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Temperature Difference Between Transformer Windings and Oil Surface
Time:
2026-03-18
The differences in temperature rise between transformer windings and the oil surface, and their impact on service life, are primarily manifested in the sequence of heat transfer, non-uniform temperature distribution, and the rate of insulation aging.
In oil-immersed transformers, the windings are the primary heat source, with their temperature typically 10–15°C higher than that of the upper oil layer. This is because electrical energy losses (copper losses) first generate heat within the windings and then gradually dissipate through the insulating oil. Consequently, the winding temperature reflects the “hot-spot” condition inside the equipment and directly influences the rate of insulation material aging.
According to the “8°C Rule,” for every 8°C increase in winding temperature, the transformer’s service life is halved:
1. When the windings operate at 95°C, their service life is approximately 20 years;
2. When the temperature rises to 105°C, the service life decreases to approximately 7 years;
3. If the temperature reaches 120°C, the service life is reduced to approximately 2 years.
In contrast, the oil temperature rise (the difference between the top-oil temperature and the ambient temperature) is an externally measurable indirect indicator, with a national standard limit of 55 K. Although the oil temperature does not directly reflect the hottest-spot condition, it serves as an important basis for determining whether to activate the cooling system or trigger an alarm.
In addition, if an abnormal condition occurs in which the oil temperature exceeds the winding temperature, possible causes include:
1. The winding thermometer is not connected to the load current compensation circuit;
2. Malfunction of the measuring device or deviation in sensor positioning;
3. Abnormalities in the cooling system lead to localized overheating.
Therefore, monitoring oil temperature alone is insufficient for a comprehensive assessment of equipment health; it must be combined with winding hotspot monitoring—such as fiber-optic temperature measurement—to accurately predict the risk of insulation aging and ensure long-term safe operation.
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Address: No. 113, Huancheng Road, Huangchuan County, Henan Province
Tel: +86-376-3985688
Email: zhjvip001@163.com
Address: No. 113, Huancheng Road, Huangchuan County, Henan Province
Tel：+86-376-3985688
Email：zhjvip001@163.com
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