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Monday, 5 January 2015

Faults in Power System

"A fault in an electrical circuit is defined as the defect in the electrical circuit because of which the current in the circuit is diverted from the intended path."
For example suppose a circuit has two parallel paths, opening up of a path will divert the current to the other path and in the process it may damage the path or the conductor. Thus, faults can damage or disrupt the power system in many ways.


In a power system, the faults occur because of insulation failure which may be because of a system over-voltage such as switching surges or lightning stroke. Faults may also be due to a broken insulator or a conductor. Various other reasons such as improper operating habits may also lead to a fault; for example, loading a distribution transformer beyond its normal rated capacity. 

Nearly one half of the faults occur on power lines which are widely branched, have greater length, operate under variable weather conditions and are more exposed to atmospheric disturbances.  

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Effects of fault

Faults give rise to abnormal operating conditions. When a fault occurs at any point in the power system large currents, large forces and or abnormal voltages are developed. The excessive current because of the fault is determined by the internal e.m.f.s of the machines in the network, their impedances, and the impedance in the network between the machines and the fault.

Faults currents, also called short circuit currents, are many times greater than the normal currents. Large voltage stresses the insulation of the various equipments, which are on the way, beyond their breakdown value causing the failure. 

Similarly large currents overheat the equipment or the element of the power system. Sometimes faults lower the system voltage below the permissible voltage limit causing unwanted and teasing interruption of various equipments and components. Faults can also cause a three-phase system to become unbalance.

Action to be performed during a fault

It is necessary that the faults or the faulty section should be removed immediately so that the normal operation of the rest of the system is maintained. The protective relays employed in the power system or network should immediately detect the faults or the faulty section without fail and send trip signal for the operation of circuit breakers.

To obtain proper setting of the protective relays and the interrupting capacities of circuit breakers, the values of these fault currents and voltages should be known with great accuracy. Short circuit studies and calculations provide currents and voltages on a power system during fault conditions.