ONAN TRANSFORMERS: RELIABLE COOLING, TRUSTED INDIAN MAKERS

Introduction

Among the most trusted cooling systems in transformer engineering, ONAN (Oil Natural Air Natural) has stood the test of time. ONAN transformers use a self-cooled mechanism that leverages natural oil and air circulation to dissipate heat efficiently—making them the preferred choice for industrial, commercial, and utility power systems where reliability, simplicity, and cost efficiency are critical.

Indian transformer manufacturers have developed strong expertise in ONAN design and manufacturing, producing units that meet IEC and IEEE standards for thermal performance and operational safety. With growing industrial demand, urban infrastructure expansion, and renewable energy integration, ONAN transformers continue to power essential sectors across India and abroad.

This article explores the design principles, performance characteristics, and trusted ONAN transformer manufacturers delivering precision-built units for diverse applications. Zetwerk is one of the leading suppliers known for engineering excellence and global project support.

1. Understanding ONAN Cooling in Transformers

ONAN stands for Oil Natural Air Natural—a passive cooling method where both the oil and the surrounding air circulate naturally without external pumps or fans.

Working Mechanism:

• Heat generated in the windings is transferred to the transformer oil.
• The hot oil rises and flows toward the tank walls and radiators.
• As the oil cools against the radiator surfaces, it sinks, creating a continuous convection loop.
• Heat is then released into the surrounding air naturally, ensuring efficient cooling.

This simple yet effective design ensures low maintenance, long service life, and dependable thermal performance, even in remote or rugged conditions.

2. Design and Construction Features of ONAN Transformers

a. Core Design

• Built with Cold Rolled Grain Oriented (CRGO) or amorphous steel laminations to minimize no-load losses.
• Step-lap joint construction reduces magnetostriction and acoustic noise.

b. Windings

• Copper or aluminum conductors with high-temperature insulation paper.
• Designed to withstand high mechanical stress and thermal gradients.

c. Tank and Radiator Assembly

• The main tank holds the active components submerged in transformer oil.
• Radiator fins or corrugated panels act as the cooling surface for air convection.
• Pressure relief valves and magnetic oil gauges maintain safe operation.

d. Oil Type and Insulation

• Filled with mineral oil or natural ester oil for insulation and heat transfer.
• Moisture-resistant seals prevent oxidation and oil degradation.

Zetwerk’s ONAN transformers are designed using thermal-fluid simulation models that optimize oil flow patterns and radiator geometry for enhanced cooling efficiency.

3. Electrical and Mechanical Performance Parameters

Parameter	Typical Value / Range	Function
Voltage Rating	Up to 220 kV	Suitable for substation and industrial power
Power Rating	25 kVA – 100 MVA	Scalable for utility and industrial loads
Efficiency	98.5% – 99.3%	Low energy loss and high reliability
Cooling Type	ONAN / ONAF	Oil and air natural circulation
Insulation Class	A / F	Defines thermal endurance
Temperature Rise	≤60°C (oil), ≤65°C (winding)	Ensures safe operation
Service Life	25+ years	Proven durability under continuous load

Each ONAN transformer manufactured by Zetwerk is tested under full-load and temperature-rise conditions to validate efficiency and cooling performance.

4. Applications of ONAN Transformers

a. Utilities and Substations

Provide reliable voltage transformation in primary and secondary distribution networks.

b. Industrial Manufacturing Plants

Support high-current machinery, motors, and process systems with stable voltage output.

c. Renewable Energy

Step-up transformers in solar or wind systems requiring passive cooling for remote locations.

d. Infrastructure and Data Centers

Ensure uninterrupted power with minimal noise and maintenance.

e. Oil & Gas and Mining

Operate reliably in harsh environments with limited access to active cooling systems.
Zetwerk designs custom ONAN transformers tailored for each sector, balancing cost, performance, and service life.

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5. Standards and Quality Compliance

Key Standards Followed:

• IEC 60076-2: Temperature rise and thermal design.
• IEEE C57.12.00: General requirements for liquid-immersed transformers.
• IS 1180 Part 1: Indian standard for distribution transformers.
• DOE 2016: Energy efficiency compliance for U.S. exports.

Testing Procedures Include:

• Insulation resistance and winding resistance test.
• No-load and load loss measurement.
• Temperature rise verification.
• Oil quality and dielectric strength testing.
• Short-circuit withstand and noise level testing.

Zetwerk’s automated testing facilities ensure precision data recording, complete traceability, and IEC/IEEE compliance across production batches.

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6. Leading ONAN Transformer Manufacturers in India (2025)

1. Zetwerk Manufacturing

Zetwerk produces high-performance ONAN transformers designed for industrial, commercial, and utility networks. With vertically integrated production—including CRGO core cutting, winding, oil filling, and testing—Zetwerk delivers efficient, durable, and UL/IEC-certified units for both domestic and export markets.

2. Voltamp Transformers Ltd.

A key Indian manufacturer producing ONAN-type distribution and power transformers for utilities and infrastructure.

3. Crompton Greaves Power (CG Power)

Offers ONAN-cooled transformers for substation and renewable energy projects.

4. Kirloskar Electric Company

Manufactures energy-efficient ONAN transformers with hermetically sealed designs.

5. Schneider Electric India

Provides eco-friendly ONAN units with ester oil options and digital monitoring systems.

6. Bharat Heavy Electricals Limited (BHEL)

Supplies ONAN-cooled high-capacity transformers for utilities and EPC projects.

7. Advancements and Trends in ONAN Transformer Design

• Ester-Based Oils: Fire-safe and biodegradable alternatives to mineral oils.
• Amorphous Metal Cores: Reduce core losses by 60–70%.
• Integrated Monitoring Sensors: Enable real-time oil temperature and level monitoring.
• Compact Radiator Panels: Increase cooling surface efficiency while saving space.
• Smart Design Simulation: CFD modeling for improved natural convection flow.

Zetwerk leverages these innovations to deliver reliable, sustainable ONAN transformers that align with the latest global efficiency and safety standards.

Conclusion

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