To test the performance of very fast transient overvoltage（VFTO） sensors under square wave voltage with high amplitude and steep rising edge

a square voltage generator which can produce square voltages with nanosecond rising edge and amplitude up to 100 kV was developed. Firstly

the circuit model of the square wave generator was established. The estimation formulas of the rise time and the wave tail time of the output voltage were derived. Then

the effects of the pulse capacitor

the load resistance

the total resistance and the inductance of the discharge circuit on the output waveform were analyzed by simulation. Finally

the 100 kV square wave generator with compact structure was designed and developed

and its performance was tested. The test results indicate that the rise time is shorter than 5 ns and the voltage drop on the flat top is less than 1% at 1 μs. The generator also has a good repeatability

and is suitable for the calibrations of VFTO sensors.