Considerations in the selection of primary 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 and/or connected equipment. Proper overcurrent protective devices are essential for the reliable operation of the plant. This paper discusses protective system philosophies, and illustrates 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 is discussed. The emphasis is on transformers applied within an industrial facility that are rated 2500 kVA and below, where the overcurrent device is normally a fuse