The power grid can sustain many different types of faults in the system. A fault can occurs when there is an unexpected rapid change in current. Faults can occur for many different reasons such as lighting strikes. Other causes are failure of insulating material, salt spray, sagging lines under high current conditions, and physical disruption such as a tree touching or falling on transmission lines or other parts of the grid. Fig. 1(a) shows a tree branch across two nodes in an experiment. Fig. 1(b) shows a tree touching a power line and catching fire.
Figure 1. Examples of faults in the electric power grid.
In addition, faults can be further categorized as either symmetrical such as a bolted three phase fault or an unsymmetrical fault such as a single line to ground or a line to line fault.
The power system designer must design the system so that the grid will be properly protected in the event of a fault.