Design, Implementation and Control of A High Efficiency Interleaved Flyback Micro-Inverter for Photovoltaic Applications.

摘要

Photovoltaic (PV) micro inverters have been gaining attention for the grid-connected PV systems because of improved energy harvest, friendly "Plug-N-Play" operation, and enhanced modularity and flexibility. Various inverter topologies for PV micro-inverters applications have been introduced in the literature that perform the maximum power point tracking (MPPT) of PV module, high step-up voltage amplification, output current shaping, and galvanic isolation. Among them, the flyback based micro-inverter is one of the most attractive solutions due to its simple structure and control and also its inherent galvanic isolation.;The conventional flyback micro-inverter consists of decoupling capacitor, interleaved flyback converter, unfolding bridge, and CL filter. The unfolding bridge is switched at line frequency by a simple square-wave control, generating a rectified sinusoidal waveform at the dc-link between the interleaved flyback converter and unfolding bridge. The decoupling capacitor maintains the power balance between the constant input power and variable output power oscillating at double-line-frequency. All the other functionalities required in PV micro-inverter are performed by the flyback converter. Therefore, the flyback converter has been widely scrutinized to improve its performance in terms of efficiency, reliability, and cost.;The aim of this thesis is to develop a new control and clamping mechanism in order to increase the efficiency of the flyback micro-inverter at the lowest possible cost. To achieve that goal, a hybrid switching strategy is adopted for the inverter. The adopted switching strategy controls the inverter in the Boundary Conduction Mode (BCM) in order to exploit the natural resonance of the flyback transformer to achieve Zero Voltage Switching (ZVS) during the turn-on process. At low load and near the zero-crossing of the grid voltage, the switching frequency of the inverter is then limited by transitioning to Discontinuous Conduction Mode (DCM) to limit the switching loss. Although this hybrid switching strategy ensures the ZCS turn-on for every switching cycle and ZVS turn-on for most of the grid cycle, it does not provide any mechanism to limit the turn-off switching loss, which is the major source of loss in the flyback converter.;In order to limit the turn-off switching loss, a novel adaptive snubber is developed in this thesis which ensures soft switching during the turn-off process. The developed adaptive snubber requires the minimum number of components and operates only at double-line frequency, which makes the control system easy and straight forward. Using the proposed adaptive snubber technology, a maximum efficiency of 96% is achieved.;Based on the proposed adaptive snubber with the associated hybrid switching method, the operation of the inverter is then further optimized in order to achieve the maximum CEC efficiency. The presented efficiency optimization procedure accurately takes into account the transformer and switching losses of the inverter and optimizes the parameters of the hardware and controller. The optimized hardware achieves the CEC efficiency of 94.96%, which is on-par with the available commercial products.