Phase Power Balancing of Interphase Grid-Connected CHB-QAB PV Systems

The Cascaded H-Bridge (CHB) converter is an appealing solution for large-scale grid-connected photovoltaic (PV) systems. However, the phase power imbalance is one of the main challenges due to the non-uniform irradiance, unequal temperature and parameter mismatch, which may result in imbalanced grid current. The interphase Quadruple Active Bridges (QAB)-based CHB (CHB-QAB) converter is one of the promising converter architectures to overcome the issue of the phase power imbalance. Nevertheless, the existing control strategy for the interphase CHB-QAB PV systems can not ensure the phase power balancing when the mismatch exists in the CHB stage. If the sinusoidal zero-sequence voltage injection (SZVI), the conventional control strategy can exchange the power among three phases to keep grid currents balanced. However, in the condition of high mismatch, the control strategy with SZVI may cause over-modulation. In this paper, a control strategy for the interphase CHB-QAB PV systems is proposed, which can ensure the phase power balance as well as individual MPPT, even in the condition of high mismatch. Based on the derived average and small-signal models of the overall system, the control system is designed. As a consequence of the proposed control strategy, the bandwidth of the dc-link voltage controller can be increased for reducing the dc-link voltage ripple of H-bridge cells. This enables to reduce the dc-link capacitance. Simulation results validate the effectiveness and superiority of the proposed control strategy.