| In order to achieve the friendly accession and coordination control of the distributed renewable energy sources into the grids,improve the efficiency of the energy and meet the high quality,high reliability power supply requirements of the users,microgrids are rapidly developed so as to effectively integrate and use the renewable energies for power generation.One of the most important characteristics of the microgrid is that it can operate in an islanded mode.The islanded microgrid needs to establish the voltage and frequency of the system due to the lack of support from the main grid.However,most of the distributed generations(DGs)are integrated into the microgrid through inverters as interfaces,which results in low inertia characteristics and increases the difficulty of the stable grid-forming control.What's more,the loads fluctuations,the plug-and-play requirements of the DGs and the existing power quality issues will lead to great challenges of the safe operation and coordinated control of the microgirds,which makes traditional methods less effective.This thesis focuses on the following issues,and obtains corresponding results:1)Hierarchical grid-forming mechanism of the islanded microgrids.The mechanism of the existing hierarchical control structures including their limitations has been analyzed.The new characteristics of the modern islanded microgrids control structure have been summarized from control point of view.Thereafter,the concept of point of common coupling(PCC)voltage synchronization(PCC-VS)control is proposed to establish a novel control architecture of the microgrids for various application scenarios,which is the foundation of the following research.2)Voltage vector synchronization control strategy of multiple grid-forming inverters.For breaking the inherent trade-off between voltage regulation and power sharing accuracy of the existing droop based control methods,the dynamic model of the multiple parallel-connected grid-forming inverters system is established,and a voltage vector synchronization control method is proposed based on the concept of PCC-VS.The amplitude and the frequency of the PCC voltage can be precisely regulated in both steady-state(e.g.,line impedance mismatch)and transient-state(e.g.,loads variation,DG plug in/off,grid connected/islanded transition),meanwhile,the active and reactive power can be shared automatically and proportionally.3)Voltage synchronous tracking based microgrid hierarchical control strategy.In order to reduce the dependence on centralized communication,improve the reliability of the islanded microgrid and realize the plug and play control of the DGs,a voltage synchronous tracking control strategy is proposed for decentralized control of the multiple parallel-connected inverters.For the proposed control method,there is no centralized controller and intercommunication between inverters,thus reliability of the system is enhanced.Furthermore,compared with the traditional droop based hierarachical control methods,the transient performance of the system can be improved,as no extra restoration control layer is needed.Meanwhile,the power sharing ratios among DG units can be changed online without affecting the voltage regulation performance,which increases the power management flexibility.4)Grid-tied inverter stability control under non-ideal grid conditions.For the issues of stable operation of the grid-tied inverters and the power quality improvements in an islanded microgrid,a robust current control strategy is proposed.An uncertainty and disturbance modeling based non-linear control law is developed for improving the stability when the grid impedance changes and achieving the robustness against the grid voltage harmonics,threephase unbalanced faults and phase/frequency variations,also,the high dynamic responses of the inverters can be ensured.Furthermore,the inverter can be capable of compensation for local nonlinear and unbalanced loads,which improves the power quality of the islanded microgrid. |
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