The operation condition of modular multilevel converter（MMC）

as the core device of flexible DC transmission

is related to the safety and stability of HVDC system. For monitoring the operation condition of Insulated gate bipolar transistor（IGBT）in MMC

a kind of whole process accurate measurement method of transient voltage of IGBT

which is suitable to the actual modular structure

is proposed in this paper. The wide frequency voltage sensing module is designed in accordance with the MMC structure

with the voltage division ratio at 10 000:1

the maximum measurement voltage at 10 kV and the frequency response range at DC～40 MHz. The acquisition module is divided into high and low potential modules

with the sampling rate at 100 Ms/s and the analog bandwidth at 20 MHz.The two modules are connected by optical fiber

which effectively solves the insulation problem in high potential measurement. It is verified by the simulated step test and the real MMC platform test that such each performance index as division ratio and frequency response characteristic of the measurement has reached the design requirement and the real-time monitoring of the transient voltage of IGBT in MMC module in the flexible DC transmission project has been achieved

which are conducive to detect the defects of MMC in time and improve the operation stability.