An AC Voltage Balancer and its Improved Modulation Strategy for CHB Based PV Inverters

Cascaded H-bridge (CHB) inverters have been widely used in distributed photovoltaic (PV) power generation systems due to their attractive features in terms of power quality and easy to expand compared to two-level or three-level inverters. Especially the modular inverters with an interphase structure composed of four-port isolated DC/DC converters and CHB inverters are preferred by academia and industry because it can naturally balance the power difference between phases. However, such modular inverters still suffer from some issues that need to be addressed, such as power mismatch between modules within the same phase. This will lead to overmodulation of some H-bridge inverter units and grid-connected current distortion, and even threaten the safe and stable operation of the entire modular inverter. In view of this problem, an AC voltage balancer (ACVB) is proposed in this article to achieve power balance between adjacent H-bridge inverter units. Furthermore, an improved modulation strategy is also presented to achieve soft switching of ACVB. Finally, the effectiveness of the proposed ACVB and its improved modulation strategy is verified by experimental results.