Research on the module power equalization control strategy of three-phase common DC-bus cascaded H-bridge multilevel inverter for large-scale PV power plants

Abstract

Featured with the characteristics of expandable modular structure, no need for line-frequency transformer directly connected to medium voltage grid and the same active power of each phase, three-phase common dc-bus cascaded H-bridge multilevel inverter has great advantages in high-voltage and high-power photovoltaic (PV) power generation application. However, if the number of modules in three phases is different, the active power of each module in the phase with fewer modules will be higher, and the active power transmitted by each module in the phase with more modules will be lower. Unbalanced active powers among modules lead to different heat dissipation requirements, which does not contribute to the modular design of the system. Aiming at this problem, this paper proposes an active power equalization control strategy based on zero-sequence voltage compensation. According to the actual number of modules in three phases and power factor of inverter, the compensated zero-sequence voltage can be calculated, by which all modules are capable of transmitting the almost same active power no matter whether the number of modules in three phases are same or not, and whether the inverter operates at unit power factor or not. The effectiveness of the proposed strategy is verified by simulation and experimental results.