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Gas-insulated Substations play an important role in power systems today but are adversely affected by the generation of Very Fast Transient Overvoltages (VFTO). This paper presents an analysis of VFTOs generated in a 245kV GIS caused by the operation of the circuit breaker and disconnector. The entire substation is modelled in PSCAD-EMTDC using equivalent circuits to simulate the operation of the disconnector and circuit breaker. The VFTO is observed for its peak voltage, rise time and voltage gradient. The results from the simulation were discussed and it was found that the VFTO does not damage the system insulation.