Micro grid-islanding detection, autonomous operation, synchronization and re-synchronization and battery based energy storage

The penetration of micro-grid system containing renewable energy generation is increasing exponentially for alleviating the energy source crisis and environmental pollution. Micro-grids are expected to achieve two priority aims, which are increase in stability and improvement of renewable energy generation efficiency. In terms of the first aim, micro-grid needs to keep operating with or without main-grid connecting. In grid disconnected condition, which is also called islanded condition, micro-grid system has to ensure its reliability and energy quality, to balance power between generators and loads. For the second aim, micro-grid with renewable energy sources is expected to extract maximum available energy at any moment. For realizing this aim, the energy storage system should be embedded in a micro-grid to help sharing the surplus energy from generators or missing energy required by loads.Based on above two aims, this thesis presents a comprehensive modeling of a micro-grid with the functions of islanding detection, autonomous operation, synchronization and re-synchronization and energy storage. Topology of local distribution generation is modeled by a grid-tied inverter with LCL filter. Proportional resonant controller is applied to control the inverter. A new islanding detection method which is a combination of two passive detection methods has been adopted in this research. A new method for autonomous operation of islanded micro-grid which combines weak droop control and PQ control has been used in this research. An effective and fast 50Hz-signal extracting filter, Windowed-sinc band-pass filter has been used for synchronizing and re-synchronizing between micro-grid and main grid. A combination of bi-directional buck/boost converter and DC/AC converter is applied to connect the energy storage block with micro-grid to fulfill the discharging/charging function. A full code-based implementation of this micro-grid modeling has been conducted in Matlab Simulink environment. The simulation results have verified that this new micro-grid model could operate well. Specifically, firstly, by combining two passive islanding detection methods, one can reach error-free islanding detection and protection. Secondly, autonomous operation of micro-grid containing three DGs could be realized by employing weak droop control and PQ control. The weak droop control could share the unbalanced energy between generators and loads automatically. Thirdly, smooth synchronization and re-synchronization could be obtained by extracting fundamental signal of each inverter output by the Windowed-sinc band-pass filter. Lastly, a battery energy system has been adopted to inject or absorb energy into or from micro-grid