Contributing Factors in Determining the Ratings of Power Transformers for Use on the Systems of The Hydro-Electric Power Commission of Ontario

Transformers like most other commodities change in style from time to time as the years pass. It is the intent of this paper to delve back 40 years into the history of the Hydro-Electric Power Commission of Ontario and, in regard to transformers, compare the mode of thinking then with that of the present time, and to follow the changes that have occurred in four decades and the governing factors that have influenced these changes. Today a transformer can be considered to be one of the most reliable pieces of equipment in operation. It has been developed partially from operating experience and partially to satisfy conditions and circumstances that presented themselves. Tappings in a transformer can be extremely hazardous and therefore should be kept to a minimum. The indoor transformer has given place to the outdoor. Water cooling has been largely superseded by self-cooling, then forced-air cooling, finally augmented with forced-oil. Single-phase transformers once used exclusively are not now preferred to 3-phase units. Capacities have been stepped up from 750-kva to 100,000-kva units. Autotransformers are used to advantage whenever they can be applied. Both manually operated off-load and automatically controlled under-load tap changing equipment has been developed. Refinements have been made to temperature indicating devices Current transformers used for relaying and control are now economically located on the bushings within the transformer tanks. Interchangeability of the various manufacturers´ bushings has become a reality and is of great advantage to the operating companies.