An Optimized Third Harmonic Injection Method for Three-Phase Cascaded H-Bridge Photovoltaic Inverter

The uneven irradiation and temperature in photovoltaic (PV) modules of three-phase cascaded H-bridge (CHB) inverters will cause the unbalanced output power between solar panels, resulting in unbalanced grid current between three phases. In addition, since each phase can be regarded as a single-phase inverter, the second harmonic voltage pulsation of dc-side capacitors is a problem to be solved. To solve these two problems, a third harmonic injection optimization method on the basis of fundamental frequency zero sequence is proposed in this paper. The third harmonic with the same amplitude of fundamental modulated wave signal is injected into the modulated wave signal when the amplitude of synthesized wave signal is no greater than unity. Otherwise, the amplitude of third harmonic is decreased correspondingly to ensure the amplitude of synthesized wave signal being unity. With this method, the dc-side voltage pulsation is effectively restrained and the power balancing limit of CHB inverter is expanded. The feasibility and effectiveness of this method are verified by an experimental prototype of a three-phase seven-level CHB inverter.