Research On The Control Of Grid-connected Inverters Of Microgrid

In order to take full use of the distributed power, a number of Distributed Resources (DRs), energy storage systems and loads were connected to constitute a micro-grid (Microgrid). How to make the grid-connected inverters work coordinately is one of the key issues to ensure the system's stability and highly efficient operation. It is also the focus of this paper.The control of the inverters consists of underlying control which named Voltage Control controls the output voltage and upper control which named Power Control controls power distribution. The Voltage Control module receives the voltage command from the Power Control module and generates the corresponding voltage output.The inverters with LC filter and LCL filter are discussed respectively. The main circuits are analyzed and parameters of the filters are designed. In order to simplify the control systems, control strategies in stationary coordinate system are used. Double closed-loop control strategy is proposed and new types of positive and negative sequence fundamental signal extractors are designed based on complex coefficient transfer function. By using this control method, the output voltage can be controlled under both balanced and unbalanced conditions. Simulation models are built in Matlab/Simulink to analyze and verify the performance of controllers designed above.The conventional droop method is improved by drawing on the frequency and voltage features of large synchronous generators. Active power-frequency and reactive power-voltage controllers are designed by using the improved method. The small-signal models of the power control system are built. The stability analysis of the microgrid is made and the range of the controller parameters can be got. The above-mentioned methods of Power Control are verified through simulations of single DR system and single DR with grid system, two DRs system and two DRs with grid system under various operating conditions.The experimental prototype is set up. The voltage control as well as power control of single DR system and single DR with grid system is verified through the experiments.