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Reference: ISH2015_320

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Oscillatory overvoltage in compact medium voltage switchgear during impulse surge tests



All power network components have to be designed in such a way that they are able to withstand all stresses during their normal operating life including transient impulse voltages. For this purpose, all power network components including switchgears have to pass high voltage impulse tests. Although the amplitude, rise and fall time of applied voltage impulses are given in standards, there is no guarantee that the voltage imposed to all internal parts of components is the same. If the spectrum of an applied voltage pulse is in the range of the Eigen frequencies of a system, resonance effects may occur. Such resonances may lead to overvoltages on internal elements of the system. Medium voltage switchgear is one of those complex systems, where different energy storing elements are unintentionally combined together; these include stray capacitances between different phases, capacitances of high voltage parts to earth and stray inductances of the different current paths within the switchgear. Especially in case of compact medium voltage switchgears, the reduced distance between different conductors leads to high capacitances, and the resonant frequencies of such systems are shifted to lower ranges. As a result, even a comparatively slow impulse voltage with a rise time of few microseconds may excite the internal modes of medium voltage switchgears. In this paper, observations on the oscillatory overvoltage in medium voltage switchgears are reported. The results indicate that under some circumstances, remarkable oscillations may be generated. The experimental measurement results and transient simulations of the whole switchgear are presented and discussed. The influence of different parameters of the test circuit as well as switchgear on the amplitude of those oscillations are investigated in detail.

File Size: 581,3 KB

Year: 2015

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