Advanced Controls for Modular-multilevel Cascaded Two-stage Grid-connected Photovoltaic System

The growing demand for solar photovoltaic power generation has pushed the capacity of photovoltaic (PV) power plants towards MW power levels. The power electronics conversion system of a two-stage grid-connected PV (TGC-PV) power system needs to meet the stringent requirements in terms of reliability, efficiency, scalability, and ease of maintenance, power quality. The multilevel converters, including the multilevel dual active bridge converter, neutral point clamped DC-AC converter meet most of the requirements. However, the solar irradiance and temperature conditions may vary significantly in a large-scale solar power plant. As a result, the bridges connected to different PV arrays are expected to generate different amounts of power. This research presents systematic modeling and control of the ML-DAB connected with NPC converter to enable it to be a potential converter topology for grid integration of full-scale TGC-PV's. Based on the developed models for TGC-PV system, the appropriate control system is proposed to provide balanced grid currents when the power imbalance problem occurs. Individual maximum power point tracking control for each port in TGC-PV system under different irradiance conditions is proposed and validated on a hardware prototype. Performance/effectiveness of the proposed control systems for the TGC-PV systems are valuated/verified as controller-hardware-in-loop in OPAL-RT Real-Time simulator and NI-myRIO controller.