Fault Analysis And Power Quality Improvment Technology Of Micro-grid

The frequent happenings of energy shortage, environment pollution and cascadedfailure in power grid have become problems that cannot be evaded for electric powerproduction and its secure operation. Meanwhile, the upsurge of distributed powersupply technology featured mainly by renewable resources have provided newsolution to save investment, to lower down energy consumption, pollution, and toboost up power system security and flexibility. As an organizing form ofgrid-connection for distributed generators, micro-grid can be operated either with orwithout connection to the main grid. So micro-grid could be used to the enhancereliability of power supply and improve the power quality, even to provide the abilityof black-start when the main power grid collapses. The combination of micro-grid andtraditional power grid is considered to be the developing trend for power systems.However, the protection and control technology of the micro-grid is the bottleneck forthe further application of distributed generation technology. Therefore, the research onthe protection and control technology of the micro-grid in this dissertation is of vitalimportance.Firstly, the dissertation introduces the development and status quo of micro-gridtechnology, discusses the structure and configuration of each component in micro-grid,and analyzes different types of inverter-based distributed power supply.Secondly, the fault characteristics of micro-grid at both grid-connection andislanding operation conditions are studied, which include the following: the effects ofboth the connection capacity and the connection site for a micro-grid on the faultcharacteristics and on the protective relays; the fault characteristics for a micro-gridduring islanding operation and its effects on protective relays. Two protectionschemes of a tie-line or feeder are proposed for the grid-connection and islandingoperation, respectively, which can meet the protective requirements of the electricpower system.Third, the islanding detection technology is studied. The existing islandingdetection methods are not suitable for the micro-grid settings. The frequencycharacteristics of both positive and negative feedback control are compared in the isolated micro-grid, and a lag-loop-based islanding detection method is proposed. Thenew algorithm does not destroy the islanding operation conditions, has a smallnon-detection zone and owns a fast detection speed. So it is quite suitable forislanding detection in micro-grid.Lastly, the control method for improving power quality in micro-grid is studied.The importance of improving power factors and lowing the harmonic ratio and thetotal harmonic distortion, as well as three-phase voltage unbalance, is explained. Theproposed control method is for the inverter-based distributed power supply on thebasis of Park transformation. The compensation accuracy is irrelevant to eitherthree-phase voltage unbalance or wave distortion, while the output power of thedistribute power supply does not change. The implementation of this controlalgorithm needs not to alter the primary electrical loop, with both the real-time andeffect validated bysimulation.