1.0 Introduction
An electric power system is made up of generators, transformers, Transmission line and consumer equipment (loads). The system must be protected against the flow of heavy short-circuit currents which can cause permanent damage to the major equipments. And this is achieved by disconnecting the faulty section of the system by means of circuit breakers, switch gears, isolators and protective relays.
The need to know the maximum short-circuit current that can occur at the different points of the system in order that the breakers selected are adequate to withstand the current and operate successfully, to cut the faulty section, and also in order that the protective relays may be selected for correct operation. The design of machines, bus bars, isolators, circuit breakers etc is based on the consideration of normal and short-circuits currents.
It is also important to be able to calculate approximately at least the size of the protective reactors which must be inserted in the system to limit the short circuit current to a value which can be handled by the circuit breakers.
The short-circuit current is an A.C system are determined mainly by the reactance of the alternators, transformers, and lines up to fault in the case of phase-to-phase faults. When the fault is between phase and earth, the resistance of the path plays an important role in limiting the current.
The rupturing capacities of a circuit breakers are based on the symmetrical short-circuit current which is the most simple calculation among all types of short-circuits. However, for determination of setting of relays, it is absolutely necessary to know fault current due to unsymmetrical fault condition for which knowledge of symmetrical components etc is required.
1.1 Definition of Fault
A fault in an electrical equipment/apparatus is defined as a defect in the electrical circuit to which current is diverted from the intended part.
Also fault is said to have occurred when a system fails to perform the required function at a given time. A fault occurs when two or more conductors that normally operate with a potential difference come in contact with each other or when a conductor that supposed to be continuous losses its continuity.
The nature of a fault simply implies any abnormal condition which causes a reduction to the basic insulation strength between phases conductors or between phases conductors and earth or any earthed screen surrounding the conductors. The reduction of insulation strength is not considered as a fault until it creates some effect on the system i.e, until it results either in excessive current or in between conductor and earth to a value below that of the lowest impedance normal to the circuit.
The possibility of occurrence of abnormal condition or failure is more on the power line.
1.2 Classification of Faults
Balanced Or Symmetrical Faults
This is a kind of fault that gives rise to symmetrical fault current. In this type of fault, current in three phases are equal and have 120o displacement
Example, when all the three phase of conductors of a three phase line are brought together simultaneously into a short-circuit condition.
Unbalanced Or Unsymmetrical Fault
This is also another type of fault in a power system that gives rise to unsymmetrical fault current. In this case, the line current is unequal and the displacement is unequal.
IY ? IB ? IB
Examples of unbalanced fault.
1. A single line to ground
2. Line-to-line
1.3 Types of Fault
Three Phase Short-circuit Fault: This type of fault results due to breakdown of insulation between all the three phases or when there is a bridge involving all the three phases at a time. Its occurrence is 2 to 3 percent.Single Phase to ground (L-G) short-circuit fault: This is caused as a result of breakdown of insulation between one of the phases and earth. Its occurrence is 70 percent.Phase to ground (L-L-G) short-circuit fault: Due to breakdown of insulation between either of the two phases and earth. Its occurrence in power network is 10 percent.Phase to Phase (L-L) short-circuit fault: This is a type of short-circuit fault that occur due to the breakdown of insulation between either of the two phases, its occurrence is 15%.
1.4 Reasons For Carrying Out Fault Analysis in the Power System
One of the reasons for carrying out fault analysis in power system is to know the maximum short-circuit current that can occur at different points of a system in order to select a circuit breaker that can adequately withstand the current and operate successfully to cut-off fault section.
The design of machine bus-bars, isolators, circuit breaker etc. is based on the consideration of normal and short-circuit current.
Also to know the size of protective reactor while must be inserted in the system to limit the short-circuit current to a value which can be handled by the circuit breaker or protective devices.
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