This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE JOURNAL ON EMERGING AND SELECTED TOPICS IN CIRCUITS AND SYSTEMS 1 An Effective Method for Low-Frequency Oscillations Damping in MultiBus DC Microgrids Nasim Rashidirad, Mohsen Hamzeh, Member, IEEE, Keyhan Sheshyekani, Senior Member, IEEE , and Ebrahim Afjei Abstract—This paper proposes a new active method for low frequency (LF) current/power oscillations damping in droop- controlled dc microgrids. Since, LF oscillations are mainly affected by droop controllers of voltage controlled (VC) DGs, detailed small-signal analysis of VC-DGs is provided. Analysis shows that each droop-controlled VC-DG creates a pair of LF complex conjugate zeros. In the proposed method, these zeros are damped by a negative feedforward of the disturbance variables (output currents) of VC-DGs. Stability analysis of the overall dc microgrid reveals that the LF zeros of VC-DGs can affect the LF modes of the system. Therefore, in the proposed method, the effective tuning of feedforward gain of each VC-DG can increase the damping factor of microgrid LF modes and consequenctly improve the dynamic response of the whole system. Moreovere, to gurantee the plug-and-play performance of DGs, a coordinanted tuning criterion for adjusting the proper feedforward gains is presented. It is shown that the proposed method is also robust against structural changes in dc microgrids. A complete set of simulation studies using MATLAB/Simulink is provided which further supports the effectiveness of the proposed active damping method. Index Terms— Active damping, dc microgrid, feedforward gain, low-frequency (LF) oscillations, plug-and-play. I. I NTRODUCTION D C MICROGRIDS have recently attracted a great deal of attention mainly due to the increasing tendency toward using renewable energy resources along with proliferation of dc loads [1]–[7]. This necessitates exploring different tech- nical aspects such as protection, stability and power sharing associated with dc microgrids. Some of these challenges are rather subtle and are not still very well understood. As a common practice, droop control is used for power sharing among dispatchable sources of islanded microgrids [7]–[11]. However, there are still several techni- cal challenges associated with droop strategies which need to be further addressed. There have been a number of Manuscript received October 14, 2016; revised December 16, 2016; accepted January 19, 2017. This paper was recommended by Guest Editor H. Ho-Ching Iu. N. Rashidirad, M. Hamzeh, and E. Afjei are with the Department of Electrical Engineering, Shahid Beheshti University, Tehran 19834, Iran. K. Sheshyekani is with the Department of Electrical Engineering, École Polytechnique de Montréal, Montréal, QC H3T 1J4, Canada. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JETCAS.2017.2664899 droop approaches proposed for the ac microgrids [12]–[18]. These include methods to improve stability and to accu- rately share the power among DGs within the ac microgrids (e.g., [14]–[18]). However, the compelling tendency toward utilization of droop-controlled dc microgrids [2], [9], [19]–[23] calls for further investigations to better understand technical challenges of this type of microgrids. One of the challenges of droop-controlled dc micro- grids is lack of accuracy of existing power sharing strategies [2], [5], [23] which is mainly attributed to line impedances. In fact, in the presence of line impedances, voltage controlled (VC)-DGs cannot provide the same output voltage and consequently the accuracy of power sharing is degraded. To circumvent this problem, some improved control methods have been proposed in [2] and [23] which are based on a low-bandwidth communication system. A new adaptive control method is also presented in [5], which satisfies accurate current sharing, without using any communication link among DGs. Another important challenge of droop-controlled dc micro- grids, is the low-frequency (LF) oscillation of power/current mainly arising from droop controllers of VC-DGs [9]. This implies that a droop-controlled dc microgrid is likely to subject to LF eigenvalues with low damping factor, which can be excited by different factors such as load changes. A con- ventional method for damping the LF oscillatory modes of the system is based on the augmentation of passive elements. Therefore, according to [2], [5], [9] and [19], increasing the line impedances can be viewed as a passive damping method. However, since each droop controller acts as a virtual series impedance for each DG, the increase of line impedances can reduce the accuracy of the power sharing. It also decreases the overall system efficiency. Therefore, the use of active damping methods for increasing the damping factor of LF eigenvalues is inevitable. In [9], an active damping method whose principle is based on a virtual inductive impedance loop has been proposed. Although this method is only applicable to current droop types, it can eliminate LF oscillations in dc microgrids. Within the context alluded above, this paper proposes a new active damping method which is applicable to both types of current (linear) and power (nonlinear) droop controllers of dc microgrids. The proposed method relies on the principle of 2156-3357 © 2017 IEEE. 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