As a high-performance reactive power compensation device

the Cascade H-Bridge Static Synchronous Compensator (CHB STATCOM) is more and more widely used in the field of power quality control in medium and high voltage distribution networks. However

due to its star-shaped connection structure

it is prone to interphase DC voltage imbalance under unbalanced power grid conditions. The traditional voltage equalization method generally adopts the zero-sequence voltage injection method. Still

its regulation ability is weak

and the DC voltage margin of the system is high

resulting in poor adaptability to the working conditions of unbalanced power grids. In this paper

a CHB STATCOM topology based on a dual-frequency power loop is proposed

utilizing an interphase RLC resonant circuit to adjust the unbalanced power. This approach reduces the system's DC voltage margin requirements

thereby improving the STATCOM's adaptability under unbalanced power grid conditions. Firstly

the working principle of the topology is analyzed to reveal the mechanism of power transmission by the resonant circuit

and the relationship between the resonant transmission power and unbalanced power is discussed. The resonant circuit parameters are designed for unbalanced grid conditions. The intrinsic connection between the resonant circuit parameters and transmitted power is analyzed

and feed-forward and phase-shift control strategies are proposed. To analyze the compensation characteristics of the topology and control method under an unbalanced grid and to compare it with the zero-sequence voltage injection method. Finally

the effectiveness and advantages of the proposed method are verified by hardware-in-the-loop simulation.