During the charging test on pole 1 lower-voltage converter transformer in Tongli Station of ±800 kV UHVDC project from Jinping to Sunan, when the reactive power control system gave a filter input command as converter transformer charging, twelve groups of AC filters in hot standby state were connected one by one. Then an over-voltage protection trip occurred at AC busbar, but the control system was automatically input again. The reason is that the voltage of 500 kV AC busbar used for reactive power control is less than the setting value of low voltage monitoring for Ac system (Umax), so the reactive power control continue gives a command to input filter. The above problems caused by measurement circuit fault are solved by adding the low-voltage limited function, and the interlock function is added in AC filter control program, in order to prohibit the AC filter connecting again when it is disconnected while its discharge is not finished. The tests on site prove the change of software is correct and reliable.