oil-immersed transformers are often subjected to temporary overload conditions due to fluctuations in power demand. Understanding the permissible overload capacity is for maintaining reliability, extending service life, and avoiding thermal degradation.
International standards IEEE C57.91-2011 and IEC 60076-7 define permissible overload percentages based on transformer insulation class and cooling conditions.
55°C rise oil-immersed transformers: Up to 30% overload for limited durations.
65°C rise oil-immersed transformers: Up to 15% overload for short periods.
These limits ensure that the transformer does not exceed its designed thermal capabilities under momentary peak loads.
Up to 50% overload may be permissible for a limited time, depending on insulation life considerations and the oil temperature profile.
Short-duration overloads exceeding 100% are sometimes tolerated during transient events such as motor starting, fault clearing, or system contingencies.
| Previous continuous loading | oil temperature | duration(min) of overloading for specific levels of overloading | ||||
| % of rated power | ℃ | 10% Min | 20% Min | 30% Min | 40% Min | 50% Min |
| 50 | 55 | 180 | 90 | 60 | 30 | 15 |
| 75 | 68 | 120 | 60 | 30 | 15 | 8 |
| 90 | 78 | 60 | 25 | 15 | 8 | 4 |
Several factors influence the permissible overload of an oil-immersed transformer:
Forced cooling mechanisms such as ONAF (Oil Natural Air Forced) or OFAF (Oil Forced Air Forced) enhance heat dissipation, allowing higher overload tolerances.
Higher ambient temperatures reduce thermal margins, lowering permissible overload levels. A lower ambient temperature may allow slight extensions of overload duration.
Prolonged overload operation accelerates insulation aging due to excessive hot-spot temperatures. IEC standards recommend keeping the hot-spot temperature below 110°C for longevity.
Short-duration overloads (minutes to hours) are generally acceptable within the recommended limits.
Permissible overload percentages for oil-immersed transformers depend on design factors, cooling effectiveness, and insulation aging considerations. Engineers should implement monitoring and cooling strategies to optimize transformer performance while ensuring long-term reliability.
