload capacity, typically 1.1-1.3 times the rated load, but some large-capacity units can achieve higher overload capacity through optimized cooling systems.

3. Efficiency and Noise: Both types of transformers can achieve efficiencies of 98%-99%. However, oil-immersed transformers, due to the high heat dissipation efficiency of their insulating oil, can achieve efficiencies up to 99.5% in large-capacity models, slightly better than dry-type transformers. In terms of noise, oil-immersed transformers have a noise level of 50-60 dB, lower than dry-type transformers (55-65 dB), making them more suitable for noise-sensitive applications.

4. Lifespan and Recycling Value: Under proper maintenance, oil-immersed transformers can have a lifespan of 25-30 years, and their insulating oil is recyclable, resulting in high recycling value. Dry-type transformers have a lifespan of 20-25 years, limited by the aging of solid insulation materials, resulting in lower recycling value.

 

III. Scenario-Based Selection Guide (Precisely Matching Facility Needs)

The core of selection is "matching the actual needs of the facility." Based on the performance comparisons above and the core requirements of different scenarios, the following are clear selection recommendations, covering mainstream scenarios such as industrial, civil, and special locations:

(I) Scenarios Where Dry-Type Transformers are Preferred

1. Indoor High-Density Locations: Such as commercial complexes, office buildings, hotels, hospitals, schools, subway stations, airports, etc. The core requirement is fire safety. Dry-type transformers pose no fire hazard and emit no toxic gases. They can be directly installed in areas close to the load center, such as distribution rooms and basements, saving transmission losses and simplifying fire safety approval processes.

2. Space-Constrained Areas: Such as electrical shafts in high-rise buildings, equipment mezzanines, small distribution rooms, etc. Dry-type transformers have a compact structure and small footprint. They do not require a separate machine room and can be flexibly integrated into existing equipment layouts. A subway station case shows that embedding a dry-type transformer in a cable mezzanine can save 20 square meters of equipment space.

3. Scenarios with limited operation and maintenance capabilities: such as small and medium-sized enterprises, community power distribution, small office buildings, etc. Dry-type transformers are easy to maintain and do not require a professional oil maintenance team. They only need to be cleaned and inspected regularly, which can significantly reduce operation and maintenance costs and manpower input. After an industrial park was converted to dry-type transformers, the total cost of ownership was reduced by 35% over ten years.

4. Scenarios with high fire and explosion protection and environmental protection requirements: such as chemical explosion-proof areas, data center main server rooms, hospital operating rooms, etc. Dry-type transformers are flame-retardant, explosion-proof, and leak-free, causing no environmental pollution. They can adapt to clean, high-temperature environments and meet N+1 or 2N system redundancy requirements, ensuring continuous power supply to critical equipment.

(II) Scenarios where oil-immersed transformers are preferred

1. Outdoor large-capacity power supply scenarios: such as outdoor substations, industrial park distribution stations, wind power/photovoltaic booster stations, railway traction substations, etc. Oil-immersed transformers have strong weather resistance, can be installed outdoors, and can meet the requirements of large capacity and high voltage levels. In one wind power project, three 200MVA oil-immersed transformers supported the entire wind farm's grid connection and power generation.

2. Long-distance power transmission and centralized load scenarios: such as power plants, large industrial and mining enterprises (steel plants, chemical plants), rural power grids, etc. Oil-immersed transformers have high efficiency, long service life, and can withstand continuous and stable operation. They are suitable for large-capacity centralized load power supply, and the unit capacity manufacturing cost is relatively low, making them suitable for cost-sensitive projects.

3. Scenarios with professional operation and maintenance capabilities: such as professional power supply companies and large industrial enterprises, which have a complete operation and maintenance team and spare parts supply system, can meet the maintenance needs of oil-immersed transformers such as regular oil quality testing and oil replacement, and can give full play to their advantages of long life and high recycling value, thereby reducing the total life cycle cost.

(III) Selection Considerations for Special Scenarios

1. Data Centers: Dry-type transformers are mandatory. They must meet fire safety requirements and employ an N+1 redundancy configuration. Some high-end data centers may opt for 2N system redundancy to ensure continuous power supply to IT equipment and prevent data loss or business interruption due to transformer failure.

2. Chemical Plants: Dry-type transformers are preferred in explosion-proof areas. Outdoor oil-immersed transformers can be used in ordinary areas, but their corrosion resistance must be improved to withstand chemical corrosion. In harsh outdoor environments such as mines and ports, weather-resistant oil-immersed transformers are preferred, with enhanced sealing and heat dissipation design.

3. High-Rise Buildings: Dry-type transformers are required for basements, rooftops, and refuge floors. Rooftop installations must use waterproof transformers, and refuge floor installations must use fire-resistant transformers to ensure compliance with building fire safety codes and avoid safety hazards.

IV. Core Selection Principles and Summary

The core of choosing between dry-type and oil-immersed transformers lies in balancing four key factors: safety, cost, operation and maintenance, and scenario suitability. There's no need to pursue high-end or low-priced options; the optimal choice is one that best meets the actual needs of the facility. The core principles can be summarized in three points:

1. Scenario Priority: Indoor, densely populated areas with high fire safety requirements → Dry-type transformers; Outdoor, large-capacity, long-distance power transmission → Oil-immersed transformers. This is the core premise of selection and crucial for avoiding safety hazards and resource waste.

2. Cost Balance: Dry-type transformers have a 20%-40% higher initial investment than oil-immersed transformers of the same capacity, but lower operation and maintenance costs and smaller space requirements, making them suitable for scenarios with long-term operation and limited maintenance capabilities. Oil-immersed transformers have a lower initial investment, but higher operation and maintenance costs and larger footprint, making them suitable for large-capacity scenarios requiring specialized operation and maintenance. A comprehensive consideration of the entire lifecycle cost is necessary, rather than just focusing on the initial construction cost.

3. Compliance and Adaptation: Must comply with local power regulations, fire protection regulations and environmental protection requirements. For example, indoor installations must meet fire protection standards, and outdoor installations must meet waterproof, antifreeze and anti-corrosion requirements. Special locations (such as explosion-proof areas) require the selection of dedicated models. If necessary, consult professional design institutes or equipment suppliers to develop customized solutions.

 

In summary, dry-type transformers offer core advantages of "safety, convenience, and environmental friendliness," making them suitable for indoor, small-to-medium capacity, and low-maintenance scenarios. Oil-immersed transformers, on the other hand, offer core advantages of "large capacity, high efficiency, and low cost," making them suitable for outdoor, large-capacity scenarios requiring specialized operation and maintenance.

 

When selecting a transformer, facility managers should comprehensively evaluate their facility's installation environment, load characteristics, safety requirements, and maintenance capabilities to ensure long-term stable operation and provide a reliable power supply for the facility.

 

Today I'm going to introduce a very important concept to all readers:

TOC about Dry-Type Transformer vs. Oil-Immersed Transformer.

1. WHAT IS TCO?

First let's clarify the Core Components of TCO (Not Just Purchase Price)

TCO = Initial Procurement Cost + Installation & Civil Construction Cost + Operating Loss Cost + Maintenance Cost + Compliance & Insurance Cost + Residual Value / Disposal Cost + Hidden Downtime Risk Cost

This is the key logic when comparing dry-type and oil-immersed transformers.
Many customers only look at the equipment price, ignoring long-term copper and iron losses, civil/fire protection costs, O&M, and fire risks.

 

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2. Key TCO Differences: Item-by-Item Comparison、

a. Initial Procurement Cost (CAPEX)

Oil-Immersed (Liquid-Filled):

For the same capacity and voltage level, lower unit price. Mature silicon steel and winding design gives strong economies of scale.

Dry-Type:
Epoxy resin cast or open-type structure, higher material and process cost, significantly higher upfront equipment price.

b. Installation, Civil Works & Supporting Infrastructure

Oil-Immersed:
Requires oil containment pit, firewalls, oil drainage pit, fire suppression system, independent transformer room or outdoor fencing.
→ High civil, fire protection, and anti-leakage investment, larger footprint.

Dry-Type:
No oil, no explosion protection, minimal fire protection requirements.
Can be installed indoors (distribution rooms, floors, basements) without oil pits or fire compartments.
→ Significantly lower civil costs and smaller footprint.

c. Operating Loss Cost (Largest TCO Component, Most Critical Long-Term)

No-load and load losses are similar, but dry-type has slightly poorer heat dissipation, leading to higher temperature rise under the same load.

Oil-filled units have better cooling and overload capability → slightly better long-term efficiency under full load.

TCO must be calculated based on 20–30 years of energy loss cost.
Differences are more pronounced under low-load or standby conditions.

d. Maintenance Cost (OPEX)

Oil-Immersed:
Requires regular oil testing, filtration, refilling, leak inspection, bushing cleaning, cooling fan maintenance.
→ Higher maintenance frequency and ongoing labor/material costs.

Dry-Type:
Almost maintenance-free. Only basic dust cleaning and insulation checks.
No oil handling or leakage risks → very low lifecycle O&M cost.

e. Safety, Compliance & Hidden Risk Costs

Oil-Immersed:
Uses flammable insulating oil → fire risk, leakage pollution, environmental compliance issues.
Restricted in high-rise buildings, malls, hospitals, and underground spaces.
High fire inspection cost and potentially huge downtime losses in case of fire.

Dry-Type:
Flame-retardant or non-flammable, no oil leakage, environmentally friendly.
Suitable for indoor, high-rise, underground, explosion-proof, and densely populated areas.
→ Near-zero hidden costs related to fire, environmental penalties, or downtime.

f. Lifespan, Residual Value & Disposal Cost

Oil-Immersed:
Lifespan 25–30 years. Transformer oil, core, and windings have high recycling value.
However, disposal requires professional oil treatment → environmental cost.

Dry-Type:
Lifespan 20–30 years. No oil disposal required.
Epoxy windings have slightly lower recycling value.

 

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3. Selection Conclusion 

When Oil-Immersed Has Better TCO?

Outdoor substations or industrial parks with ample space and no strict fire restrictions;

Large capacity, high-load, long-term operation → benefit from lower upfront cost and better cooling;

Sufficient land and acceptable civil/fire investment, less concern about maintenance labor.

When Dry-Type Has Better TCO?

High-rise buildings, malls, hospitals, subways, basements, and densely populated areas;

Explosion-proof, chemical plants, cleanrooms, environmentally strict facilities;

Avoid high civil/fire investment, prefer low maintenance and reduced fire/environmental risk;

Although higher upfront cost, over 20 years the savings in civil works, O&M, and risk far exceed the price difference → lower total TCO.

 

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4. Standard Approach to Building a TCO Report (Template Logic)

a. Set unified assumptions: same capacity, voltage, loss class, service life (20/30 years), and electricity price.

b. Break down costs: equipment cost, civil/fire cost, annual energy loss cost, annual maintenance, risk premium, residual value.

c. Apply discounted lifecycle cost calculation and determine payback period(How many years it takes to recover the higher dry-type cost through savings in energy, O&M, and civil works).

As an important type of electrical steel, silicon steel plays a critical role in the power industry. In recent years, China's silicon steel industry has made remarkable progress and emerged as a leader in the global market. This article introduces three technological breakthroughs in the rise of China's silicon steel industry, showcasing the outstanding contributions of Chinese electrical steel manufacturers and producers in technological innovation.

First Technological Breakthrough: Development and Production of High Magnetic Induction Silicon Steel
Chinese electrical steel manufacturers actively engage in the research, development, and production of high magnetic induction silicon steel to meet the growing demand. High magnetic induction silicon steel exhibits higher magnetic induction strength and lower iron losses, effectively reducing energy loss in power equipment. By adopting advanced production techniques and precise alloy design, Chinese electrical steel manufacturers have achieved breakthrough results, elevating the magnetic properties of silicon steel to new heights.

Second Technological Breakthrough: Promotion and Application of Amorphous Silicon Steel
Amorphous silicon steel, as a novel silicon steel material, features extremely low hysteresis losses and iron losses, offering higher operational efficiency and reduced energy consumption. Chinese electrical steel producers promote the application of amorphous silicon steel by introducing advanced production lines and manufacturing processes, effectively improving the quality and performance of silicon steel. Amorphous silicon steel has been widely employed in power equipment such as transformers, making significant contributions to the efficient operation of China's power industry.

Third Technological Breakthrough: Innovative Manufacturing Processes for Thin-Gauge Silicon Steel
Thin-gauge silicon steel holds immense potential for applications in the power industry, but its manufacturing process is relatively complex and imposes high requirements on production technology and equipment. Chinese silicon steel manufacturers have successfully developed efficient manufacturing technologies for thin-gauge silicon steel through continuous innovation and process improvement. These technological innovations not only enhance the production efficiency and quality of thin-gauge silicon steel but also reduce production costs, providing users with more competitive product options.

With the continuous development and innovation in China's silicon steel industry, Chinese electrical steel manufacturers and producers have achieved tremendous technological breakthroughs. The promotion and application of high magnetic induction silicon steel, amorphous silicon steel, and thin-gauge silicon steel have provided robust support for the development of the power industry and energy efficiency improvement. The Chinese silicon steel industry will continue to strive for technological innovation and development, making even greater contributions to the prosperity of the global electrical steel market.

Transformers are essential electrical devices that facilitate the efficient transmission and distribution of electrical energy. At the heart of every transformer lies its core, which plays a crucial role in transforming voltage levels. One commonly employed technique in constructing transformer cores is lamination. In this article, we will explore why lamination is used and delve into its significance in the design and performance of transformer cores.

 Why COGO Lamination?

The primary reason for incorporating CRGO laminations in transformer cores is to mitigate energy losses caused by magnetic characteristics while maintaining optimal performance. Laminated cores consist of numerous thin layers of a magnetic material, typically silicon steel, stacked together and insulated from each other. This technique introduces several benefits that enhance the efficiency and reliability of transformers.

 

Reducing Eddy Current Losses:

When an alternating current flows through the primary winding of a transformer, it induces a magnetic field in the core. However, this varying magnetic field can induce small circulating currents, known as eddy currents, within the solid core material. These eddy currents generate heat and consume a significant amount of energy, leading to undesirable energy losses. Lamination effectively addresses this issue by breaking up the solid core into thin insulated layers, thus interrupting the flow of eddy currents and minimizing energy dissipation as a result.

 

 

Controlling Magnetic Flux:

Lamination also helps in controlling the flow of magnetic flux within the transformer core. By dividing the core into multiple layers, each with its own magnetic path, laminations ensure that the magnetic flux follows a desired and efficient route. This controlled flux path minimizes magnetic leakage and maximizes the coupling between the primary and secondary windings, leading to improved transformer performance.

 

Reducing Hysteresis Losses:

Hysteresis loss occurs when the magnetic field within the core material repeatedly reverses its polarity with each alternating cycle. By using laminations, the size of the hysteresis loop, and thus the associated hysteresis losses, can be significantly reduced. This is achieved by carefully selecting the thickness and composition of the laminations, optimizing the magnetic properties and reducing energy losses within the core.

SO…

Lamination is a fundamental technique employed in transformer core design to enhance efficiency and reduce energy losses. By effectively controlling eddy currents, magnetic flux, and hysteresis losses, laminated transformer cores ensure optimal performance and improve the overall energy efficiency of electrical power distribution systems. As technologies continue to advance, the use of advanced laminated materials and designs will further contribute to the evolution of efficient and sustainable transformers.

Transformer core plays a crucial role in the efficient and reliable transmission of electrical power. As a key component, it provides a low reluctance path for the magnetic flux generated by the primary winding to be transferred to the secondary winding. Among various materials used for transformer cores, oriented silicon steel, also known as CRGO (Cold-Rolled Grain-Oriented) silicon steel or electrical steel, stands out for its exceptional magnetic properties and widespread application in different power ratings of transformers.

CRGO Silicon Steel: A Superior Core Material:

CRGO silicon steel is specifically engineered to exhibit grain orientation, enabling it to maximize its magnetic properties when subjected to an alternating magnetic field. The manufacturing process involves a controlled cold rolling technique that aligns the crystal grains within the steel in a specific direction. This grain orientation reduces the occurrence of magnetic domains and minimizes hysteresis losses and eddy current losses, making CRGO silicon steel the preferred choice for transformer cores.

Applications in Different Power Ratings:

1. Low-Power Transformers:
   In low-power transformers, such as those used in residential and small-scale commercial applications, CRGO silicon steel is utilized to enhance energy efficiency. The material's low core losses and high magnetic permeability contribute to reduced power wastage and improved voltage regulation, ensuring optimum performance in household appliances, lighting systems, and electronic devices.

2. Medium-Power Transformers:
   Medium-power transformers, commonly employed in industrial settings and power distribution networks, require reliable and efficient core materials. CRGO silicon steel offers excellent magnetic properties at intermediate power ratings, enabling enhanced energy transmission and minimal power losses. These transformers find application in areas such as manufacturing facilities, commercial buildings, and utility substations.

3. High-Power Transformers:
   For high-power transformers, such as those used in large-scale power generation and transmission systems, CRGO silicon steel provides superior performance. With its advanced grain orientation and optimized magnetic characteristics, it minimizes core losses and enhances efficiency, ensuring reliable power transmission over long distances. These high-power transformers are crucial components of electrical grids, enabling the efficient distribution of electricity to cities, industries, and infrastructure projects.

 

 

The selection of the core material plays a vital role in the performance and efficiency of transformers. CRGO silicon steel, also known as oriented silicon steel or electrical steel, stands out as an ideal choice for transformer cores across different power ratings. Its unique grain orientation and magnetic properties significantly reduce energy losses, ensuring optimal power transmission. Whether in low-power, medium-power, or high-power transformers, CRGO silicon steel demonstrates its superiority in enhancing efficiency and reliability in the transmission and distribution of electrical energy.