Selecting Overcurrent Protection for Three-Phase Transformers

Transformers provide the electrical power that drives most industrial processes. Even though continuous service is important, situations may occur that require the unit to be removed from service to prevent damage to the transformer or connected equipment. Proper overcurrent protective devices are essential for the reliable operation of the plant. In this article, we will discuss protective system philosophies and illustrate the selection alternatives through examples. Transformer inrush currents, overload capability, the protective device performance for different fault types, winding connections, and the impact on operating concerns will be discussed. Our emphasis will be on transformers applied within an industrial facility that are rated 2500 kVA and below, where the overcurrent device is normally a fuse.