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All transformers have nameplates, and all manufacturers use the same nomenclature for capacity, ratings, impedance, and other pertinent information. This article focuses on dry- or oil-type transformers, which have both ventilated self-cooled or forced-air cooling (i.e., Class AA/FA).
Forced-air cooling is typically achieved with fans and increases the capacity of the transformer by 33%. For these transformers, the capacities are typically shown as 1,000/1,333kVA. Here, the 1,000kVA capacity refers to the ventilated self-cooled rating, and 1,333kVA refers to the forced-air rating (i.e., when the fans are on to generate forced air to cool the windings). The nameplate also shows impedance as a percentage such as 5.85%. This impedance percentage relates to the ventilated, self-cooled 1,000kVA rating only. These percent impedance values can mislead electrical engineers in two ways:
1. The percent impedance (Z) value of 5.85% in the Photo is actually associated with the voltage that needs to be applied to either primary or secondary windings in order for the full load current (1,202.9A) to flow in the other winding when it is shorted.
2. The percent impedance (Z) value of 5.85% actually pertains to the base kVA capacity of the transformer only. If the transformer has forced-air cooling, then the percent impedance (voltage) value should increase proportionally to the increased capacity obtained...