4088 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 10, OCTOBER 2009 Adaptive Droop Control Applied to Voltage-Source Inverters Operating in Grid-Connected and Islanded Modes Juan C. Vasquez, Josep M. Guerrero, Senior Member, IEEE, Alvaro Luna, Pedro Rodríguez, Member, IEEE, and Remus Teodorescu, Senior Member, IEEE Abstract—This paper proposes a novel control for voltage- source inverters with the capability to flexibly operate in grid- connected and islanded modes. The control scheme is based on the droop method, which uses some estimated grid parameters such as the voltage and frequency and the magnitude and angle of the grid impedance. Hence, the inverter is able to inject independently active and reactive power to the grid. The controller provides a proper dynamics decoupled from the grid-impedance magnitude and phase. The system is also able to control active and reactive power flows independently for a large range of impedance grid values. Simulation and experimental results are provided in order to show the feasibility of the control proposed. Index Terms—DC–AC power conversion, distributed genera- tion (DG), droop control, impedance estimation, power generation control, voltage-source inverters (VSIs). I. I NTRODUCTION D ISTRIBUTED generation (DG) systems and microgrids are becoming more and more important when trying to increase the renewable energy penetration. In this sense, the use of intelligent power interfaces between the electrical generation sources and the grid is mandatory. These interfaces have a final stage consisting of dc/ac inverters, which can be classified in current-source inverters (CSIs) and voltage-source inverters (VSIs). In order to inject current to the grid, CSIs are commonly used, while in island or autonomous operation, VSIs are needed to keep the voltage stable [1]. VSIs are very interesting for DG applications since they do not need any external reference to stay synchronized [2], [3]. In fact, they can operate in parallel with other inverters by using frequency and voltage droops, forming autonomous or isolated microgrids [4]. Furthermore, VSIs are convenient since they can provide to distributed power generation systems perfor- mances like ride-through capability and power quality enhance- Manuscript received November 24, 2008; revised July 9, 2009. First pub- lished July 28, 2009; current version published September 16, 2009. This work was supported by the Spanish Ministry of Science and Technology under Grants CICYT ENE 2006-15521-C03-01/CON and ENE2007-67878-C02-01/ALT. J. C. Vasquez and J. M. Guerrero are with the Department of Auto- matic Control Systems and Computer Engineering, Universitat Politecnica de Catalunya, 08036 Barcelona, Spain (e-mail: juan.carlos.vasquez@upc.edu; josep.m.guerrero@upc.edu). A. Luna and P. Rodríguez are with the Department of Electrical Engineering, Universitat Politecnica de Catalunya, 08222 Terrassa, Spain. R. Teodorescu is with the Power Electronics Section, Institute of Energy Technology, Aalborg University, 9220 Aalborg, Denmark. 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/TIE.2009.2027921 ment [5], [6], [25]–[31]. When these inverters are required to operate in grid-connected mode, they often change its behavior from voltage to current sources [7]. Nevertheless, to achieve flexible microgrids, i.e., able to operate in both grid-connected and islanded modes, VSIs are required to control the exported or imported power to the mains grid and to stabilize the mi- crogrid [8], [9]. In this sense, the droop method can be used to inject active and reactive power from the VSI to the grid by adjusting the frequency and amplitude of the output voltage [2]–[4]. However, the conventional droop method needs the knowledge of some parameters of the grid in order to control independently the active and reactive power flows. In this sense, the estimation of the grid impedance can be interesting not only for injecting P and Q into the grid with high precision but also for islanding detection. In this paper, we propose a control scheme based on the droop method which automatically adjusts their parameters by using a grid-impedance estimation method based on analyzing the voltage and current variations at the point of common coupling (PCC) resulting from small deviations in the power generated by the VSI [11]. The VSI is able to operate in both grid-connected and islanded modes, as well as to seamlessly transfer between these modes. This paper is organized as fol- lows. In Section II, the grid parameters are obtained by using an estimation algorithm. With these estimated parameters, an adaptive droop-control method is presented in Section III, which is able to decouple active and reactive power flow. Simulation results are presented in Section V to illustrate the feasibility of the proposed controller. Section VI shows the experimental results. Finally, Section VII gives the conclusion. II. ESTIMATION OF THE GRID PARAMETERS The grid characterization technique used in this paper is based on processing the voltage and current phasors at the PCC between the power converter and the grid. A frequency- locked loop based on the second-order generalized integrator (SOGI-FLL) is used to monitor such voltage and current pha- sors. As shown in Fig. 1, two cascaded integrators working in closed loop are used to implement the SOGI [10], [11]. This grid monitoring technique provides high precision, low compu- tational cost, and frequency adaptation capability [11], [24]. The aforementioned SOGI-FLL is also applied to monitor the current injected into the PCC in order to obtain the current phasor  I = i d + ji q . The SOGI-FLL acts as a selective filter 0278-0046/$26.00 © 2009 IEEE Authorized licensed use limited to: UNIVERSITAT POLITÈCNICA DE CATALUNYA. Downloaded on November 25, 2009 at 15:40 from IEEE Xplore. Restrictions apply.