Managed DC Voltage Utilization Technique for the Renewable Energy Source Based on the Star-Connected Cascaded H-bridges Converter

The modular multilevel cascaded converter with single-star bridge-cell (MMCC-SSBC) is very suitable for next generation high-scale renewable energy resource system for the low switching loss, low output current distortion, and high reliability. The voltage balancing control is an important issue for MMCC-SSBC to maintain system operation which can be accomplished by negative sequence current and/or zero sequence voltage injection. The pure negative sequence current injection results in unbalanced output current, and thus increasing the stress of power electronic devices. On the other hand, pure zero sequence voltage injection increases the DC voltage utilization, and thus increasing the risk of over-modulation. This paper provides a flexible voltage balancing by regulating the effectiveness between these two injections to manage the DC voltage utilization and reduce the imbalance of output currents. The laboratory test results verify the proposed method, which are tested under unbalanced output power between each phase of MMCC-SSBC and asymmetrical voltage faults.