Cascade energy storage system and method for improving power quality of power grid

The invention discloses a cascade energy storage system and method for improving power quality of a power grid. The system comprises distributed energy storage units, a low-voltage module group and ahigh-voltage module group, wherein the low-voltage module group comprises a plurality of cascaded H-bridge circuits; the low-voltage module group is connected to a low-voltage alternating current busthrough a filter; the high-voltage module group comprises a plurality of the cascaded H-bridge circuits; the high-voltage module group is connected in parallel between a low-voltage bus and a high-voltage bus through the filter; each distributed energy storage unit is connected to the low-voltage module group or the high-voltage module group through one of the H-bridge circuits; and each distributed energy storage unit is connected to the corresponding module group through the H-bridge circuit to form a three-phase independent structure. Through the cascade H-bridge series structure, the energy storage system can be di