Operation Control And Analysis Of Micro-Grid With Different Types Of Distributed Generations

ABSTRACT With the further development of micro-grids, researching on it is becoming more and more necessary. As an important complement to the large grid, the micro-grids still have many areas to be improved, and the frequency stability is an important indicator of power quality, which is essential to the stable operation of micro-grid. Based on the research of micro-grids in recent years, this dissertation focuses on the frequency control strategies when the micro-grid in islanding mode and the smooth transition control between two modes of micro-grid.This dissertation constructed a micro-grid structure which contains different types of distributed generations, through analyzing the work characteristics of different DGs, building the corresponding dynamic models. Because micro-sources are all inverter interface type, in the light of different operation characteristics of DGs, this dissertation designed different control strategies, including double close loop decoupled PQ control and V/f droop control. In different operation mode, micro-sources will use different control strategies to enable the system to operate stably. Considering the output voltage contains harmonic, this dissertation designed a LC filter with a small resistor in series, which improves the quality of inverter output voltage.When the micro-grid operates in the islanding mode, in order to ensure better frequency stability of micro-grid, combining with the principle of secondary frequency regulation of power system, the new strategy introduces the proportional-integral (PI) control into control flow, which better commits frequency deviation caused by load changes. Taking into account the low inertia of micro-grid, a first-order inertia link from the motion equation of rotor of synchronous generator is added to the control strategy, achieving a more smooth frequency transition. This dissertation also made a theoretical analysis on the power distribution of active micro-sources during adjusting the frequency, which proved the correctness of the control strategy.Considering the large grid failure and the grid-connected micro-grid will switch to the islanding operation mode, this dissertation design a passive islanding detection method based on detection the frequency and voltage of PCC, and analyzed the effect of micro-sources with developed V/f droop control strategy in the switching. This dissertation use three-phase software phase-locked loop (SPLL) technology to complete the pre-synchronization of voltage and frequency between micro-grid with large grid when the micro-grid operates from islanding mode to the grid-connected mode, which will reduce the oscillation during switching.Finally, it verified the dynamic models of different types of DGs on the Matlab/Simulink platform, based on the micro-sources models it constructed a micro-grid structure with different types of DGs, and verified the effectiveness of frequency control strategy and smooth transition strategy.