In recent years, the significant increase in the penetration of renewable energy sources, such as photovoltaic wind power, has challenged the safe and stable operation of the power grid. Therefore, a large number of Battery Energy Storage Systems (BESS) are connected to the power grid, mainly used to improve the grid’s frequency regulation and voltage regulation capabilities. The Cascaded H-bridge (CHB) topology of Power Conversion System (PCS) can connect low-voltage DC components directly to medium-voltage grid or even highvoltage grid, without a power transformer. Due to the high number of voltage levels in the cascaded topology, the equivalent switching frequency is high and thus the system output waveform is more satisfactory. In addition, CHB-PCS has the advantages of modularity and high scalability, so this topology scheme is very promising and has received widespread attention.
However, BESS based on CHB-PCS (CHB-BESS) faces the issue of inter-phase SOC imbalance. The conventional zero-sequence voltage injection strategy is inefficient when DC-side voltage is insufficient. To address this issue, an innovative self-adaptive inter-phase SOC balancing control strategy is proposed. In this strategy, injecting a three-phase common-mode max-min average signal significantly reduces the modulation voltage magnitude by 13.4%, thereby allowing for a remarkable increase in the injected zero-sequence voltage magnitude. This strategy dynamically adjusts the injected zero-sequence voltage amplitude and phase based on the real-time state of CHB-BESS. It optimally utilizes the DC-side voltage while avoiding overmodulation and remains effective even when conventional strategies are inefficient. To validate the effectiveness of this strategy, a 10kV 5MW/11.2MWh BESS was designed, and simulation was conducted using Matlab/Simulink for various operating conditions within its power capability. The results indicate that this strategy does not affect the output power quality of the BESS. Moreover, it maintains satisfactory balancing efficiency even when the DC-side voltage is not abundant.
Moreover, due to the large number of sub-modules and thus the increased probability of sub-module failures, CHB-BESS needs to have a certain degree of fault-tolerance to improve system reliability. this research proposes an innovative fault-tolerance strategy for CHB-BESS to operate under fault conditions with power balance among all submodules and without voltage overmodulation. Theoretical analyses show that the modulation voltage amplitude of the new strategy is significantly lower than that of the conventional strategy when coping with the same fault and exhibits a wider range of applicability. Simulation results based on 6kV 2MW/6.72MWh CHB-BESS and a 10kV 5.5MW/11.2MWh CHB-BESS in Matlab/Simulink confirm that, under various operating conditions, the proposed solution exhibits negligible differences in output power quality compared to the conventional method. Compared with the conventional approach, the new strategy has significantly enhanced sub-module power equalization capability, demonstrating superior efficiency and applicability.
