To achieve the goal of reactive power compensation and capacitor voltage balance of modular multiple-level converters（MMC） used in static synchronous compensator（STATCOM）

the three-level energy balance control strategy is used. Firstly

we analyzed an energy transform and transfer for MMC in detail

which explains how the energy can be transmitted from one phase to another phase

between the upper and lower arms

and among the SMs. Then

we discussed the determinant of influencing energy balance of MMC is. Thirdly

we adopted the circulating current suppression strategy based on quasi-PR controller. Thereby

the three-level energy balance control strategy is used for reference to realize the balance control of capacitor voltage. The simulation results show that the capacitor voltage large fluctuations caused by load unbalance between phases can be smoothed by utilizing the three-level energy balance control of MMC. The occurrence of the STACOM’s unstable operation which is caused by the load unbalance between phases can be avoided as well. Therefore

the proposed capacitor voltage balance control strategy can effectively enhance the security and stability of the MMC-STATCOM.