Operating and control of cascaded photovoltaic systems suffering module-mismatch

Modular cascaded multilevel converter is a promising option for high power solar photovoltaic (PV) system due to its modularity, scalability, and distributed maximum power point tracking (MPPT) capability, etc. 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 cause over-modulation or even destroy the system stability. In this paper, a novel modulation method for cascaded modular multilevel converter with two modes which are low-loss mode for normal operation and extended mode for module-mismatch operation is firstly proposed. With the former mode, the switching times are reduced greatly and then low the switching loss. When it is encountered by module-mismatch, extended mode is able to re-distribute power and then to balance dc-link voltages. Then a reactive power compensation algorithm (RPCA) is analyzed and applied to get a larger operating range based on the proposed modulation method. 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 technologies.