CONTROL METHOD FOR CASCADED H-BRIDGE MEDIUM-VOLTAGE DIRECT-MOUNTED PHOTOVOLTAIC INVERTER

The present invention discloses a control method for a cascaded H-bridge medium-voltage direct-mounted photovoltaic inverter, including the steps of: S1, determining two phases of a power grid where a short circuit occurs; S2, calculating an adaptive zero-sequence voltage of a phase A by means of an adaptive zero-sequence voltage compensation strategy, wherein the phase A refers to a phase of three-phase the power grid where no short circuit occurs; S3, compensating for the adaptive zero-sequence voltage, then calculating an amplitude of a modulation voltage of the phase A, and calculating the modulation voltage of the phase A based on the amplitude of the modulation voltage of the phase A; S4, expanding the modulation voltage of the phase A to symmetrical three-phase voltages based on the modulation voltage of the phase A; S5, calculating a harmonic zero-sequence voltage by means of a maximum-minimum harmonic zero-sequence voltage compensation strategy; and S6, acquiring three-phase modulation voltages based on the adaptive zero-sequence voltage and the harmonic zero-sequence voltage, to suppress active power backflow. The present invention can significantly reduce a region with the active power backflow, and is more applicable to conditions with higher voltage sag depth and lower system output power.