| Under the pressures of resource shortages, environmental issues, the grid security and reliability requirements, the traditional power production and transmission are being changed gradually. Basd on the use of renewable energy resources, the distributed generation(DG) is being penetrated into grid system rapidly. In recent years, with its higher energy efficiency, less losses of transmission and distribution, higher reliability, less pollution, etc, DG has received much attention. In order to reduce the adverse effects of DG access to the grid, while maximizing its strengths to better promote the integration and application of large-scale DG system, the concept of microgrid was raised.The inverter is used as the interface to connect most of DGs to the grid. As a micro-grid basic power electronics interface, its reasonable and effective control would impact on the security and stability of microgrid operation directly. With virtual synchronization control technology and good performance in the distributed generation control, it has drawn great attentions. This paper includes:(1) From a mathematical model of the synchronous generator, virtual synchronization control strategy is analysed. Then combining with droop characteristecs of synchronous generator, the implementation process of the two control loops including active/frequency, reactive power/amplitude of virtual synchronization control is derived in detail. Small signal model of virtual synchronization control strategy is build. Moreover, based this control a microgrid model is built in PSCAD/EMTDC, from which the phase response is compared with small signal model from MATLAB, the consistency between both verifies the correct of the small signal model built.(2) Microgrid needs to be operated in the grid-connected or islanded mode, and the seamless transfer between both is requred. A clearly analysis of implementation process of the virtual impedance manner is presented, as well as operation characteristics of transferring from grid-connected mode to islanded mode, to achieve seamless transfer of microgrid operation modes based on virtual synchronization control. For the problem that grid-connect can not achieved with load based on this method, a non-impact method to connect to the grid with load is proposed without any didecate synchronization units, which uses the phase angle of output voltage of synchronization inverter as the reference angle of rotating frame, and a extra compensation quantity of reference angle frequency of inverter could be obtained by the PI close-loop control in rotating frame, so as to track the phase angle of grid voltage automatically. The effectiveness and feasibility were verified by the simulation results on PSCAD/EMTDC.(3) When a balanced voltage sag occurs in the grid, virtual synchroniztion control strategy is changed and the output power reference values are adjusted according to the sag magnitude, to achieve low voltage ride through function for the microgrid, and reactive power is outputed to support the recovery of the grid voltage. When an unbalanced failure, to extract positive and negative components according to the symmetrical component method, and to add negative sequence current suppression unit, thus output power of the inverter is ensured to follow their preset value respectively. The effectiveness of the proposed method is verified by simulation results on PSCAD / EMTDC(4) The design and implementation of software and hardware of microgrid inverter experiment platform. Through testing the prototype built, output voltage of the inverter can be self-synchronized quickly with that of the grid from the experimental results under the condition of power supply to the load. And the load is connected to the system with small impact, which shows good dynamic performance. The proposed control strategy is verified effectively. |
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