Oil-Immersed Transformer Capacity Parameters Table: Key Points for Selection

Oil-immersed transformers are one of the most widely used transformer types in power systems. They rely on transformer oil for both insulation and cooling, ensuring stable performance under continuous operation. When engineers, project managers, or procurement specialists need to select an oil-immersed transformer, the most critical reference is the capacity parameter table. By mastering these data and understanding how they affect performance, one can ensure reliable power supply and reduce long-term operation costs.

1. What is an Oil-Immersed Transformer?

An oil-immersed transformer is a type of power or distribution transformer that uses mineral oil or synthetic insulating oil. The oil not only enhances dielectric strength but also transfers heat generated during operation. This structure provides excellent insulation, cooling, and protection for transformer windings and cores. Compared with dry-type transformers, oil-immersed units are better suited for outdoor environments, high-capacity loads, and large-scale grid applications.

2. Importance of Capacity Parameters

The transformer capacity parameters determine how much electrical load the device can handle without overheating or efficiency loss. If the capacity is too small, the transformer may overload, leading to insulation aging, overheating, or even accidents. If the capacity is too large, initial investment and idle losses increase unnecessarily. Therefore, accurate sizing and parameter selection is a critical step in transformer engineering.

3. Oil-Immersed Transformer Capacity Parameters Table

Below is a commonly referenced capacity parameter table for oil-immersed transformers. It lists the rated capacity, no-load loss, load loss, short-circuit impedance, and cooling method. These parameters are based on IEC and GB standards, and are widely applied in distribution and industrial power systems.

4. Explanation of Key Parameters

Rated Capacity (kVA): Indicates the maximum continuous load the transformer can carry. Selection should be based on actual system demand plus a reasonable growth margin.

No-Load Loss: The energy loss when the transformer is energized without load. This affects long-term operating costs, especially for base-load transformers.

Load Loss: The loss caused by current passing through windings during load operation. This is proportional to the square of the current and directly affects efficiency.

Short-Circuit Impedance: Determines the voltage drop during load and affects system short-circuit current levels. Higher impedance reduces fault current but increases voltage regulation.

Cooling Method: ONAN and ONAF cooling methods are widely used. ONAN relies on natural oil and air circulation, while ONAF uses fans to force air cooling, improving thermal performance for high-capacity units.

5. How to Select Oil-Immersed Transformer Capacity

When selecting an oil-immersed transformer, several factors must be considered:

First, calculate the total load capacity of the facility, including lighting, HVAC, motors, and special equipment. Ensure that the selected transformer has a capacity 20–30% higher than the maximum estimated load to handle future expansion.

Second, consider load characteristics. For fluctuating loads such as industrial motors, a transformer with higher overload tolerance should be selected. For continuous stable loads, efficiency and loss performance are more important.

Third, evaluate installation environment. For outdoor substations or high-temperature regions, cooling capacity and protection grade should be prioritized. Oil-immersed transformers generally require oil conservators and protection systems against leakage and fire.

Finally, refer to energy efficiency standards. New national and international standards demand reduced no-load and load losses, which directly impact operational savings. High-efficiency oil-immersed transformers can reduce energy bills significantly over their lifecycle.

6. Oil-Immersed vs. Dry-Type Transformers

Although this article focuses on oil-immersed transformers, it is worth comparing them briefly with dry-type transformers. Oil-immersed units are more suitable for high-voltage, high-capacity, and outdoor applications due to better cooling and lower costs. Dry-type transformers, on the other hand, are safer in fire-prone or indoor environments such as subways, hospitals, and commercial buildings. Understanding these differences helps engineers choose the right transformer technology for specific projects.

7. Practical Selection Case

For example, an industrial plant with a total load demand of 1200 kVA plans to expand production in the next five years, expecting an additional 400 kVA. Instead of choosing a 1250 kVA transformer, the design engineer selects a 1600 kVA oil-immersed transformer. This ensures sufficient margin, avoids frequent overload operation, and extends service life. In addition, by selecting a high-efficiency model with lower no-load loss, the company saves energy and reduces costs during continuous operation.

The oil-immersed transformer capacity parameters table is an essential tool for engineers and procurement professionals. By understanding rated capacity, no-load loss, load loss, short-circuit impedance, and cooling method, one can make informed choices tailored to project requirements. Correct transformer selection not only guarantees reliable power supply but also optimizes long-term economic benefits. Whether for industrial plants, substations, or commercial projects, mastering these selection key points ensures safety, efficiency, and sustainability.