What is Fault in the Electrical power system:
In an electric power system, a fault or fault current is nothing but a any abnormal electric current flow in the circuit. Never forget this “ Current always flows in short circuit path”. For example, a short circuit or earth fault is a fault in which the Normal current bypasses the normal load. An open-circuit fault occurs if a circuit is interrupted by some failure; electrically to say any one of the conductor gets open due to some electrical phenomenon such as heavy load current or physical phenomenon such as conductor may get opened physically due to lose contact or it may get cut. In three-phase systems, a fault may involve Ground faults (L-G, L-L-G, L-L-L-G) or Short Circuit Faults. In a “ground fault” or “earth fault”, current flows into the earth. The prospective short-circuit current of a predictable fault can be calculated for most situations. In power systems, protective devices can detect fault conditions using instrument transformer such as voltage transformer/current transformer and operate circuit breakers and other devices to limit the loss of service due to a failure.
In a polyphase system, a fault may affect all phases equally which is a “symmetrical fault”. If only some phases are affected, the resulting “asymmetrical fault” becomes more complicated to analyses. The analysis of these types of faults is often simplified by using methods such as symmetrical components.
The design of systems to detect and interrupt power system faults is the main objective of power-system protection.
Reason for Faults:
Faults may occur in the three phase or single phase power system due to the number of reasons like natural disturbances (lightning, high-speed winds, earthquakes), equipment insulation failure, falling of a tree, bird shorting, Line Over loads etc.
Types of Fault:
Transient fault: The fault occurs in Very Short time or an insulation fault which only temporarily affects a device’s dielectric properties which are restored after a short time.
Example: lightning strike on Transmission Line.
Persistent fault: Fault in the Under Ground cable
Example: cable drench broken due to heavy loads (JCB or Lorry going on the drench closing plates) on the drench shield.
Symmetric fault: Symmetrical fault involves all the three phases, and these phases carry the identical fault current which makes the system balance.
Asymmetric fault: it means the fault involves one phase to ground or between the phase. In asymmetric faults all three phase lines become unbalanced. Asymmetric faults are L-G, L-L-G, L-L-L-G, L-L, L-L-L
Bolted Fault: One extreme is where the fault has zero impedance, giving the maximum prospective short-circuit current.
Ground fault & earth fault: A ground fault and earth fault is any failure that allows unintended connection of power circuit conductors with the earth.
Realistically, the resistance in a fault can be from close to zero to fairly high relative to the load resistance. A large amount of power may be consumed in the fault, compared with the zero-impedance case where the power is zero. Also, arcs are highly non-linear, so a simple resistance is not a good model. All possible cases need to be considered for a good analysis.
Arching Fault occurs due to the high system voltage, Such an arc can have a relatively high impedance (compared to the normal operating levels of the system) and can be difficult to detect by simple overcurrent protection.