How to choose the cooling method for oil-immersed and dry-type forced air-cooled transformers?

Oil-immersed forced air-cooled transformers offer two selectable cooling methods: oil-immersed forced air cooling (ONAF) and oil-immersed self-cooling (ONAN). Generally, ONAF cooling is chosen when the transformer operates at 100% rated capacity, while ONAN cooling can be selected when operating at 67% (or other percentages specified by the manufacturer) of the rated capacity. This selection is based on the principle of keeping the winding average temperature rise and the top oil temperature rise within the allowable limits. Without forced air cooling, the heat dissipation efficiency is lower, resulting in a corresponding reduction in output capacity. On the other hand, forced air cooling enhances heat dissipation, allowing for an increase in the permissible output capacity.

When selecting the cooling method based on the principle of maintaining temperature rise close to the allowable limit, the load loss depends on the average temperature of the corresponding winding. Choosing ONAN cooling at lower capacities can lead to savings by reducing fan losses. Even when operating at 67% of the rated capacity or below, the forced air cooling method is still preferred due to its higher heat dissipation efficiency, resulting in a lower winding average temperature rise below the specified limit. Consequently, the actual load loss is reduced, despite no reduction in fan losses. Therefore, selecting forced air cooling minimizes the actual operating losses.

However, when operating near the rated capacity, ONAF cooling must be chosen. During ONAF operation, if the fan experiences a malfunction requiring replacement or maintenance, the output capacity should be reduced; otherwise, the operational lifespan of the transformer will decrease.

For dry-type transformers with various temperature resistance levels, adding forced air cooling can increase the transformer's output capacity. From an installation size perspective, smaller spaces can accommodate transformer capacities increased by 40% to 50%.

Although forced air cooling still meets the allowable temperature rise limits, it is uneconomical from an energy-saving perspective. Consider the following example: a 10kV, 800kVA dry-type transformer without forced air cooling, when the fan is activated, can output 1.4 times the capacity, i.e., 1120kVA. At this output, the operating losses are 1500+1.4²×8950=19042W. In comparison, choosing a 10kV, 1250kVA dry-type transformer without forced air cooling results in losses of 2300+0.896²×11300=11372W at the same output capacity of 1120kVA. The total losses for the two different capacity transformers, under different cooling methods, differ by 19042-11372=7670W. Additionally, forced air cooling introduces fan power consumption and a 1.4 times increase in impedance voltage for the 800kVA dry-type transformer, which is unfavorable for voltage regulation. Hence, as a regular operational method, forced air cooling is uneconomical for dry-type transformers, and the larger the capacity, the greater the difference in total losses.

Another example is a 10kV, 10000kVA dry-type transformer without forced air cooling, which can output 15000kVA when using forced air cooling. Comparing the total losses during forced air cooling with two 8000kVA transformers not using forced air cooling to achieve the same output capacity of 15000kVA, the forced air cooling results in total losses of 13850+1.5²×43500=111725W. In contrast, the total losses for two 8000kVA transformers without forced air cooling are 2×13000+2×38900×0.9375²=94379W, resulting in a difference of 17346W in total losses. Therefore, forced air cooling can serve as a cooling method for dry-type transformers during emergency operation beyond the nameplate capacity.

LuShan, est. 1975, is a Chinese professional manufacturer specializing in power transformers and reactors for 49 years. Leading products are single-phase transformer, three-phase transformers, DC inductors, AC reactors, filtering reactor, expoxy resin high-voltage transformer and intermediate, high-frequency products. Our transformers and reactors are widely used in 10 application areas: rapid transit, construction machinery, renewable energy, intelligent manufacturing, medical equipment, coal mine explosion prevention , excitation system, vacuum sintering, central air conditioning.

Know more about power transformer and reactor : www.lstransformer.com