Design and Convergence Analysis of an Improved Droop Controller With Adaptive Virtual Impedance

Microgrids are used to incorporate distributed generators (DGs) together so that DGs can be scheduled to satisfy the load demand. Usually droop controllers are adopted so that each DG shares the load equally. To improve the flexibility in system configuration with DGs of different capacities, droop controllers should be updated. In order to share the load among DGs in proportion to each unit's capacity and improve power supply quality, in this paper an improved droop controller is presented which is based on adaptive virtual impedance. By means of low bandwidth communication, two variables, i.e., the average distribution ratio of active and reactive power, are introduced. And through two integrators, the voltage reference and virtual impedance of every DG are tuned online. For the proposed controller, the system dynamic is discussed. It is shown that active or reactive power sharing convergences can be guaranteed when some inequalities are satisfied. Furthermore, the design of controller coefficients is discussed while considering convergence speed and system stability at the same time. The proposed droop controller reduces the amplitude and frequency shift usually caused by the traditional droop controller. It also improves the output voltage quality of the whole microgrid system. Finally, through simulations and experiments the effectiveness of the proposed droop controller is verified.