Transient behavior of line-connections between GIS and transformers

Summary

It is obvious in EHV/ UHV- range, that a grid transformer is connected with a gas- insulated switchgear (GIS) by a certain kind of a transmission line. The distances from GIS to transformer can be hundreds meters dependent on the grid configuration. Especially for longer distances of such line-connection at switching operation in the GIS dangerous overvoltages may occur caused by travelling wave phenomena on the line and within the transformer. They are influenced not only by the speed of the pswitching operation (GIS) but they are also depending on the type of the line-connection and the wave properties of the connected transformer. The mentioned very fast transient phenomena were investigated for three line types: overhead line, cable and GIL, for various values of switching speed and different line length. Because of the very short rise time of switching operation it has been necessary to consider the wave propagation within the entire system. Therefore, the system consisting of the switching source (GIS), the line-connection and the transformer winding was modelled like a circuit with distributed parameters. Its analysis was carried-out numerically in time domain and an algorithm based on FDTD has been developed. It provides the time and space distribution of voltage and current waves along the line and the transformer windings. The impact of the line type and its length on the propagating voltage and current waves could be studied for various rise time of the switching operation. The analysis has shown, that in the case of the shorter rise time like for SF6- insulation type of GIS the connected transformer will be stressed significantly more than in the case of a lower switching speed. The obtained results enable deeper understanding of the behaviour at very fast transients not only along the line but also within the transformer winding. That can be used even for more secure design of electrical insulation and for protection systems.

Additional informations

Authors

NIRGUDE, DE FARIA

Keywords

Fast transients, time- space characteristic, modelling, distributed line parameters