Cascade STATCOM (Static Synchronous Compensator)-based optimized voltage support control method under asymmetric fault of power grid

According to the cascaded STATCOM-based optimized voltage support control method under the asymmetric fault of the power grid, when the power grid breaks down, on the basis that the power condition constraint that the sum of the positive-sequence active power and the negative-sequence active power of a cascaded H-bridge STATCOM is 0 is met, the positive-sequence current and the negative-sequence current injected into the power grid are reasonably designed, the grid-connected point voltage is supported, and the optimal voltage support control effect is achieved. And meanwhile, the amplitude of the injected current is limited within a safe range to ensure reliable operation of equipment. And finally, zero-sequence voltage is injected into the alternating current output side of each phase of the cascaded STATCOM system to balance the power of each phase, the capacitor voltage is stabilized, and the situation that the system cannot operate normally due to over-voltage and under-voltage of the direct current side