This paper deals with a crucial aspect in the control of grid-connected power converters, i.e., the detection of the fundamental-frequency positive-sequence component of the utility voltage under unbalanced and distorted conditions. Specifically, it proposes a positive-sequence detector based on a new decoupled double synchronous reference frame phase-locked loop (DDSRF-PLL), which completely eliminates the detection errors of conventional synchronous reference frame PLL's (SRF-PLL). This is achieved by transforming both positive-and negative-sequence components of the utility voltage into the double SRF, from which a decoupling network is developed in order to cleanly extract and separate the positive-and negative-sequence components. The resultant DDSRF-PLL conducts then to a fast, precise, and robust positive-sequence voltage detection even under unbalanced and distorted grid conditions. The paper presents a detailed description and derivation of the proposed detection method, together with an extensive evaluation using simulation and experimental results from a digital signal processor-based laboratory prototype in order to verify and validate the excellent performance achieved by the DDSRF-PLL.Index Terms-Grid-connected converters, phase locked loop (PLL), positive sequence signals detection, synchronous reference frame (SRF).

This paper examines distribution systems with a high integration of distributed energy resources (DERs) and addresses the design of local control methods for real-time voltage regulation. Particularly, the paper focuses on proportional control strategies where the active and reactive output-powers of DERs are adjusted in response to (and proportionally to) local changes in voltage levels. The design of the voltage-active power and voltage-reactive power characteristics leverages suitable linear approximations of the AC power-flow equations and is network-cognizant; that is, the coefficients of the controllers embed information on the location of the DERs and forecasted non-controllable loads/injections and, consequently, on the effect of DER power adjustments on the overall voltage profile. A robust approach is pursued to cope with uncertainty in the forecasted non-controllable loads/power injections. Stability of the proposed local controllers is analytically assessed and numerically corroborated .

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Abstract-1 This paper presents a centralized protection strategy for medium voltage dc (MVDC) microgrids. The proposed strategy consists of a communication-assisted fault detection method with a centralized protection coordinator and a fault isolation technique that provides an economic, fast, and selective protection by using the minimum number of dc circuit breakers (DCCBs). The proposed method is also supported by a backup protection which is activated if communication fails. The paper also introduces a centralized self-healing strategy that guarantees successful operation of zones that are separated from the main grid after the operation of the protection devices. Furthermore, to provide a more reliable protection, thresholds of the protection devices are adapted according to the operational modes of the microgrid and the status of distributed generators (DGs). The effectiveness of the proposed protection strategy is validated through real-time simulation studies based on the hardware in the loop (HIL) approach.Index Terms-Adaptive protection, centralized protection, smart dc microgrids. I. INTRODUCTIONDue to the increasing penetration of DGs, especially in the form of renewable energy systems (RES), the concept of microgrids has been proposed as a method for DG integration into the electrical grids. Microgrid is a common concept in both ac and dc systems and is defined as a smallscale low or medium voltage grid consisting of loads and DGs. Such a system is capable of operating in both islanded and grid-connected modes [1]. Because of the advantages of the dc networks over the ac grids, and also because of the new developments in the technology of voltage source converters (VSCs), nowadays there is a major interest in dc grids in both research and industrial realms [2][3][4][5].At the present moment, protection is one of the most important challenges in the development of dc microgrids. Protection issues mainly arise due to the particular behavior of the fault current in VSC-based networks [6]. When a fault occurs in a dc grid, firstly, the dc-link capacitor is discharged causing the voltage of the main dc bus to drop precipitously. Then, the energy stored in the cable This work was supported in part by the Spanish Ministry of Economy and Competitiveness under Project ENE2013-48428-C2-2-R. The work of M. Monadi was supported by the Ministry of Science, Research, and Technology, Iran.M. Monadi is with Technical University of Catalonia (UPC) Barcelona, Spain and Shahid Chamran University of Ahvaz, Ahvaz, Iran (e-mail: meh_monadi@yahoo.com).C. Gavriluta is with the Grenoble Electrical Engineering Laboratory (G2ELab), France (email: catalin.gavriluta@g2elab.grenoble-inp.fr).A. Luna, J. I. Candela are with Technical University of Catalonia (UPC) Barcelona, Spain. (e-mails: luna@ee.upc.edu, candela@ee.upc.edu) P. Rodriguez is with Technical University of Catalonia (UPC) Barcelona, Spain and Abengoa research, Sevilla, Spain (e-mail: prodriguez@ee.upc.edu).inductance is also discharged through the freewheeling dio...

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