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Power System Fault Analysis using Fault Reporting Data

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INTRODUCTION

Any abnormal conditions which causes flow of huge current in the conductors or cable through inappropriate paths in the circuit can be defined as a fault. In normal operating conditions all the circuit elements of an electrical system carry currents whose magnitude depends upon the value of the generator voltage and the effective impedances of all the power transmission and distribution system elements including the impedances of the loads usually relatively larger than other impedances.
Modern electric systems may be of great complexity and spread over large geographical area. An electric power system consists of generators, transformers, transmission lines and consumer equipment. The system must be protected against flow of heavy shortcircuit currents, which can cause permanent damage to major equipments, by disconnecting the faulty section of system by means of circuit breaker and protective relaying. Such conditions are caused
in the system accidentally through insulation failure of equipment or flashover of lines initiated by a lightning stroke or through accidental faulty operation. The safe disconnection can only be guaranteed if the current does not exceed the capability of the circuit breaker. Therefore, the short circuit currents in the network must be computed and compared with the ratings of the circuit breakers at regular intervals as part of the normal operation planning.

TRANSIENTS ON A TRANSMISSION LINE :

Let us consider the short circuit transient on a transmission line. Certain simplifying assumptions are made at this stage:
1. The line is fed from a constant voltage source.
2. Short circuit takes place when the line is unloaded.
3. Line capacitance is negligible and the line can be represented by a lumped RL series circuit.

SYMMETRICAL THREE PHASE FAULT ANALYSIS :

In normal operating conditions, a three-phase power system can be treated as a single-phase system when the loads, voltages, and currents are balanced. If we postulate plane-wave propagation along the conductors (it is, however, known from the Maxwell equations that in the presence of losses this is not strictly true), a network representation with lumped elements can be made when the physical Dimensions of the power system, or a part of it, are small as compared with the wavelength of the voltage and current signals. When this is the case, one can successfully use a single line lumped-element representation of the three-phase power system for calculation. A fault brings the system to an abnormal condition. Short-circuit faults are especially of concern because they result in a switching action, which often results in transient overvoltage. In the case of a symmetrical three-phase fault in a symmetrical system, one can still use a single-phase representation for the short-circuit and transient analysis.
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