Study of protection of transformer from very fast transient over-voltage in 750 kV GIS

Gas insulated switchgears, also called gas insulated substation, is applied to power system widely. Running experiences have shown that switching operations generates very fast transient overvoltage, which has steep front and is followed by high frequency oscillations. VFTO spreads out of GIS and endangers the equipments connected to GIS. The transformer is the main casualty of VFTO. In this paper a 750 kV GIS system has been set up. VFTO at the transformer terminal of different running modes have been calculated using electro-magnetic transient program (EMTP). This paper has studied the protective effect of shunt resistor of the switches, MOA and R-C surge suppresser. From a large amount of simulated calculations by electro-magnetic transient program, it can be found that the shunt resistors could accelerate the attenuation of overvoltages, so the peak and gradient reduce greatly. MOA only has effect on the peak value, whereas has no effect on the gradient. R-C surge suppresser not only could limit the peak of VFTO, but also could reduce its gradient. The effect of R-C surge suppresser is better than MOA, if the values of the resistor and the capacitor are selected suitably.