Flexible DC converter valve submodule DC capacitor fault on-line detection method

The invention discloses an online fault detection method for a DC capacitor of a submodule of a flexible DC converter valve, and relates to the field of flexible DC converter valve detection. In a normal operation mode of the flexible direct current converter valve, module redundancy in the flexible direct current converter valve is utilized, an instruction is given to a to-be-monitored sub-module, a removal mode is entered, sub-module capacitor undervoltage fault protection logic is shielded, and a sub-module capacitor enters a discharge process; at the moment, starting to measure the discharge time constant in the discharge process; when the capacitor discharges to lower voltage, the triodes connected in parallel with the oscillation inductor are turned off, and oscillation period measurement is carried out; after the measurement is finished, the sub-modules can be controlled to enter an input process, and the sub-modules can be input after being cut off; and finally obtaining the capacitance value of the c