The very fast transient（VFTO）is a serious threat to the insulation performance of gas insulated switch-gerar（GIS）and the coil equipment

for example the transformer

especially when GIS and the transformer are di-rectly connected through oil/gas insulated bushing. Therefore

in order to accurately and timely master the impact ofVFTO on the operation status of GIS and transformer

it is necessary to constantly monitor the VFTO level causedby the operation of circuit breakers and disconnectors of GIS. In this paper an VFTO on-line monitoring systembased on the ultra-wide band capacitive sensing is designed for 500 k V transformer and GIS equipment of a largehydropower station. Capacitance sensing transformation is carried out on the cover plate of the connecting cylinderof GIS equipment and transformer. Signal conversion and acquisition are implemented locally at the monitoring ter-minal

and uploaded to the upper storage control unit through optical fiber. The effective measurement bandwidth of the monitoring system ranges from 2.1 Hz to 230 MHz

the sampling rate is 250 MS/s

and the analog bandwidth is100 MHz. The abrupt voltage waveform can be accurately captured by customized steep trigger mode

and the re-cording time is not less than 200 ms. The vert fast transient overvoltage waveforms caused by the disconnector andcircuit breaker are recorded successfully and accurately by the on-line monitoring system. Through data analysis

itis found that the duration of VFTO waveform caused by disconnector operation is less than 5 μs

the rising edge isless than 60 ns

the main spectrum is between 1 MHz and 10 MHz

and the maximum instantaneous frequency at therising edge can reach 100 MHz;The duration of VFTO waveform caused by circuit breaker operation is less than30 ms

the rising edge is less than 100 ns

the main spectrum is between 10 k Hz and 1 MHz

and the maximum instan-taneous frequency at the rising edge also can reach 100 MHz. The VFTO simulation model of the GIS and transformerequipment can be optimized by the measured waveform of the overvoltage on-line monitoring system

which lays a sol-id foundation for improving the protection level of VFTO and analyzing the causes of equipment failure.