Research On Phase-Locked Method And Control Strategy For Dynamic Voltage Restorer

As the proliferation of voltage-sensitive equipments used in industrial processes, voltage sags has become one of the most important issues in power quality. Dynamic Voltage Restorer (DVR) is referred to as the most effective solution to eliminate voltage sags, voltage harmonic and voltage unbalance. The phase-locked section and control section take an important role in the control flow of the DVR, so this thesis lays a strong emphasis on the study of the two sections.This thesis first introduces the principle of operation and configuration of the DVR, and then presents the basic system topologies for DVR and compares them with respect to rating in power and voltage. The analysis and comparison among several inverters used in DVR about performance, cost and difficulty degree of realization is also given.The phase locked technique as a common technique for many electrical devices is investigated and summerized, which includes open loop and closed loop phase locked technique. The Adaptive Software Phase Locked Loop (SPLL) and three other SPLLs based on synchronous reference frame are analyzed in particular. Finally, simulation and experiment is conducted to confirm the analysis and the decoupled double synchronous reference frame SPLL is used in DVR in this thesis.The equivalent circuit and math model of the DVR is established. Because of the poor performance of the feed-forward control strategy and the disadanvantages of the conventional compound control strategy, a double-loop compound control strategy based on outer load voltage loop ,inner filter current loop and DC bus voltage feedforward control is adopted. Qualitative comparison between the two control strategies in terms of stability margins, frequency response and damping performance is given to confirm that the compound control strategy has better stability, dynamic response and load adaptability. The controller parameters based on compound control strategy are designed according to root locus method. Furthermore, the effectiveness of the compound control strategy is validated by simulation studies.In this thesis, a 3KVA supply voltage simulator and a 3 KVA DVR experimental system is established. The inverter control results, the SPLL results, the supply voltage simulator results and the DVR results when supply voltage has balanced sags, unbalanced sags and harmonics are given. The experimental results and simulation results show great consistency. They verify the effectiveness of the system design.