Solution Overview

ONAF Cooling for Oil-Immersed Transformers

TengQi Technology’s ONAF solution addresses limited cooling capacity and temperature-rise challenges in medium and large oil-immersed transformers, including space-constrained outdoor substations. It combines natural oil circulation with large-diameter, low-speed fans and reinforced supports to enhance air-side heat transfer, with cooling and noise performance subject to project-specific design and verification.

Performance Confirmation: Cooling capacity, temperature rise and noise require project-specific verification
Specified Fan Speed: 720 r/min; subject to project configuration
Airflow Design Basis: Radiator arrangement, site clearances and surrounding obstructions
Maximum Supported Impeller Diameter: Up to 1250 mm; project configuration may vary
Cooling Method: ONAF: natural oil circulation with forced-air cooling
Structural Reinforcement: Thick-wall tubes and welded reinforced fan brackets

Solution Highlights

Coordinate fan selection, structural reinforcement and airflow layout while retaining natural oil circulation.

Strengthen Air-Side Heat Transfer

Natural oil circulation retained, fan-driven airflow across radiator surfaces

Expand Airflow Coverage

Large-diameter impellers, low-speed operation, broader swept area

Reinforce Fan Mounting Areas

Thick-wall tube structure, welded reinforced brackets, support for larger fans

Match Airflow to Radiator Layout

Coordinated fan positions, aligned airflow direction, site-specific ventilation paths

TengQi Technology · Transformer Cooling Solutions

ONAF Temperature Rise and Heat Dissipation Capacity Solutions for Medium and Large Oil-Immersed Transformers

TengQi Technology (TQT) provides ONAF solutions combining natural oil circulation with forced-air cooling for medium and large oil-immersed transformers and outdoor substations facing insufficient cooling capacity, restricted airflow, and temperature-rise control requirements within limited installation space. The solution enhances air-side convective heat transfer through reinforced panel radiator structures, large-diameter low-speed fans, and air-side airflow management.

Keywords:ONAF forced-air cooling solutions, oil-immersed transformer cooling, transformer temperature-rise control, panel radiators, outdoor substation thermal management, TengQi Technology

ONAF Cooling: Combining Natural Oil Circulation with Forced-Air Cooling

ONAF is a commonly used cooling method for oil-immersed transformers, referring tonatural oil circulation and forced-air cooling. Temperature differences drive the natural circulation of insulating oil inside the transformer: heated oil enters the radiators and releases heat, then returns to the transformer tank as its density changes during cooling. Outside the radiators, fans generate forced airflow to enhance air-side convective heat transfer.

Compared with ONAN cooling, which relies primarily on the natural convection of ambient air, ONAF reduces air-side thermal resistance by improving airflow over radiator surfaces. Under the specified design conditions, this method can provide greater cooling capacity and help the system meet thermal management requirements at higher loads.

Transformer Cooling Challenges in Limited Space

Limited Heat Dissipation Area

Equipment spacing and installation clearances in outdoor substations are limited, making it difficult to meet increased cooling requirements simply by adding more radiators or expanding their installation footprint.

Insufficient Airflow and Coverage

Small-diameter impellers have a limited effective airflow coverage area and may not adequately cover radiator surfaces, restricting air-side heat transfer performance.

Structural and Vibration Risks

Large-diameter fans place greater demands on the load-bearing capacity and vibration resistance of their mounting structures. Conventional supports may not meet the requirements of long-term outdoor operation.

Temperature-Rise Control Challenges

When transformer loads increase without sufficient air-side heat transfer capacity, the ability to control oil temperature and winding temperature rise may be affected.

Solution Core Logic

Rather than simply adding fans to ONAN radiators, TengQi Technology's ONAF forced-air cooling solution coordinatesradiator structures, fan supports, impeller dimensions, speed control, and airflow managementas interconnected parts of a system. The aim is to enhance air-side heat transfer without changing the fundamental principle of natural oil circulation, while accounting for airflow, noise, vibration, and mechanical stability during outdoor operation.

Preserving Natural Oil Circulation:Natural oil circulation provides the underlying heat transfer mechanism, without requiring an oil pump as the primary means of circulation.
Enhancing Air-Side Convection:Fans force air across radiator surfaces to accelerate heat rejection to the surrounding environment.
Expanding Effective Airflow Coverage:A large-diameter impeller design increases the swept area and airflow coverage across radiator surfaces.
Strengthening Structural Compatibility:Thick-walled tubes and welded reinforced brackets provide the mechanical support required for the installation and operation of large-diameter fans.

Typical application scenarios

Application scenarios	Main Issues	Applicable Applications
Medium and Large Oil-Immersed Power Transformers	At higher loads, the cooling capacity of ONAN natural-air cooling may be insufficient.	A forced-air cooling system supports operation in, or switching to, ONAF mode according to design requirements.
Outdoor Substations with Limited Installation Space	Radiator installation footprints cannot be expanded indefinitely, yet thermal management capacity still needs to increase.	Enhanced air-side heat transfer within the available installation space allows clearances, ventilation, and maintenance access to be considered together.
Projects Requiring Large-Diameter Fans	Insufficient strength in existing supports can limit the stable installation and operation of larger impellers.	Thick-walled tubes and welded reinforced brackets meet the structural support requirements of large-diameter fans.
Noise- and Vibration-Sensitive Projects	High-speed, small-impeller configurations may generate more noticeable aerodynamic noise and mechanical vibration.	A large-diameter, low-speed fan design operating at 720 r/min balances airflow coverage with operating performance.

Solution Value: A Systematic Design Approach for Predictable Thermal Management

Increased Cooling Capacity

Forced airflow enhances air-side convective heat transfer, providing a basis for greater cooling capacity under ONAF conditions than under natural-air cooling.

Improved Conditions for Temperature-Rise Control

Expanded airflow coverage and stronger airflow over radiator surfaces help improve the conditions for controlling oil temperature and winding temperature rise.

Balancing Airflow, Noise, and Vibration

Combining large impellers with low rotational speeds avoids relying solely on higher speeds to increase airflow, helping balance airflow coverage with operating performance.

More Efficient Use of Limited Space

Where additional heat dissipation area is constrained, forced-air cooling offers one technical approach to increasing thermal management capacity within the available installation space.

TengQi Technology's Project Collaboration Capabilities

Building on its capabilities in panel radiator R&D and design, intelligent manufacturing, and thermal management services, Jiangsu Tengqi Electric Power Technology Co., Ltd. can coordinate technical consultation, R&D and design, product selection, sample development, customized manufacturing, quality verification, volume supply, and global delivery to meet the project needs of transformer manufacturers, EPC contractors, power utilities, and new energy customers. Specific structural parameters, performance requirements, inspection and testing items, delivery schedules, and service arrangements for ONAF cooling systems are subject to the project's technical specifications, product documentation, and mutually confirmed requirements.

ENGINEERING & MANUFACTURING

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