Abstract: The key to maintain secure and stable operation of DC grid is to keep the instantaneous energy balance of DC system. Considering the energy balance control strategy, the literatures more focus on the energy balance control among converters while ignoring the instantaneous energy balance control inside the converter. This paper, considering the modular multilevel converter（MMC） based DC grid, investigateed the energy balance control inside MMC. Firstly, according to energy transfer in the arms, it analyzed the commutation process of the MMC. Then according the relationship between the submodule（SM） voltage and instantaneous energy flow in the arms, a control strategy to regulate the SM voltage was proposed to maintain the energy balance in the MMC. A hierarchical energy control architecture for DC grid was proposed by combining the system level energy control and internal energy balance control of MMC. The simulation results verified the effectiveness of the proposed control methodology.