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What is the permissible overload percent for oil-immersed transformers?

The rated overloading of a transformer depends on the transformer's previous load, the corresponding windings or oil temperature at the beginning of the overloading.
Examples of the permissible duration and the respective levels of the acceptable over loadings are shown below for oil immersed transformers.
For example, if the transformer is loaded with 50% of its rated power continuously, then the transformer can be overloaded to 150% or to 120%, only the time will be the difference. 


Note: It should also be noted that the oil temperature is not a reliable measure for the winding temperature, since the time constant of the oil is 2 to 4 hours, while the time constant of the winding is 2 to 6 minutes. Therefore, the determination of the permissible duration of the overloading must be done very carefully, since there is a danger of the winding temperature exceeding the critical temperature of 105°C, without being visible for the oil temperature.
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