Research On The Low-Voltage Micro-Grid Decoupling Droop Control Strategy

Recently the development and application of microgrid has drew much attention, that’s because the microgrid is one effective form of the clean, pollution-free and intermittent DG (Distributed Generation). As the most important DG control method of microgrid, the traditional droop control method which usually applies to the inductive microgrid has been used widely because of its advantage that it can achieve power distribution without communication system. However, the low-voltage microgrid where the line impedance is usually resistive does not meet the traditional droop control method application requirement. The coupling will occur between the DG’s active power and reactive power and the power distribution will become hard to achieve if we use traditional droop control method in a low-voltage microgrid. The thesis study the droop controlled DG’s power decoupling and the restoration of frequency and voltage under the low-voltage islanded microgrid environment.First of all, the traditional droop control method are analyzed. Then the power droop controller model and the voltage and current double loop controller model are built and tested in the Matlab/Simulink. The DG’s power transmission theory, the automatic power distribution theory between droop-controlled DG and the inherent limitation of the traditional droop control method are analyzed.Secondly, the low-voltage microgrid decoupling droop control strategy is established on the base that the virtual impedance loop is added to the traditional droop control method. The virtual impedance theory is analyzed and the impact to equivalent output impedance before and after adding the virtual impedance is studied. Besides, the virtual impedance parameter is selected according to the closed-loop transfer function and the root locus plot. Then verify the virtual impedance in Matlab/Simulink, which can change the DG equivalent output impedance and achieve the decoupling power control of a droop-controlled DG under the low-voltage microgrid environment.Finally, the hierarchical control of a microgrid is introduced on the base that the virtual impedance-based droop control is used as the primary control level. The central secondary control and the distributed secondary control are analyzed. The impact of the secondary inserting time and the communication delay are also analyzed and verified.The thesis use the virtual impedance-based droop control as the primary control, then add a secondary control, so the low-voltage microgrid decoupling control strategy is established. The strategy can not only achieve the power decoupling and the automatic power distribution but also achieve the restoration of frequency and voltage very well in the low-voltage microgrid where the line impedance is resistive, so it has certain significance on keeping the system stable and improving the power quality of the islanded low-voltage microgrid.