Modeling and control of a cascaded NPC/H-Bridge inverter with LCL filter in PV- Grid application

The output voltage and current of renewable energy sources such as Photo- voltaic are not controlled. Consequently PV- Grid interface system is employed condition output voltage and current into the grid. This call for efficient and well designed control strategies to ensure reliability, stability and quality power output. This paper proposes nine- level cascaded NPC/H-bridge inverter as the PV- Grid interface. Cascaded NPC/H-Bridge inverter is modeled and a control law for realizing nine- level output with reduced harmonic content is developed. Modeling is easily achieved by considering the whole inverter as having four cascaded three level legs, two positive and two negative. Then a mathematical model is formulated for LCL filters assuming that the resistances associated with the inductances have negligible effect on the system steady state variables. Finally a control technique is developed for the whole system and its robustness tested by carrying out several simulations tests in MATLAB under different operating conditions.