Level Doubling Network and Ripple Correlation Control MPPT Algorithm for Grid-Connected Photovoltaic Systems

The implementation of ripple correlation control (RCC) algorithms for maximum power point tracking (MPPT) schemes in PV generation systems is presented and discussed in this PhD thesis in order to improve static and dynamic performances. Improvements in RCC are introduced first, considering fast irradiance transients and a hybrid RCC scheme is proposed. Power quality of the PV generation system is improved by multilevel inverter implemented by level doubling network (LDN), reducing output voltage and current harmonics. Reference is made to single-phase single-stage multilevel PV generation systems, when the inverter input variables, actually PV voltage and PV current, have multiple low-frequency (ripple) harmonics. The harmonic analysis is carried out with reference to a multilevel configuration consisting in H-bridge inverter and level doubling network (LDN) cell, leading to a multilevel inverter having the double of output voltage levels compared to the basic H-bridge inverter topology (i.e., five levels vs. three levels). The LDN cell is basically a half-bridge fed by a floating capacitor, with self-balancing voltage capability. The multilevel configuration introduces additional PV voltage and current low-frequency harmonics, perturbing the basic implementation of the RCC scheme (based on the 100 Hz component in case of 50 Hz fundamental), leading to malfunctioning. The proposed RCC algorithm employs the PV current and voltage harmonics at a specific frequency for the estimation of the voltage derivative of the power dP/dV (or dI/dV), driving the PV operating point toward the MPP in a more precise and faster manner. Detailed analytical expressions of peak-to-peak voltage and current harmonic amplitudes are given as function of the modulation index for both H-bridge and LDN cells. Maximum peak-to-peak values of both low-frequency and switching frequency voltage ripple components are effectively adopted to design the capacitors of both Hbridge and LDN cells basing on the desired dc-link voltage ripple requirements. Both simulation and experimental results are carried out to prove the validity of the analytical developments. The analysis of the dc-link voltage and current harmonics has been extended also to three-phase configurations. The steady-state and transient performances of the proposed RCC-MPPT schemes have been tested and compared by MATLAB/ Simulink. Results have been verified by experimental tests considering the whole single-phase multilevel PV generation system, including real PV modules, multilevel IBGTs inverter, and utility grid.