The voltage stability and balance control at DC side are the prerequisites to ensure safe and reliable operation of the cascaded H-bridge STATCOM. For the issue of voltage at DC side between the phases of STATCOM due to imbalance of grid voltage of the distribution grid

the three-phase power model of the system is set up on the basis of the dq coordinate system and the active power exchange process between phases is analyzed. The intrinsic relationshope between negative sequence current and phase-to-phase active power is proved theoretically. Then

a kind of control strategy using negative sequence current feedforward is proposed to achieve the control strategy of DC voltage balance between phases of cascaded H-bridge STATCOM under unbalanced conditions. On the other hand

such issue as unequal active power throughput of each H bridge sub-model in the unit time is proved throughout the analysis of the CPS-SPWM modulation method.Although the use of a fundamental frequency non-integer multiple carrier can reduce the active power difference of each sub-module to a certain extent

yet the effect of DC voltage equalization control of the submodule in the phase is still not ideal. In view of this shortcoming

a kind of improved voltage balance control strategy of the H-bridge sub-module in the phase based on the independent control idea of the sub-modules is proposed. The correctness and feasibility of the proposed control strategy are proved by the simulation and verification.