A novel modulation method applied in quasi-Z-source based cascaded pv system suffering module mismatch

Quasi-Z-source based modular cascaded multilevel converter featuring both the advantages of multilevel converter like modularity, scalability, distributed maximum power point tracking (MPPT) capability and the advantages of quasi-Z-source network (qZSN) like high efficiency, no consideration for annoying switching dead-zone, wide dc voltage boosting range etc. is a promising option for high power solar photovoltaic (PV) system. Nevertheless, inherent mismatch and varied operating conditions (both called module-mismatch) are still the issues to be disclosed. Asymmetrical active power harvesting among individual PV converter modules resulting from module-mismatch will fluctuate dc voltages and polarize the distribution of the alternating terminal voltage and may derivatively destroy the coordination between the QZS and the H-bridges (HB). In this paper, a novel modulation method for qZSN based cascaded modular multilevel converter with two modes which are low-loss mode for normal operation and extended mode for module-mismatch operation is proposed. With the former mode, the switching times are reduced greatly and then low the switching loss. When encountered by module-mismatch, extended mode is able to re-distribute power and then to balance dc-link voltages. Based on the modulation method, a reactive power compensation algorithm (RPCA) with larger operating range is applied in order to mitigate the polarization of ac voltage. Finally, a 3MW/12kV system with the proposed modulation and control strategy is developed in Matlab/Simulink platform. Simulation results are presented to demonstrate the effectiveness of the proposed technology.