deionized is the solution to this problem. This is called high speed reclosing.Transformer failures can be due to deterioration caused by aging or over-voltages as aresult of lightening or switching transients. Faults may be external to transformer but the canstill cause large mechanical forces internal to transformer. These forces can cause windings orother parts to move or damage insulation or actually cause structural failures. These types offaults may be in generators also.Two parallel conductors carrying current can subject a force of attraction or repulsion,depending on the direction of current. Magnitude of this force is given asFWhere I = current in each conductorD = separation between conductorsCurrents in same direction cause attraction whereas currents in opposite direction causerepulsion. In EHV this force can be enough to cause the movement of conductors if circuitbreaker does not operate in reasonable time. Other effects include corona, corona losses,radio noise and radio interference (RI).1.3FAULT ANALYSIS AND ITS IMPORTANCEThe Power System analysis may be classified as: Power flow --- evaluate normal operating conditionsFault analysis --- evaluate abnormal operating conditions3
The purpose of fault analysis is to have the maximum and minimum fault currents andvoltages at different locations of power system for various types of faults so that a properprotection scheme can be designed. Fault analysis results are used for specifying ratings for circuit breakers and fusesprotective relay settingsspecifying the impedance of transformers and generatorsMagnitude of fault currents depend on:the impedance of the networkthe internal impedances of the generatorsthe resistance of the fault (arc resistance)Network impedances are governed bytransmission line impedancestransformer connections and impedancesgrounding connections and resistancesGenerator behavior is divided into three periodssub-transient period, lasting for the first few cyclestransient period, covering a relatively longer timesteady state periodFault StudiesSub-transient period, XG= Xd"determine the interrupting capacity of HV circuit breakersdetermine the operation timing of the protective relay system forhigh-voltage networksTransient period, XG= Xd’determine the interrupting capacity of MV circuit breakersdetermine the operation timing of the protective relay system formedium-voltage networkstransient stability studies1.4TRANSIENT ON A TRANSMISSION LINELet us consider the short circuit transient on a transmission line. Certain simplifying assumptions are made at this stage.The line is fed from a constant voltage sourceShort circuit takes place when the line is unloadedLine capacitance is negligible and the line can be represented by II lumpedRL series circuit.