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 which cannot be resolved by the use of conventional time-current curves. Relays in different locations will see 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 the setting of 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, and which also has two utility ties is examined in order to illustrate these points.