Abstract—Synchronization is a key issue in distributed power generation and storage (DPGS), fast and accurate estimation of the grid voltage phase angle being requested. Synchronization methods can be roughly divided among a) zero-crossing-methods, b) Phase-Locked-Loop (PLL) and c) Frequency-Locked-Loop (FLL) methods, with PLL implemented in a synchronous reference frame (SRF-PLL) likely being the most popular choice. A drawback of these methods is that, in general, they do not implement harmonic rejection strategies, their performance in polluted networks being therefore compromised; also their implementation and tuning is not straight forward, often being made by trial and error. This paper proposes a pre-filtering based synchronization method using cascaded complex-coefficient-filters (CCCF) and a complex PLL. The CCCF allows selection of the harmonic isolated/rejected, while the CPLL obtains the phase of the positive sequence component of the fundamental voltage needed for synchronization. The proposed method provides high rejection/adaptation capabilities against unbalances, frequency drift and harmonic distortion of the line voltages. Index Terms — Grid synchronization, complex PLL, complex coefficients filters, harmonic rejection. 1 I. INTRODUCTION UE to the growing interest in renewable energy resources, the traditional generation paradigms have changed [1]. New generation systems include large centralized power plants that coexist with small distributed and decentralized generation spots, located as close as possible to the consumers [1], thus reducing the transmission losses as well as the size and number of power lines needed, the resulting distributed system being referred as distributed power generation (DPG) [2]. To deal with DPG and distributed power storage (DPS) issues, the concept of microgrid was introduced in [3]. While this new concept can potentially increase the efficiency, bring economic improvements and emissions reduction [4, 5], new concerns arise related to power quality communication, dynamic behavior, stability, control, protection and secure operation, [6]. Operation of a microgrid requires knowledge of the magnitude and phase of the positive sequence component of the fundamental voltage, which is needed for grid synchronization as well as for operation -power factor correction, power flow calculation, islanding detection…. . This work was supported in part by the Research, Technological Development and Innovation Programs of the Spanish Ministry of Science and Innovation- ERDF under grant MICINN-10-ENE2010-14941 and the Ministry of Science and Innovation under grant MICINN-10-CSD2009-00046, and by the Personnel Research Training Program funded by the Regional Ministry of Education and Science of the Principality of Asturias under grant BP11-107 Development of a fast, precise and robust synchronization method is therefore a critical issue. Synchronization methods can be roughly divided into a) zero-crossings based methods [12], b) frequency-locked-loop based methods (FLL) [14, 15] and c) Phase-Locked-Loop (PLL) based methods [16-21], they are briefly discussed following. a) The zero-crossing based method. Likely the simplest option to track the fundamental frequency of the grid voltage [12], they detect the zero crossings of this signal. However, the estimated frequency can only be updated each half period of fundamental voltage, also the reliability of the method being compromised under distorted conditions of the line voltage or when the measured signals are contaminated with noise [13]. b) FLL based methods. These methods track the frequency of the fundamental voltage, option for doing this are: • Dual Second Order Generalized Integrator using frequency-locked-loop (DSOGI-FLL) [14] - This method separates the negative and positive sequence components using two quadrature-signal-generators (QSG) that act as a band-pass filter. The frequency of the input voltage is obtained from the relationship between the quadrature signal of the fundamental-QSG and the signal error between fundamental-QSG input and output. • Multiple SOGI-FLL (MSOGI-FLL) [15] - The idea behind the MSOGI-FLL is to combine a set of SOGI- QSGs tuned at different frequencies, to remove specific harmonics present in a distorted network. c) PLL based methods. These methods track the phase angle of the fundamental voltage, a variety of implementations can be found in the literature [16-25]. The PLL, or a combination of PLLs, can be implemented using phase (abc) quantities [16] or dq components in stationary [21] or synchronous reference frame [17-20]. Methods that implement a PLL in synchronous reference frame can be further divided into three different categories: • Synchronous reference frame PLL (SRF-PLL) [17] - They transform the abc voltages to stationary dq voltages and use a simple PLL in the q-axis to obtain the grid frequency. The bandwidth of the PI controller looks for a compromise between dynamic response and disturbance rejection (see Fig. 1b). PLL (SRF-PLL) are the base for other methods described below. • Pre-filter stage SRF methods [18, 22] - In order to isolate the positive sequence voltage, these methods insert a pre- filter stage, either in the stationary or synchronous reference frame, before the SRF-PLL. Options of these type of methods include: Cristian Blanco, David Reigosa, Fernando Briz, Juan M. Guerrero and Pablo García University of Oviedo. Dept. of Elect., Computer & System Engineering, Gijón, 33204, Spain blancocristian@uniovi.es, reigosa@isa.uniovi.es, fernando@isa.uniovi.es, guerrero@isa.uniovi.es, pgarcia@isa.uniovi.es Grid Synchronization of Three-Phase Converters Using Cascaded Complex Vector Filter PLL D