Substation-Transformer Emergency Overloading Practice

Although much has been written in recent years regarding allowable over-loading of transformers, the practical matter of matching existing load curves with such allowable limits has continued to present some difficulties. The interim report of the AIEE transformer subcommittee gives one approach to the problem, taking into account the ambient temperature and the duration of the overload. However, in order to take full advantage of the transformer's inherent load-carrying ability, it is necessary to consider additional factors such as the individual transformer characteristics and the specific shape of the overload curve. This paper shows a method by which all of the influencing factors can be taken into account easily. Allowable overloads thus determined are often considerably higher than those indicated by the AIEE method. The application of the proposed method is sufficiently simple to justify a semiannual analysis of expected overloads with respect to substation-transformer emergency ratings. It consists in 1. Determining the allowable 24-hour steady overload that can be carried by the transformer without exceeding certain agreed-upon limits. 2. Converting the specific load curve to a 24-hour steady load which would cause equal aging of the transformer insulation. In this manner both the transformer load-carrying ability and the actual irregular load are reduced to a common basis for ready comparison. A 24-hour overload period is ordinarily of ample duration to allow for replacement or repair of faulty equipment.