Advanced Adaptive Control and Formal Stability Analysis of the N-Level Cascaded H-Bridge Multilevel Inverter fed Single-Stage Grid-Tied PV System Applications with an LCL Filter

This paper deals with the control process of a single-stage grid-tied photovoltaic (PV) system, consisting of N separate PV panels, N cascaded H-bridge (CHB) inverters, and an LCL filter. The objective is to design a novel controller based on a new decoupling 5th order model to achieve threefold purposes: (i) power factor correction (PFC) by forcing the grid current tracking; (ii) providing maximum power from PV panels and controlling the sum of the squared capacitor voltages; (iii) power exchange balance by regulating the involved capacitor voltages. To fulfill the former objectives, an adaptive nonlinear controller composed of three-loops is synthesized: (i) the inner loop is designed using integral backstepping and Lyapunov approaches for the PFC objective; (ii) the outer loop is formulated using a filtered proportional-integral (PI) regulator for voltage regulation; (iii) the balance loop is designed using PI controllers for voltages balancing. Furthermore, the controller includes a sampled-data adaptive observer to provide online estimation of the grid state variables and the unknown impedance. The stability analysis of the closed loop system is formally proven involving the system averaging theory. The developed controller is evaluated in a 5-level CHB, through simulations in MATLAB/Simulink under different operating conditions. Hence, demonstrating the supremacy of the proposed approach over the PI-based controller.