Protective device coordination in an industrial power system with multiple sources

Protective device coordination in an industrial power system with electric utility ties and multiple in-plant generators presents challenges that cannot be resolved by the use of convectional time-current curves. Relays in different locations will detect greatly different currents during the same fault. This may be resolved by the use of the conventional per-unit time-current curve. However, many computer-aided time-current curve programs do not support this technique. The coordination of time overcurrent relays in this case can best be performed numerically using tables or spreadsheets. In addition, spreadsheet techniques may be used for setting other relays, such as feeder protection relays. An example of the coordination of an industrial power system with multiple generators connected through a synchronizing bus, which also has two utility ties, is examined to illustrate these points.