Research On Control Strategy Of Micro-grid Converter

It is attracting more and more attention worldwide as Micro Grid is becoming the pioneer practical solution of the sustainable power. This thesis focuses on variety of control strategies of the inverters used in Micro Grid and conducted simulations and experiments to analyze them according to different control objectives. Firstly it introduces different topologies of the inverters and the control theories and algorithms that can be used with them. Then it focuses on the control strategies of the common DC-bus during synchronization status and off-grid status which uses grid-side inverter control strategy and power storage unit control strategy respectively. At the same time considering the working condition of the grid, it proposes a control strategy which can stabilize the Micro Grid and dispatch the power of the grid more reasonably at the same time. It also provides simulation results to testify the effectiveness of the strategy and experimental results which realizes the control strategy of the common DC-bus.Secondly it analyzes the topology and control strategy of the inverter in the Micro Grid. A variety of DC-DC converters both insulated or not are introduced and according to the control objectives of constant current working status it introduces the control strategies for the bidirectional DC-DC converters either half-bridge non-insulated ones or full-bridge insulated ones, and a simulation was conducted to testify them. An experiment was also conducted to realize the constant current discharge of the batteries and unity power factor on a low power half-bridge non-insulated bidirectional DC-DC converter which can prove the feasibility of the control strategies.Thirdly it analyzes the voltage restoration of grid while asymmetrical or non-linear load is connected. It introduces the problems that can be caused by this situation and focuses on the asymmetrical load with neutral line situation, with a three phase four bridge arm topology and a flexible control of the fourth bridge arm it can eliminate the zero-sequence factor of the load voltage, and a simulation is also provided. Also a simulation was conducted to analyze the effectiveness of the control strategy in eliminating the negative-sequence factor of the load voltage that is known as direct negative-sequence method and indirect negative-sequence method. While facing a non-linear load, a proportional-integration-resonate (PIR) controller is introduced to replace the conventional PI controller which can handle the harmonic factor of the load voltage during non-linear load situation; a simulation was also conducted to testify the effectiveness and feasibility in this part.In the meantime, it studies the switching techniques of the Micro grid and the main grid. It firstly introduces the control strategies of the Micro Grid in both synchronization status and off-grid status and then the switching control strategies which can realize the flexibility and stability during the switch. Both simulation and experimental results are provided to testify the control strategy under synchronization status, off-grid status and switching status.