Abstract: In the ?800kV UHVDC system, the SF₆ low pressure trip circuit will start the function of emergency switch off sequence(ESOF) in order to make the inverter unit return to the standby status when the SF₆ pressure gauge terminals of the converter transformer valve bushing stick together. In order to eliminate the defects, the first scheme is to stall relay K205 and K305 in parallel with the trip circuit 1 and trip circuit 2 respectively, then the improved signal circuit and trip circuit can monitor refusing action of single element or misoperation, then to start trip function correctly. The second scheme is to increase a couple of new mechanical contacts, then the question of refusing action and misoperation of mechanical contacts and relays can be avoided by using of three out of two logic. The third scheme is to combine with real operation situation through series connecting warning circuit and trip circuit, which can avoid inverter unit ESOF caused by the contacts adhesion. The comparision shows that the second scheme is the best one.