Research On Power Sharing Strategy And Smooth Switch Method In A Microgrid

Micro-grid is an independent power supply system, made up of distributed generation (DG) and local load, which can operate not only in grid mode but also in island mode. In addition, rational allocation of power between the DGs is an important goal in island mode. Since the droop control can achieve power allocation without communication system, it receives widespread attention. However, as the conventional droop control affected by the line impedance mismatch, partial load and other factors, it is difficult to achieve precise control of power allocation.Therefore, this dissertation focuses on the characteristics of power allocation and its improvement methods based on droop control strategy, and smooth switching method between two operating modes of micro-grid, the specific contents are as follows:Firstly, the inverter model is established; the outer voltage loop and inner current loop controller is designed; the droop controller model is also established. On the basis of the droop controller model, the stability of the active power-frequency and reactive power-voltage droop controller is analyzed and verified by a simulation.Secondly, the characteristics of power allocation in parallel inverter system are analyzed in this dissertation. What’s more, the influences of line impedance, sagging coefficient and partial load on the power allocation are researched.Thirdly, the dissertation has studied a virtual impedance control method based on Q-U droop characteristic in order to eliminate the effects of deviation of the equivalent output impedance on reactive power distribution through the control method, so as to improve the reactive power distribution characteristics And the validity of the control method has been testified.Finally, the smooth switching method in the micro-grid based on droop control is researched. This method can achieve smooth switching between grid mode and island mode without changing the control method. The simulation results verify the correctness and validity of this method. In this dissertation, theoretical analysis and simulation verification show that: virtual impedance can effectively compensate for the effect on reactive power allocation, caused by line impedance mismatch. Thus the accuracy of reactive power allocation is improved. The smooth switching method based on droop control in micro-grid reduces the complexity and increases the flexibility, as it does not need to change the control mode. This conclusion has some value for power allocation strategy and smooth switching method based on droop control in micro-grid.