3414 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 51, NO. 4, JULY/AUGUST 2015 Grid Voltage Synchronization for Distributed Generation Systems Under Grid Fault Conditions Alvaro Luna, Member, IEEE, Joan Rocabert, Member, IEEE, J. Ignacio Candela, Member, IEEE, Juan Ramón Hermoso, Remus Teodorescu, Fellow, IEEE, Frede Blaabjerg, Fellow, IEEE, and Pedro Rodríguez, Fellow, IEEE Abstract—The actual grid code requirements for the grid connection of distributed generation systems, mainly wind and photovoltaic (PV) systems, are becoming very demanding. The transmission system operators (TSOs) are especially concerned about the low-voltage-ride-through requirements. Solutions based on the installation of STATCOMs and dynamic voltage regulators (DVRs), as well as on advanced control functionalities for the existing power converters of distributed generation plants, have contributed to enhance their response under faulty and distorted scenarios and, hence, to fulfill these requirements. In order to achieve satisfactory results with such systems, it is necessary to count on accurate and fast grid voltage synchronization algo- rithms, which are able to work under unbalanced and distorted conditions. This paper analyzes the synchronization capability of three advanced synchronization systems: the decoupled double synchronous reference frame phase-locked loop (PLL), the dual second order generalized integrator PLL, and the three-phase enhanced PLL, designed to work under such conditions. Although other systems based on frequency-locked loops have also been developed, PLLs have been chosen due to their link with dq0 controllers. In the following, the different algorithms will be pre- sented and discretized, and their performance will be tested in an experimental setup controlled in order to evaluate their accuracy and implementation features. Index Terms—Electric variable measurements, electrical engi- neering, frequency estimation, frequency-locked loops, harmonic analysis, monitoring, synchronization. I. I NTRODUCTION T HE power share of renewable energy-based generation sys- tems is supposed to reach 20% by 2030, where wind and photovoltaic (PV) systems are assumed to be the most outstand- ing examples of integration of such systems in the electrical Manuscript received September 2, 2013; revised February 4, 2014 and April 12, 2014; accepted May 15, 2014. Date of publication January 13, 2015; date of current version July 15, 2015. Paper 2012-SECSC-582.R2, presented at the 2012 IEEE Energy Conversion Congress and Exposition, Raleigh, NC, USA, September 15–20, and approved for publication in the IEEE TRANSACTIONS ON I NDUSTRY APPLICATIONS by the Sustainable Energy Conversion Systems Committee of the IEEE Industry Applications Society. This work was sup- ported by the Spanish Ministry of Economy and Competitiveness under Project ENE2013-48428-C2-2-R. A. Luna, J. Rocabert, J. I. Candela, and J. R. Hermoso are with the De- partment of Electrical Engineering, Technical University of Catalonia, 08222 Barcelona, Spain (e-mail: luna@ee.upc.edu; rocabert@ee.upc.edu; candela@ ee.upc.edu; hermoso@ee.upc.edu). R. Teodorescu and F. Blaabjerg are with the Department of Energy Sys- tems, Aalborg University, 9220 Aalborg East, Denmark (e-mail: ret@et.aau.dk; fbl@et.aau.dk). P. Rodríguez is with the Department of Electrical Engineering, Technical University of Catalonia, 08222 Barcelona, Spain, and also with Abengoa, 41014 Seville, Spain (e-mail: prodriguez@ee.upc.edu; www.abengoa.com). 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/TIA.2015.2391436 network [1]. The increased penetration of these technologies in the electri- cal network has reinforced the already existing concern among the transmission system operators (TSOs) about their influence in the grid stability; as a consequence, the grid connection stan- dards are becoming more and more restrictive for distribution generation systems in all countries [2]–[6]. In the actual grid code requirements (GCRs), special con- straints for the operation of such plants under grid voltage fault conditions have gained a great importance. These requirements determine the fault boundaries among those through which a grid-connected generation system shall remain connected to the network, giving rise to specific voltage profiles that specify the depth and clearance time of the voltage sags that they must withstand. Such requirements are known as low voltage ride through (LVRT) and are described by a voltage versus time characteristic [7]. Although the LVRT requirements in the different standards are very different, as shown in [8], the first issue that generation systems must afford when a voltage sag occurs is the limitation of their transient response, in order to avoid its protective discon- nection from the network. This is the case, for instance, of fixed speed wind turbines based on squirrel cage induction generators, where the voltage drop in the stator windings can conduct the generator to an overspeed tripping, as shown in [9]. Likewise, variable speed wind power systems may lose controllability in the injection of active/reactive power due to the disconnection of the rotor side converter under such conditions [10], [11]. Likewise, PV systems would also be affected by the same lack of current controllability. Solutions based on the development of auxiliary systems, such as STATCOMs and dynamic voltage regulators (DVRs), have played a decisive role in enhancing the fault ride through (FRT) capability of distributed generation systems, as demon- strated in [12]–[16]. Likewise, advanced control functionalities for the power converters have also been proposed [17], [18]. In any case, a fast detection of the fault contributes to improving the effects of these solutions; therefore, the synchronization algorithms are crucial. In certain countries, the TSOs also provide the active/reactive power pattern to be injected into the network during a voltage sag; this is the case for the German E-on [2] and the Spanish Red Eléctrica Española (REE) [3]. This trend has been followed by the rest of the TSOs; moreover, it is believed that this operation requirement will be extended, and specific demands for balanced and unbalanced sags will arise in the following versions of the grid codes worldwide [19]. 0093-9994 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.