Study On The Fault Analysis And Protection Scheme Of Microgrid

With the development of renewable energy sources, the distributedgeneration(DG) units have gradually infiltrated into the power system in recent years.However, the large application of distributed generation is limited by its own defectswhich prevent the development of new energy-based distributed generation. Theconcept of microgrid emerges could be used to solve this problem. The new structureof the distributed grid explores the potential of all kinds of power generation andenergy storage. Making use of microgrid the problem of connecting DGs to the utilitygrid is solved and energy efficiency is maximized. The traditional distributionnetwork protection is no longer applicable to the microgrid which has thebi-directional power flow and small short-circuit capacity, thus it is urgent to presentthe new microgrid protection scheme.The project "Integrated control microgrid and the development and constructionof experimental platform" funded by State Key Laboratory of Power Systems as thebackground in this thesis enables the main research of micro-grids. The main issues inthis paper are the fault characteristics of the microgrid and the protection strategiesbased on local information and communication.Firstly, the mathematical model of the distributed energy unit is established. It isbased on the three-phase voltage source inverter circuit and controlled by the PQ andV/f control strategies. After the influence of the system grounding, static switch andcommunication on the microgrid operation control and protection is discussed, thestudied microgrid structure and parameters in this paper is present and the range ofprotection zones are set according to the requirements of the protectionSecondly, the inverter output characteristics under the asymmetric voltagecondition are given. In the case of asymmetric earth fault the certain harmonics of thevoltage is outputted from the inverter and the multitude of voltage dips. The triplefrequency harmonic content of voltage is used as the protection criterion according to these fault characteristics. Then the zero-sequence current protection is set based onthe microgrid structure. Finally the integrated earth fault protection of the Microgridis established and the simulation results verify the effectiveness of the proposedprotection strategy.Lastly, a new kind of invert model which considers the control method under theasymmetric voltage condition is designed. The simulation results point out that theimproved inverter could not output a large mounts of harmonics in the case ofasymmetric fault, and then verify that the protection criterion. The triple frequencyharmonic content of voltage loses its sensitivity. According to the basic faultcharacteristics of the Microgrid, the different protection strategies undergrid-connected mode and islanded mode are given respectively depending on thecommunication.