3500 kVA 35kV Three-Phase Oil Immersed Transformers with ONAN Cooling System

Time：2026-03-18    Auther：ZTelec-www.ztelectransformer.com

The 3500 kVA 35kV three-phase oil immersed transformer with an ONAN cooling system is widely used in modern medium-voltage power distribution networks. Known for its stable operation, simple structure, and low maintenance requirements, this transformer type is commonly installed in industrial power systems, utility substations, renewable energy plants, and infrastructure projects.

Thanks to its reliable oil-immersed insulation structure and natural cooling mechanism, the 3500kVA 35kV oil-filled transformer provides long-term operational stability while maintaining high energy efficiency. It is considered one of the most cost-effective transformer solutions for medium-capacity power distribution applications.

Overview of the ONAN Cooling System in 3500kVA 35kV Oil-Immersed Transformers

The cooling method used in many medium-capacity transformers is ONAN, which stands for Oil Natural Air Natural. This cooling method dissipates heat through the natural circulation of insulating oil combined with natural air convection, without requiring additional mechanical cooling equipment.

During transformer operation, the electrical losses generated in the core and windings produce heat. The insulating oil inside the transformer tank absorbs this heat and rises naturally as its temperature increases. As the hot oil moves upward, it flows through external radiators or corrugated tank walls, where the heat is transferred to the surrounding air.

After releasing heat, the oil cools down and becomes denser, allowing it to descend back to the lower part of the transformer tank. This continuous circulation creates a stable natural convection cooling loop that effectively maintains safe operating temperatures.

The ONAN cooling design offers several operational advantages. Because the system does not require fans or oil pumps, the overall structure is simple and highly reliable. The absence of moving parts reduces the risk of mechanical failure and minimizes routine maintenance requirements. In addition, ONAN transformers operate with very low noise levels, making them suitable for installations near residential areas or commercial facilities.

For a 3500kVA transformer, the heat dissipation area provided by radiators or corrugated tank panels is carefully designed to ensure that the transformer can operate safely under full-load conditions, even in environments with relatively high ambient temperatures.

Main Technical Specifications of a 3500kVA 35kV Three-Phase Transformer

When selecting a medium-voltage power transformer, it is essential to match the electrical parameters with the requirements of the local power system. A typical configuration for a 3500kVA 35kV oil immersed transformer includes the following characteristics.

The rated capacity is 3500 kVA, which is suitable for medium-scale industrial loads and power distribution networks. The high-voltage side is usually designed for 35kV or 34.5kV systems, depending on regional grid standards.

The low-voltage side can be configured for different voltage levels depending on application requirements. Common configurations include 0.4kV for industrial and commercial power distribution, while other options such as 6.3kV or 10.5kV may be used for sub-transmission or internal power networks.

Voltage regulation is typically achieved through a tap changer. Most transformers use a de-energized tap changer (DETC), while some projects require an on-load tap changer (OLTC) to maintain voltage stability during operation. The common adjustment range is ±5% or ±2×2.5%.

The winding connection group often adopts the Dyn11 configuration, which features a delta connection on the high-voltage side and a star connection with neutral point on the low-voltage side. This configuration helps suppress third-harmonic currents and improves system stability.

In terms of insulation performance, oil-immersed transformers typically follow Class A insulation standards, with a thermal limit of approximately 105°C. Temperature rise limits are designed according to international standards such as IEC 60076 and IEEE C57.12.00. Under rated operating conditions, the top oil temperature rise generally remains within 55K to 60K, while the average winding temperature rise does not exceed 65K.

Modern transformer designs also emphasize energy efficiency. High-permeability grain-oriented silicon steel laminations and oxygen-free copper windings are widely used to minimize both no-load and load losses. These improvements help reduce energy consumption and lower long-term operating costs for power utilities and industrial users.

Typical Applications of 3500kVA 35kV ONAN Oil-Immersed Transformers

The 3500kVA 35kV ONAN oil immersed transformer is suitable for a wide range of medium-voltage power distribution scenarios.

In industrial parks and manufacturing facilities, the transformer functions as a primary step-down unit that converts grid voltage from 35kV to usable distribution voltages such as 10kV or 400V for factory equipment and production systems.

In renewable energy projects, such as solar photovoltaic plants and wind farms, this transformer can be used for voltage transformation between generation equipment and the utility grid, ensuring stable and efficient grid integration.

The transformer is also commonly installed in mining operations, transportation infrastructure, and remote substations. Its robust oil-filled structure and sealed tank design enable it to operate reliably in outdoor environments with high temperatures, dust, or humidity.

Procurement and Installation Considerations

When purchasing a 3500kVA 35kV oil immersed transformer, several important technical factors should be evaluated to ensure safe and reliable operation.

The radiator configuration must provide sufficient heat dissipation capacity. Buyers should verify that the radiator surface area in ONAN mode is adequate for full-load operation under the highest expected ambient temperature.

An effective oil protection system is also important. Many transformers are equipped with an oil conservator and silica gel breather to prevent moisture from entering the insulation system and to extend the service life of the transformer oil.

Protective accessories are essential for monitoring transformer operation. Common devices include oil level indicators, temperature gauges, pressure relief valves, and Buchholz relays. These components help detect internal faults and ensure operational safety.

Finally, the transformer should comply with international manufacturing and testing standards such as IEC 60076, ANSI, or other relevant national standards to guarantee quality and reliability.

The 3500 kVA 35kV three-phase oil immersed transformer with ONAN cooling is a mature and widely used solution in medium-voltage power systems. Its natural cooling design, stable operating performance, and low maintenance requirements make it an ideal choice for industrial power distribution, renewable energy integration, and infrastructure projects.

With proper design, high-quality materials, and compliance with international standards, this transformer type can provide long-term reliable power supply while maintaining excellent energy efficiency and operational economy.

If you need detailed specifications, technical support, or the latest quotation for 3500kVA 35kV ONAN oil immersed transformers, please contact our engineering team for customized power distribution solutions.