For the 50% load rejection test of a supercritical unit

the rotating speed exceeds the standard requirement

which seriously affects the safety operation of the unit. Based on the analysis of the experimental mechanism

combining with the control design logic and dynamic and static test data

it is concluded that the lag of signal transmission is the main reason for the over-speed. This study proposes some measures such as increasing the hard wiring loop logic between the switch and over-speed solenoid valve and shortening the scanning cycle of the module. Finally

the unit passes the rejection test successfully. The conclusion of this study has some reference value for the control logic design and operation commissioning of the same type set.