Improvement in the Synchronization Process of the Voltage-Sourced Converters Connected to the Grid by PLL in Order to Detect and Block the Double Frequency Disturbance Term Hamidreza Mahdian 1∗ , Mohamadreza Hashemi 2 and Ali Asghar Ghadimi 3 1 Department of Electrical Engineering, Science and Research Branch, Islamic Azad University, Arak-Iran; Hamidreza_mahdian@yahoo.com 2 Department of Electrical Engineering, Science and Research Branch, Islamic Azad University, Arak-Iran; Mohamadreza_hashemi@yahoo.com 3 Department of Electrical Engineering, Faculty of Engineering, Arak University, Arak-Iran; a-ghadimi@araku.ac.ir Abstract Phase-Locked Loop (PLL) Structures have many applications in power system engineering. When required the synchro- nous performance of Voltage Sourced Converter (VSC) with Grid, such as wind power generation system, active filters, HVDC transmission lines and FACTS devices, the most common method for accurate estimation of frequency of network, is to use a PLL. However, problems such as grid disturbances due to noise, voltage unbalances, lack of coordination, changes in grid frequency and harmonics can produce a frequency disturbance, twice the grid frequency and disturbed PLL and also the control system connected to it. In this paper, common structures of PLL is introduced. After that, unbalance and disturbance in the three phase input of PLL is studied. Then for idea and invention in PLL, a new control method based on predictive control technique is used in the control strategy of converter. Furthermore, the use of type III controller in PLL to ensure optimal performance in various conditions is proposed in this paper. How to set the parameters of the proposed controller is described by the Bode stability criterion. Conducting simulations to evaluate the performance of the proposed method is done in PSIM software. According to the proposed method and the result observed, the double frequency distur- bance term is weakened and also the transient state is improved. * Corresponding author: Hamidreza Mahdian (Hamidreza_mahdian@yahoo.com) Indian Journal of Science and Technology Keywords: Phase-Locked Loop (PLL), Voltage-Sourced Converters (VSC), Synchronization, Current Control, Unbalanced Voltage. 1. Introduction In order to connect to AC network and inject power by power electronic converters, we require knowing the precise phase angle of voltage. It means that the injected voltage must be in phase with the grid voltage. So it must be synchronized when connected the voltage sourced con- verter to grid. Here, kind of VSC is grid imposed frequency VSC system. PLL is the mechanism to ensure that the con- verter is synchronized with grid [1]. So existence of PLL in the converter control system is mandatory. Current techniques that estimate the frequency are sampled voltage and current. If we consider pure sinusoidal voltage system, by measuring the time between zero cross- ings of the signal, the frequency can be easily calculated. Furthermore, the wide spread use of power electronics devices in the power generation, transmission, distribution and utilization of electrical energy, has led to change the actual shape of the voltage wave form. Consequently, in prac- tice given signal from power system is associated with noise and distortion, so we need to be able to use more advanced techniques that frequency can be calculated with reasonable