| Electromagnetic model of bushing of GIS (Gas Insulated Switchgear) is very important for the simulation of VFTO(Very Fast Transient Over-voltage), TEV(Transient Enclosure Voltage) and transient electromagnetic field. The VFTO can contain frequency components up to 100MHz. At such high frequencies, the length of the inner shielding conducting layer is comparable with the wavelength and hence the bushing can not be modeled as a lumped parameter element. In this thesis, the broadband circuit model of a 1100kV gas insulated bushing with double layer shielding is investigated. The three conductor system of bushing is modeled using transmission line theory; the influence of high order modes of electromagnetic waves excited in the discontinuities of bushing on transmission of the main mode is treated as a lumped capacitance; the two-conductor with tapered size is considered as the cascading of multiple segments of transmission lines; the electromagnetic leakage from bushing is handled with by using a radiation impedance obtained from the approximate formula; the ground capacitance parameters of top grading ring and the middle shielding layer, and the capacitance parameter between top grading ring and the middle shielding layer are extracted respectively, by using numerical computation method. The input impedance observed at the bottom of the bushing is calculated using the circuit model of transmission line and impedance model built above within the frequency range from 1MHz to 100MHz. The results from the broadband circuit model are in good agreement with those from the 3D full wall simulations solver CST-MWS. The transient simulation model of GIS bushing is built based on the EMTP simulation software.The excitation source of transient simulation circuit is a double exponential pulse. The results of voltage at the top of bushing from the transient simulation circuit model are in good agreement with those from the 3D full wall simulations solver. |
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