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978-3-8439-1411-6, Reihe Energietechnik
Untersuchung der inneren Blitzstoßspannungsfestigkeit von Vakuumschaltkammern nach Ausschaltversuchen
175 Seiten, Dissertation Technische Universität Braunschweig (2013), Softcover, A5
Switching high currents and short circuit currents places considerable demands on circuit breakers. High current densities on contact surfaces cause loss of contact material. Intensive contact erosion can lead to technical failure of circuit breaker.
This work “Investigations on the inner lightning impulse voltage performance of vacuum circuit breakers after switching operations” deals with the effect of reduced dielectric strength of vacuum interrupter (VI) after switching operations. The investigations were carried out on a VI for medium voltage.
Switching operations were carried out at different current intensities from 5 kAeff to 20 kAeff. Currents with a 50 Hz sineform for switching operations were generated in a high power testfield. This guarantees that the test conditions are similar to the technical demands of the VI. Lightning impulse voltage tests were performed to evaluate the dielectric performance of VIs. Comparison of new VIs with VIs after several switching operations reveals the reduction of dielectric performance of investigated VI.
During switching operations, contact material is being evaporated. The metal vapour condenses on inner surfaces of the VI. Shieldings inside the VI should protect the inner ceramic surfaces from being coated by metal vapour, but a small area of ceramic surface becomes coated as well. After opening the VIs, the evaporation layers on ceramics became visible. The area of coated ceramic surface grows bigger with increasing number of switching operations and increasing test currents. Measurement of layer thickness was done with “electron probe microanalysis”. Pictures with “REM” show the structure of ceramic with and without evaporation layers. The electric conductivity of coated ceramic surface was done with an electrometer.
Calculation of electric field gives information about the influence of different intensities of coatings on the maximum field strength inside the VI. Transient simulation of VI was performed to investigate the process of dielectric breakdown inside the VI with lightning impulse voltage and coated ceramics. Electric field simulation, transient simulation together with results from the investigations of coated ceramic surfaces were used to build up a model for the vacuum breakdown in VI, when segments of the ceramics are covered with contact material.