Abstract: The transient electric field (E_field) generated by the switching operation of the IGBT of the modular multilevel converter (MMC) and hybrid DC circuit breaker (CB) in the converter station is measured. Moreover, its application in the on-line condition monitoring of the equipment is analyzed. First, it is proved that the switching transient E_field in the converter station is a quasi-static field and it has a linear relationship with the equipment voltage. This provides the theoretical basis for the condition monitoring method based on the switching transient E_field. Furthermore, in the artificial short-circuit test of Zhouding converter station, the high-frequency E_field waveforms of the MMC and the switching transient E_field pulses of the DC CB are measured. The characteristics of the high-frequency E_field signal under different working conditions of MMC are analyzed. It is found that the high-frequency E_field signal contains the high-order harmonic information that is not easy to measure by the self-contained waveform recording system of the converter station. On the other hand, it is found that based on the steep pulse front of the switching E_field pulse, the blocking moment of MMC and DC CB can be measured with an error of less than 0.005ms. Therefore, the measurement method based on transient electric field can make up for the limited measurement bandwidth of the self-contained waveform recording system of the converter station to a certain extent. It can supplement the high-order harmonics and other operating information of the system and improve the measurement accuracy of the blocking moment of the MMC and CB. It has potential application value for online condition monitoring of the MMC, HVDC CB and other equipment.