Abstract—Grid synchronization is one of the most important issues of the distributed power generation system (DPGS) connected to utility network. This paper proposes a novel phase-locked loop (PLL) for synchronization of power electronic converters in unbalanced and variable-frequency environments. The method utilizes double rotating reference frames to transform the input signal and detects the positive fundamental component and negative sequence component simultaneously. It’s based on a synthesis circuit which generates the orthogonal and in-phase signals to decouple input signal. This method detects the phase without distortion even in variable-frequency environment comparing with existing synchronization methods. Structural simplicity of the method greatly simplifies its implementation in digital software and/or hardware environments. This paper gives a description and derivation of the method. Its excellent performance is verified by simulation. Index Terms—Distributed power generation system (DPGS), phase-locked loop (PLL), positive sequence component detection, synchronous reference frame (SRF). I. INTRODUCTION Distributed power generation system (DPGS) based on renewable energy sources experiences a large development worldwide, with Germany, Denmark, Japan, and USA as leaders in the development in this field [1]. Due to the increase number of DPGS connected to utility network, instability of these systems and of the grid itself can occur. Consequently, new and more stringent standards are issued in respect to power distribution. One of the important issues of the DPGS connected to the utility network is the synchronization with the grid voltage vector [2]. Specifically, the detection of the positive-sequence voltage component at fundamental frequency is essential for the control of distributed generation and storage systems. The magnitude and angle of the positive-sequence voltage is used for the synchronization of the converter output variables, power flux calculations, or for the transformation of the state variables into rotating reference. Regardless of the technique used in the system detection, the amplitude and the phase of the positive-sequence component must be fast and accurately obtained, even if the utility voltage is distorted and unbalanced. This work was supported by National Science Foundation of China No. 50707025. The use of a phase-locked loop (PLL) is a conventional technique to make the synchronization system frequency-adaptive. There are some methods based on PLL technology to synchronize the positive sequence component of the utility voltage under unbalanced condition. Enhanced phase-locked loop (EPLL) presented in [3] allows independent frequency- adaptive synchronization with each phase-voltage of the three-phase system without using synchronous reference frames. Although the EPLL based on positive-sequence detector constitutes a ingenious solution for grid synchronization in unbalanced three-phase systems, but it needs four EPLL modules including their respective 90- degree shifted versions which make it complicate. Another method based on decoupled double synchronous reference frame phase-locked loop (DSRF-PLL) [4], which completely eliminates the detection errors of conventional synchronous reference frame PLL’s. The DSRF-PLL defines an unbalanced voltage vector, consisting of both positive- and negative-sequence components, and expresses it on the double synchronous reference frame. It enables a fast and accurate phase and amplitude detection of the utility voltage positive- sequence component under unbalanced utility condition. Other methods [5-6] detect the positive-sequence component based on the second order generalized integrator (SOGI). Two SOGIs are used for filtering and obtaining the 90-degree shifted signals. This method has very good harmonics rejection capability, but needs additional circuit to make the system frequency-adaptive. This paper presents an alternative detection method in unbalanced power network. It utilizes the double synchronous reference frame method similar to DSRF- PLL, but based on a synthesis circuit. It’s frequency- adaptive without using additional circuit. A proper decoupling system gives rise to a new technique that can accurately obtain the positive and negative sequence components simultaneously under unbalanced utility conditions. II. THE UNBALANCED VOLTAGE VECTOR ON CONVENTIONAL PLL In the conventional PLL, three-phase voltage vector is translated from the abc natural reference frame to the αβ stationary reference frame by using Clarke’s transformation, and then translated to dq rotating frame by Park’s transformation, as shown in Fig. 1. The angular A Novel PLL for Grid Synchronization of Power Electronic Converters in Unbalanced and Variable-Frequency Environment FANG Xiong, WANG Yue, LI Ming, WANG Ke and LEI Wanjun Xi’an Jiaotong University, Xi’an, P.R. China 2010 2nd IEEE International Symposium on Power Electronics for Distributed Generation Systems 978-1-4244-5670-3/10/$26.00 ©2010 IEEE 466