Effect of Load Factor on Operation of Power Transformers by Temperature

The practice of loading transformers by temperature is growing among operators who desire to take advantage of the additional capacity offered by this method of loading. The standards now recognize continuous overloads in cool ambients and short-time overloads under recurrent and emergency conditions. The paper discusses the effects of different shapes of system load factors on the aging of the insulation. Then the ones that result in the greatest amount of aging are used for determining the permissible peak loads that can be carried safely with the same deterioration of its insulation as results from continuous operation at rated kilovolt-amperes. A rational method of calculating the hot-spot temperature is shown by means of examples using a general average ``time constant''. The hot-spot temperature curve is then integrated to compare the aging with a given constant temperature. From these results it is concluded that a normal power transformer can be overloaded 3 per cent for each 10 per cent that the system load factor is below 100 per cent. That is, for a 50 per cent load factor the permissible peak load is 15 per cent over the transformer rating in a standard ambient. For low ambients where 1 per cent overload for each degree that the ambient is below 30 degrees is now permitted, the permissible peak loads will vary depending on ambient and on load factor.