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Oil-immersed transformers rely heavily on cooling to ensure safe and efficient operation. Their performance is tightly bound to temperature regulation. When transformers operate, electrical losses within the windings and core generate heat. Without proper dissipation, this heat accelerates insulation aging, increases failure risk, and jeopardizes long-term reliability. Cooling, therefore, is not a luxury—it is the safeguard of service life and operational stability.
In a natural state, transformers shed heat through radiation and natural convection. However, this method often proves insufficient under heavy load. By equipping the oil tank and radiators with fans, the cooling efficiency improves dramatically. Studies and field data show that fan-assisted ventilation raises the cooling rate of the oil tank surface by 50% to 60%.
This improvement translates into increased loading capability. Compared to self-cooling operation, a transformer equipped with forced ventilation can handle more than 30% additional capacity. Yet, the relationship between cooling and loading must be respected. If the fans stop running, the transformer can only be operated at about 70% of its rated load. Any attempt to carry the full rated load without forced cooling risks excessive temperature rise, potentially surpassing the allowable limit.
Regulations stipulate clear boundaries for transformer oil temperatures. For oil-immersed transformers with forced-air cooling, operation without fans is acceptable only if the top-layer oil temperature remains below 55 °C. At this threshold, even under rated load, the winding’s hottest spot temperature does not exceed 95 °C, a level considered safe for insulation integrity.
By contrast, transformers designed with forced oil circulation—whether water-cooled or air-cooled—are not permitted to operate without active cooling. Their tanks are smooth, offering minimal surface area for natural dissipation. In such designs, even no-load operation cannot be safely sustained without the cooling system engaged, as the losses alone can accumulate heat beyond tolerable levels.
Power systems often demand uninterrupted supply, even during cooling failures. For this reason, emergency allowances exist within operational guidelines. If the entire cooling system unexpectedly shuts down, a transformer may continue to operate at rated load for a maximum of 20 minutes. This window accounts for the thermal inertia of the transformer, as the rise in temperature is not instantaneous but follows a time constant.
If the oil temperature remains below 75 °C during this period, the transformer may continue running, but never beyond one hour without cooling. These strict temporal boundaries balance the need for power continuity with the imperative of preventing thermal damage.
Cooling is the invisible guardian of transformer reliability. Whether through natural convection, forced ventilation, or oil circulation systems, effective thermal management ensures that power flows without interruption and equipment endures the test of time. Adhering to oil temperature regulations and respecting emergency allowances is essential, for they embody both engineering precision and operational prudence.
