A fault implies an abnormal condition in the electrical power system. Only the failure of insulation causes a fault. A fault causes an excess current to flow through an unintended path. Low insulation reduces system impedance. When the system impedance falls below the lowest load impedance a fault generates.
A power system network consists of generators, switchgear, transformers, transmission and distribution networks. Obviously, some failure may occur somewhere in such a large system network. An overhead power line faces many more faults than an indoor and underground system.
Causes of Faults
Breakdown at Normal Voltage
Breakdown may occur under normal operating voltage. One of the main causes is deterioration of insulation. Week and degraded insulation provides unwanted low impedance path for current. There are also many unpredictable external causes for electrical fault even at a normal voltage condition. For examples these causes are birds perching on conductors, snakes causing short circuits, tree branches touching the line, etc.
Breakdown Due to Abnormal Voltage
Even healthy insulation may fail because of an abnormal transient overvoltage. The switching surges, and lightning surges cause transient over voltages.
The modern power utility companies generally provide sufficiently higher insulation levels than nominal voltage levels. However, insulation strength may still decrease because of the following reasons. Pollution is one of the main cause of degradation of exposed insulators. The deposition of dust and chemicals on the insulator body surface degrades the insulators in industrial areas. Moist and salty atmosphere causes the deposition of salt on the insulators in coastal regions. The ageing of insulation is another cause of failure. Voids inside XLPE, PVC, and other solid insulation on cable and other equipment cause partial discharge. This also degrades the insulation badly.
Consequences of Faults
The common consequence of an uncleared fault is heat. Heat causes abnormal temperature hike. Abnormally high temperature can deform, even destroy the equipment. The most common consequence of a short circuit is fire. The prolonged fault causes severe reduction of system voltage over a large area, interrupting electrical supply. Overheating and excessive mechanical forces causing equipment damage.
Typical Distribution of Faults in a Power System
| Equipment | Percentage of Total Faults |
|---|---|
| Overhead Lines | 50% |
| Cables | 10% |
| Switchgear | 15% |
| Transformers | 12% |
| CTs & PTs | 2% |
| Control Equipment | 3% |
| Miscellaneous | 8% |