Temperature Limits
According to ANSI standards, modern distribution transformers are to operate at a maximum 65˚C average winding rise over a 30˚C ambient air temperature at rated kVA. One exception to this is submersible or vault-type distribution transformers, where a 55˚C rise over a 40˚C ambient is specified.

The bulk oil temperature near the top of the tank is called the “top oil temperature,” which cannot be more than 65˚C over ambient and will typically be about 55˚C over ambient, 10˚C less than the average winding rise.

Hottest-Spot Rise
The location in the transformer windings that has the highest temperature is called the “hottest spot.” Standards require that the hottest-spot temperature not exceed 80˚C rise over a 30˚C ambient, or 110˚C. These are steady-state temperatures at rated kVA.

The hottest spot is of great interest because, presumably, this is where the greatest thermal degradation of the transformer’s insulation system will take place. For calculation of thermal transients, the top-oil rise over ambient air and the hottest-spot rise over top oil are the parameters used.

Load Cycles
If all distribution loads were constant, then determining the proper loading of transformers would be a simple task. Loads on transformers, however, vary through the hours of a day, the days of a week, and through the seasons of the year.

Insulation aging is a highly nonlinear function of temperature that accumulates over time. The best use of a transformer, then, is to balance brief periods of hottest-spot temperatures slightly above 110􀁲C with extended periods at hottest spots well below 110˚C.

Methods for calculating the transformer loss-of-life for a given daily cycle are found in the ANSI Guide for Loading (IEEE, 1995). Parameters needed to make this calculation are the no-load and load losses, the top-oil rise, the hottest-spot rise, and the thermal time constant.

Thermal Time Constant
Liquid-filled distribution transformers can sustain substantial short-time overloads because the mass of oil, steel, and conductor takes time to come up to a steady-state operating temperature. Time constant values can vary from two to six hours, mainly due to the differences in oil volume vs. tank surface for different products.

Loading Distribution Transformers
Utilities often assign loading limits to distribution transformers that are different from the transformer’s
nameplate kVA. This is based on three factors: the actual ambient temperature, the shape of the load
curve, and the available air for cooling.

For example, one utility divides its service territory into three temperature situations for different ambient temperatures: summer interior, summer coastal, and winter. The transformer installations are divided into three applications for the available air cooling: overhead or pad-mounted, surface operable, and vault.

The load shape is expressed by the peak-day load factor, which is defined as the season’s peak kVA divided by the average kVA and then expressed as a percentage.

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