To ensure the secure

stable and reliable operation of a power system

it is necessary and important to research transient over-voltage monitoring technology countermeasure against the transient over-voltage. In this study

the transient over-voltage online monitoring technology based on 500 kV capacitive voltage transformer（CVT） was studied. The 500 kV CVT was modified by paralleling a low-voltage capacitor C3 to form a reliable coupled voltage divider

and lightning and switching impulse transfer tests demonstrated its effectiveness of transient over-voltage monitoring for power grid. On the basis of the voltage divider

an intelligent over-voltage monitoring system was developed

and performance tests were conducted in laboratory

including the waveforms records of simulated switching actions

such as single opening

single closing

reclosing

opening-closing-opening

and operating voltage coupled with switch impulse. Further

this transient over-voltage monitoring system was installed at the "string adjustment project of 500 kV Pingyao substation" of East China Electric Power

and was verified by start-debugging test of Yaoyang 5438 line. Some recorded typical transient over-voltage waveforms are discussed to illustrate the feasibility and reliability of the proposed online monitoring technology and system in long-term field application.