Abstract
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.
Highlights
With the development of alternative energy technologies, the application of distributed generators (DGs) has become more and more widespread
Tang et al.: Design and Convergence Analysis of An Improved Droop Controller based on Adaptive Virtual impedances for the practical implementations
Tang et al.: Design and Convergence Analysis of An Improved Droop Controller based on Adaptive Virtual Impedance where E and are nominal voltage amplitude and By the adaptive controller based on the sharing error, Z viri is frequency references, ni and mi are droop coefficients of active and reactive power for DGi
Summary
With the development of alternative energy technologies, the application of distributed generators (DGs) has become more and more widespread. Tang et al.: Design and Convergence Analysis of An Improved Droop Controller based on Adaptive Virtual impedances for the practical implementations. For this reason, in some research literatures, output impedance estimation and calculation strategies have been studied [13]-[15]. The output impedance of each DG can be obtained by measuring the low-frequency common coupling harmonic voltage and the DGs' output harmonic current This method is complicated and difficult to utilize. To achieve frequency restoration and accurate reactive power sharing in islanded microgrids, an improved P-f droop control is proposed [22].
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