Abstract
The energy storage units of modular multilevel converter (MMC) based on battery energy storage system (BESS) are dispersed, which leads to the problem of state of charge (SOC) imbalance between energy storage units during steady-state operation. When the energy storage module is overcharged or over discharged, it needs to be out of operation, which will affect the stability of the whole system. This paper presents a SOC equalization control strategy for submodule battery. By changing the reference value of DC component of arm current, the phase to phase SOC equalization is realized without changing the DC side transmission power. The virtual voltage feedforward is used to realize the fast equalization of SOC between arms with small amount of calculation. By adding a power scale factor to the submodule switch signal, the SOC balance of the submodule in the arm can be achieved under the charging and discharging conditions. This paper deduces the power transfer relationship of MMC based on BESS, and studies the equalization control strategy of battery SOC based on its mathematical model. Finally, the effectiveness of the control strategy is verified by simulation. The results show that the SOC balance and power transmission of the sub module energy storage unit can be realized under the three-level control strategy.
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