Method for simulating GIS transformer substation to produce very fast transient overvoltage (VFTO) and test circuit

The invention provides a method for simulating a gas insulated switchgear (GIS) transformer substation to produce very fast transient overvoltage (VFTO) by a test circuit and the test circuit. The test circuit comprises a first isolating switch gear, a second isolating switch gear, a first isolating pipe sleeve, a second isolating pipe sleeve and a branch bus, wherein one end of the first isolating switch gear is connected with one end of the second isolating switch gear via a pipeline; and the other end of the first isolating switch gear is connected with one end of th first isolating pipe sleeve via a pipeline; the other end of the second isolating switch gear is connected with one end of the second isolating pipe sleeve; one end of the branch bus is connected with the pipeline between the first isolating switch gear and the first isolating pipe sleeve and the other end thereof is overhung; and VTTO waveforms occurring on isolating switch fractures and GIS pipelines in the GIS transformer substation are simulated by changing the length of the branch bus under different GIS connection modes.