Fault current operation test method of flexible DC (Direct Current) transmission MMC (Modular Multilevel Converter) valve

The invention relates to a fault current operation test method of a flexible DC (Direct Current) transmission MMC (Modular Multilevel Converter) valve. The method comprises the following steps of: charging the DC capacitors of a decaying current injection loop, a sinusoidal current injection loop and a high-voltage injection loop; generating a resonant current and generating a decaying current by the decaying current injection loop; generating a sinusoidal current by the sinusoidal current injection loop; then, continuously superimposing the decaying current and the sinusoidal current and applying the superposed currents to a trial valve; controlling to switch off a switchable device G and withdrawing the decaying current injection loop from the circuit; withdrawing the sinusoidal current injection loop from the circuit after the sinusoidal current crosses zero and triggering a bidirectional control valve V3 to put the high-voltage injection loop into operation; and applying a reverse voltage on the trial valve. The method enables a tested converter valve to tolerate transient current, transient heat and loss strength equivalent to that of true failure working conditions, is used for realizing tests and checks on the failure operation conditions of the tested valve and is convenient and simple to realize different test modes.