MICROSOURCE INTERFACE FOR A MICROGRID

A MicroGrid is typically a small power system, which consists of several microsources and energy storage units, providing heat and electricity to local loads. The MicroGrid has the capability to island and operate autonomously from the main utility network. MicroGrids potentially enable a greater integration of small-scale renewable energy sources. The objective of this thesis is to develop a single-phase microsource interface with energy storage unit embedded into the interface. An integrated energy storage unit avoids the necessity of a separate stand-alone energy storage unit in the MicroGrid. Thus the ? plug-and-play? functionality of the MicroGrid can be improved. A collection of power electronic converter based microsources with storage units connected to form a MicroGrid is a complex system. Development of such simple controllers, which decouple the effect of sub-unit while achieving the desired ? plug-and-play? capabilities, is a complex but important challenge. A photovoltaic panel was considered as the microsource and a battery bank was used as the energy storage unit. The proposed microsource interface consists of three power electronic converters. A photovoltaic panel is connected to a unidirectional dc-dc converter and its output is connected to the input of the single-phase inverter which can be connected to the MicroGrid. Energy storage is connected to the dc-link,which is the input of the single-phase inverter, through a bi-directional dc-dc converter. A simulation model of a photovoltaic panel was developed in the EMTDC/PSCAD software. The limitations of the model and a method to reduce these limitations are discussed. For the experimental validation of the proposed system, an adjustable-voltage-regulator hardware photovoltaic emulator was designed. The characteristic curves of the hardware emulator are compared with those obtained from the simulation model.A controller was designed for the unidirectional dc-dc converter to keep the output voltage of the photovoltaic panel at a given reference. Similarly the controller of the bi-directional dc-dc converter was designed to keep the dc-link voltage approximately constant. The behaviour of the dc-dc converters, which are connected to microsource and energy storage unit, around the steady state and worst-case scenarios was analysed, simulated and experimentally validated. Simulation and experimental results are compared. A current controller, based on instantaneous measured current, was implemented. This was designed to achieve good dynamic stiffness and command tracking properties. Furthermore a smooth grid connection method with the aid of the current controller is presented. The ability of the inverter to control the active and reactive power output was also analysed and verified with the aid of the simulation model and experimental set-up. Results corresponding to current controller, grid connection and power control are presented. The integrated system was simulated in EMTDC/PSCAD. The system response to the variations in the microsource and inverter output power variations was analysed. A smooth start-up method is shown which reduces the inrush current. Simulation results corresponding to different case studies and start-up transient are also included.