The electro-optic electric field sensor has a wide frequency band

which can be used to measure the transient voltage. It has no direct electrical connection with the system and is easy to be installed and maintained

but there is a problem of three-phase electric field coupling. In this paper

a simplified decoupling method is proposed

which is suitable for non-contact measurement of 500 kV or above AC system. In the 500 kV and above substations

a long distance exists between conductors of different phases

thus the coupling of non adjacent phases can be ignored. When the field layout is relatively symmetrical

by using the phase angle and amplitude information of the measured three-phase electric field signal

the simplified electric field decoupling can be carried out. At the AC side of ±800 kV Qilian converter station

the transient voltage waveform measured by this method is highly coincident with the waveform measured by the coupling capacitor voltage divider

which proves the feasibility of this method. The method is applied to the Zangzhong connecting project

the transient voltage waveform of no-load line closing is measured as follows: at the head station

the over-voltage wave front time is about 1.5 μs and the maximum over-voltage is 1.74 per unit; at the end station

the over-voltage wave front time is about 33 μs and the maximum over-voltage is 1.32 per unit.