A Grid Frequency Support Control Strategy of the Three Phase Cascaded H-Bridge Based Photovoltaic Generation System

Due to advantages such as being clean and safe, photovoltaic (PV) generation has become an effective way to solve the energy crisis and environmental pollution problems. However, the PV inverter does not have inertia and damping characteristics. With the increased penetration of the PV generation into the power system, large-scale PV grid-connection has posed a great challenge to grid frequency stability. The Cascaded H-bridge (CHB) inverter has become a promising candidate for the PV generation system by virtue of advantages such as modular structure and direct grid-connection without power frequency transformers. An inverter control strategy with grid frequency support function is proposed in this paper for the three phase CHB based PV generation system. With the PV string active power reserve, grid frequency dynamic characteristics are improved without energy storage devices. In order to solve the common module power unbalance problem for the CHB topology, the PV string output power is controlled to be balanced with the reserved power distribution control between PV strings proposed in this paper. The H-bridge inverter input power balance is maximized, and balanced three phase grid-connected current output is achieved. The over-modulation risk of H-bridge modules is reduced. At last, simulation models are built in Matlab/Simulink, and a 10kW experimental platform is also built. The effectiveness of the control strategy proposed is validated by simulation and experimental results.