scope:

This guide is applicable to naturally cooled dry-type power transformers complying with IEC Publication 726 and operated within the limitations referred to in Clause 6. Six different insulation systems are taken into account, identified by their system temperatures. Because there are numerous combinations of different insulation systems and constructions it is possible to make loading recommendations only of a general nature. For this reason the guide is in two parts. - the first part makes no loading recommendations, but gives the method of calculating loading conditions when the variable parameters are known as the result of prototype testing of a particular construction and/or insulation system. The calculations are given in the form of an algorithm from which computer programs can be written; - the second part assumes constant values for the variable parameters, with the exception of the insulation temperature limits (Table I) and the temperature of external cooling air, irrespective of insulation system or construction, thereby enabling load curves to be produced. The guide indicates how dry-types transformers may be operated without exceeding the acceptable limit of deterioration of insulation through thermal effects. The acceptable limit of deterioration of insulation is defined as that which occurs when the dry-type transformer is operating under rated conditions at the basic temperature of the external cooling air.