IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 3, MARCH 2011 689 Mode Adaptive Droop Control With Virtual Output Impedances for an Inverter-Based Flexible AC Microgrid Jaehong Kim, Member, IEEE, Josep M. Guerrero, Senior Member, IEEE, Pedro Rodriguez, Senior Member, IEEE, Remus Teodorescu, Senior Member, IEEE, and Kwanghee Nam, Member, IEEE Abstract—A decentralized power control method in a single- phase flexible ac microgrid is proposed in this paper. Droop control is widely considered to be a good choice for managing the power flows between microgrid converters in a decentralized manner. In this work, to enhance the power loop dynamics, droop control combined with a derivative controller is used in islanded mode. In grid-connected mode, to strictly control the power factor in the point of common coupling (PCC), a droop method combined with an integral controller is adopted. Small-signal analysis of the pro- posed control is shown both in islanded and grid-connected mode. The proposed control scheme does not need any mode switching action. Thus, it is relatively simple in control for full mode of op- eration. Smooth transitions between the operation modes and the effectiveness of the proposed control scheme are evaluated through simulation and experimental results. Index Terms—Dispersed storage and generation, droop control, microgrid. I. INTRODUCTION B Y SUCCESSFULLY integrating power electronics and new emerging technologies, distributed generation (DG) has become an increasingly competitive option compared to a conventional centralized system. Among the various DG con- figurations available, the microgrid approach offers the most flexibility and reliability for power systems, and thus the mi- crogrid is generally regarded as the most attractive DG system configuration [1], [2]. A microgrid can be operated both in islanded and grid- connected mode. In the islanded mode, the control objective is to achieve accurate power sharing while maintaining close regu- lation of the microgrid voltage magnitude and frequency. Active load sharing techniques such as centralized [5], master-slave [3], average load sharing [12] and circular chain control [8] are very Manuscript received June 30, 2010; revised October 26, 2010; accepted October 31, 2010. Date of current version May 13, 2011. This paper has not been presented at any conference or journal. Recommended for publication by Associate Editor Paolo Mattavelli. The authors are with the Department of Electrical Engineering, POSTECH, Hyoja San-31, Pohang, 790-784 Republic of Korea (e-mail: jhongkim@ postech.ac.kr; josep.m.guerrero@upc.edu; prodriguez@ee.upc.edu; ret@iet. aau.dk; kwnam@postech.ac.kr). 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/TPEL.2010.2091685 intuitive and effective ways to achieve these objectives. But, due to their dependency on critical intercommunication lines among modules, these techniques could reduce system reliabil- ity and expendability [22]. The most attractive alternative is the droop method [6], [17], [18], [22], [24], [27], [31]. The droop method uses only local measurement and does not have a crit- ical high bandwidth communication link among the DG units. Thus it achieves a higher reliability level and is flexible in terms of the physical location of the modules. However, the conven- tional droop method also has several drawbacks including a slow transient response, a trade-off between power-sharing accuracy and voltage deviation, unbalanced harmonic current sharing and a high dependency on the inverter output impedance [22]. To overcome these drawbacks, modified droop control meth- ods are proposed in [13], [15], [19], [25], [26] and [30]. Ad- justable virtual output impedance was utilized in [13] and a frequency restoration loop was added in [15]. A derivative con- troller combined with conventional droop control was utilized in [19], [25] and [26]. In [30], virtual inductance was utilized to prevent coupling between the real-power and reactive-power controls. In grid-connected mode, the control objective is to achieve accurate power flow regulation at the point of common coupling (PCC), while maintaining all of the control functions in the islanded mode. Two proportional and integral (PI) controllers were utilized to regulate the grid current in [4]. In this case, the local ac bus was considered as a PCC in grid-connected mode and the control strategy needed to be switched, depending on the operating mode. In recent research on this type of flexible ac microgrid concept, a PI controller combined with conventional droop control was proposed for full mode of operation in [26]. In this method, the power references were changed depending on the operating modes. A similar control mode switching method was also proposed in [30]. A mode-adaptive droop control method is proposed in this paper. The proposed control can work both in islanded and grid-connected mode, without any control switching action in a decentralized manner. The dynamic performance is enhanced and stable transitions between operating modes are obtained with the proposed control. Small-signal analysis of the proposed control method was done for both islanded and grid-connected mode in Section V. Finally, a simulation and experimental ver- ification of the proposed control are shown in Section VI and Section VII. 0885-8993/$26.00 © 2010 IEEE