Power substations as a target of study consists of some elements which must be protected against different types of fault. These elements are Transmission Lines, Bus Bars, Power Transformers, Outgoing Feeders, and Bus Couplers.
Fault Causes :
Healthy insulation in the equipment subjected to either transient over voltages of small time duration due to switching and lightning strokes, direct or indirect. Failure of insulation may be happened, resulting in very high fault current. This current may be more than 10 times the rated or nominal current of the equipment.
Aging of power equipments may cause breakdown of it even at normal power frequency voltage.
External object such as bird, kite string, or tree branch are considered as external cause of fault. These objects may span one conductor and ground causing single line to ground fault (phase-earth) or span two conductors causing phase-phase fault .