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Abstract Electrical equipment and installations are exposed to stresses from overvoltages under normal and abnormal conditions. Some of these overvoltages may cause damage to the electrical equipment in the substations. gas insulated substations (GIS) disconnector switch operation generates fast transient overvoltages in the system. Therefore, the present investigation has been devoted to study the fast transient phenomena in the Egyptian Electricity National Grid. Since the disconnector has a relatively slow speed of the moving contact the switching problems increase in direct proportion to the system voltage, in addition to the potential stressing of the insulation relative to the earthed housing and across the open contacts. The switching of capacitive charging currents are of special significance under these conditions pre-or restikings occur and cause fast transient overvoltage on the supply and load sides. This phenomena cannot be investigated in public supply system owing to the inherent possibility of earth fault. Therefore an actual test object 220kV disconnector has been constructed in the Extra High Voltage Laboratory and the tests were carried out to record the performance of the disconncctor under different values of SF6 pressure . Also, the disconnector has been tested in open position under steady state conditions with the combined stresses across the isolating distance of rated power frequency voltage at one terminal and standard lightning impulse voltage applied to the other terminal. Field measurements on switching capacitive currents with disconnector in 220kV gas insulated substation were carried - out and compared with the calculated waveforms by EMTP. The work is described in eight chapters: • The first chapter includes a summary of the review of overvoltage types in high and extra high voltage networks and a brief explanation of both GIS and AlS relevant to the insulation behavior and overvoltage stresses. • Chapter two includes a review of the previous work in the field of very fast front overvoltage phenomena in GIS. Most of the previous investigations in the area of the research work did not consider the variation of the arc behavior with the SF6 gas pressure. • Chapter three presents the statistical analysis of GIS outages in the Egyptian Electricity E.H.Y. Grid; also it includes a comparison between the failure frequency values of GIS in Egypt and the values obtained from international surveys. • Chapter four describes the testing arrangement and instrumentation techniques, which are used in both the High Voltage Laboratory tests and field measurements. A detailed and special considerations of the experiment arrangements and test procedures are also presented. • Chapter five presents the records obtained from the experimental results and the analysis of the overvoltages including fast transient (Y-t) curves under different gas pressure. • Chapter six contains the results of the actual field overvoltage measurements 011 220k Y system and the analysis of these results. • Chapter seven offers details of digital computation model of fast transient overvoltages for the following cases: I. Switching operation of 500 kY line disconnector in GIS substation. 2. Switching operation of220 kY line disconnector in GIS substation. 3. Switching operation of220 kY GIS floating B.B 4. Switching operation of220kY test object It also includes the simulation of the arc in the disconnector during a closing operation. Computation results of the fast transient overvoltages are compared with field measurements and laboratory tests. • Finally chapter eight presents the main recommendations which are drawn through out the work and its analysis. Further some proposals for future investigations ill this field are given. |