Control Of Microgrid Converter In Grid-connected Mode And Switching Between Grid-connected Mode And Islanding Mode

The technology microgrid has been paid more and more attention. In microgrid, the distributed generation could be accepted to the maximum extend, while reliability of power supply and ability of emergency service could also be improved. The microgrid discussed in this dissertation contains battery storage system, wind power system and PV power system. In this dissertation, we focus on the control strategy of microgrid converter in grid-connected and switch method between grid-connected mode and islanded mode.Firstly, the operation performances of microgrid converter in grid-connected are investigated. The relationship between droop characteristics of microgrid converter and operation environment is discussed. The reasons adopting P-f and Q-V droop characteristics in this dissertation and matters which need to be paid attention are indicated. Based on the equivalent circuit of output voltage and output impedance, this dissertation builds the mathematic model of microgrid converter in grid-connected. The coupling of output power and immunity to a grid disturbance impact of control parameters are analyzed. Simulation results show that mathematic model of microgrid converter built in this paper is accurate and analysis results are credible. The occurrence mechanism of output harmonic current is analyzed. The output harmonic impedance is increased by improving control loop without change output fundamental impedance and the output harmonic current is suppressed, the proposed method is verified by simulation and experimental results.Secondly, the switch methods among different modes of microgrid converter are investigated. Different switch methods are discussed based on dynamic response of output power in grid-connected and smooth control of output voltage in switch moment between grid-connected mode and islanded mode. A new switch method is proposed for microgrid converter between PQ control and single power loop droop control. The proposed switch method adopts the closed-loop cracking, which can be applied to other switch method. Control block diagram of switch method is given, working principle is analyzed, and parameters of switch method are discussed. Concrete implementation way of switch method is given according to the intentional islanding and unintentional islanding. And the proposed method is verified by simulation and experimental results.Thirdly, state detection methods of microgrid are investigated. In this dissertation, active harmonic method injection based on the complex filter is adopted to estimate grid impedance. The principle of complex filter is analyzed, the parameter of complex filter is chosen andthe specific way of digital implementation is given. The grid impedance estimation is verified by simulation and experimental results. This dissertation analyzes the principle of islanding phenomenon, introduces three different traditional islanding detections, and proposes a method adopting uncharacteristic harmonic positive feedback mode to realize islanding detection. The principle of uncharacteristic harmonic positive feedback is analyzed, the positive feedback loop is designed and positive feedback coefficient is selected according to stability and response time. Different simulation and experimental results validate the performance of this islanding detection method.Fourthly, voltage synchronizing methods of microgrid system in the moment from islanding to grid-connected are investigated. This dissertation introduces the principle of quasi-synchronize paralleling control method between generator and grids, analyzes the voltage synchronizing methods and introduces the hierarchical control of microgrid. The related equipment and scope of microgrid is illustrated when voltage synchronizes based on the hierarchical control. The traditional voltage synchronizing methods of microgrid are discussed and an improved voltage synchronizing method is proposed in this dissertation. The frequency regulator and phase regulator are unified in improved method. The negative sequence regulator is added to realize adjusting the positive and negative sequence of microgrid system respectively under unbalanced grid voltage conditions. The effectiveness and feasibility of proposed method are verified by simulation and experimental results.