ITEE Innovation Expo 2008 : Project Details

Insulation Coordination in High Voltage Gas Insulated Switchgear (GIS)

Gas-insulated switchgear (GIS), as opposed to air-insulated switchgear, is a specific type of compact switchgear that is completely insulated by sulphur hexafluoride. Sulphur hexafluoride is a self-resorting medium which has a dielectric strength that is approximately three times that of air. Insulation coordination (IC) is the iterative process of assessing potential overvoltage threats to power systems assets and accordingly insulating the assets against those threats. Gas-insulated switchgear insulation coordination (GISIC) is therefore the application of the process of insulation coordination specifically to gas-insulated switchgear.

This thesis is about GISIC. By modelling GIS in the Electromagnetic Transients Program (EMTP), it is possible to subject the switchgear to overvoltages, without the destruction of expensive equipment, and then subsequently view the results. This thesis included three studies. Firstly, a sensitivity study showed that the GIS model is relatively insensitive to small variations of sensible values of model parameters. It is only when parameters that are several orders of magnitude too large or small that the model fails. Secondly, a backflashover study directly applied insulation coordination techniques to protecting a GIS switchyard and successfully managed to bring the overvoltage down to acceptable levels. Finally, a switching study showed that the GIS transformer could not have failed under the overvoltage created when the line was energised.

In conclusion, the thesis finds that GISIC does not require particularly careful modelling for small high-voltage GIS substations. It also shows that while EMTP is overall a useful program for IC, it is flawed.