The Ultimate Guide to Oil Cooled Transformers

Oil-immersed transformers can be cooled using self-cooling, forced-air cooling, forced-oil air cooling, or water cooling methods.

Oil-Immersed Self-Cooling

With the development of low-loss technology, the upper limit of capacity for oil-immersed self-cooling is increasing. Transformers with a rated capacity of 40,000 kVA and below can use oil-immersed self-cooling. The advantages are that no auxiliary power supply for fans is needed, there is no noise generated by fans, and the radiators can be directly mounted on the transformer oil tank or centrally installed near the transformer; oil-immersed self-cooled transformers are easy to maintain and can always operate at their rated capacity.

If expandable finned radiators are used, the transformer does not need an oil conservator and can be designed as a fully sealed type, requiring less maintenance. This is generally suitable for distribution transformers of 2500 kVA and below.

Forced-Air Cooling

Forced-air cooling radiators use fans to change the temperature difference of the oil entering and leaving the radiator, improving cooling efficiency, reducing the number of radiators, and reducing the footprint. Transformers with a capacity of 8000 kVA and above can use forced-air cooling. However, this introduces fan noise and requires an auxiliary power supply for the fans; when the fans are switched off, the transformer can operate in self-cooling mode, but the output capacity needs to be reduced to two-thirds of the rated capacity.

For tubular radiators, each radiator can be equipped with two fans; for finned radiators, a large-capacity fan can be used for centralized blowing, or one fan can correspond to multiple sets of radiators.

Forced-Oil Air Cooling and Water Cooling

Forced-oil air cooling and water cooling use forced-air coolers with submersible oil pumps and fans, or water coolers with submersible oil pumps, and are generally used for transformers with a rated capacity of 50,000 kVA and above. Forced-oil air coolers can be mounted on the oil tank or installed separately. According to domestic industry practice, transformers are usually equipped with an additional spare cooler for use when one cooler fails and requires maintenance. When the transformer is operating at less than its rated capacity, some coolers can be shut down; for the coolers that are shut down, the submersible oil pumps cannot reverse direction, therefore each cooler needs to be equipped with a check valve to ensure that the oil flows in only one direction.

Key Considerations for Forced Oil Cooling

• When the oil pump and fan lose power, the transformer cannot operate (even under no-load conditions), therefore the coolers need to be equipped with two independent power sources.

• The submersible oil pump must not experience stator and rotor rubbing. Once rubbing occurs, metal foreign objects entering the windings can cause a breakdown accident; the oil circuit design should avoid negative pressure in the submersible oil pump to prevent air from being drawn in and affecting the insulation strength.

• The oil surface temperature rise of forced oil cooling is low, and the oil surface temperature cannot be used to judge the winding temperature rise, especially in forced oil-water cooling systems. Even if the winding temperature rise is close to the specified limit, the oil surface temperature rise will still be very low.

• When ultra-high voltage transformers use forced oil cooling, it is necessary to prevent oil flow discharge. The oil circuit design inside the windings should avoid oil turbulence, limit the oil flow speed, select oil with appropriate resistivity, ensure that the surface of the insulating parts is smooth, and the iron core needs to reserve sufficient volume for the oil to release charges to prevent the oil flow from developing into oil flow discharge; when starting the coolers, they can be started one by one until the required number of coolers are in operation.

•  When selecting large-capacity coolers, it is necessary to avoid oil flow short circuits and ensure that the cooled oil can smoothly enter the windings.

• When selecting water coolers, attention should be paid to the quality of the cooling water. Impurities in the water can easily clog the coolers and affect the heat dissipation surface; and the water pressure must not be greater than the oil pressure.

• When there is a partition wall on the outside of a forced oil-air cooled transformer, the distance between the partition wall and the cooler should be no less than 3m to avoid interfering with the free flow of air. When using radiators or forced oil-air cooling methods, the transformer can operate at 80% of its rated capacity when the pump is stopped, and at 60% of its rated capacity when both the pump and fan are stopped, but sufficient installation area must be ensured. Conclusion

Conclusion

 the selection of cooling methods for oil-immersed transformers requires a comprehensive assessment considering various factors such as rated capacity, operating environment, maintenance requirements, and environmental regulations. For small and medium-capacity transformers, oil-immersed self-cooling is the preferred option, balancing cost-effectiveness and low maintenance. For medium and large-capacity transformers, forced air cooling can be selected based on available space and noise limitations. For ultra-large capacity or high-load transformers, forced oil cooling or water cooling is suitable to ensure cooling efficiency and equipment stability.