Research On Control Strategy To Improve The Power Quality Of Micro-Grid Based On UPQC

In order to cope with the crisis of energy and optimize energy structure, the micro-grid has obtained a rapid development in recent years. A series of key technologies associated with micro-grid has become a hot research, power quality analysis and control is one of the key technologies. Due to the particularity of micro-grid system itself, its power quality is not only connected with the operation characteristics of internal distributed power, storage device and load, but also has interactive effect of external distribution network. According to the feature of micro-grid power quality, Unified Power Quality Conditioner (UPQC) is used to comprehensive control ofmicro-grid power quality, the detection and control strategy is researched to improve micro-grid power quality based on UPQC in this paper.Firstly, the basic structure of micro-grid system based UPQC is introduced and the topology structure of UPQC hybrid energy storage unit is studied, the mathematical models of UPQC series unit, parallel unit and hybrid energy storage unit are established respectively, the control principle and goal of UPQC to improve micro-grid power quality are clarified furtherly.Secondly, the detection method of instruction voltage and current signals for UPQC output is researched. It is difficult to apply traditional phase-locked loop when distribution network voltage has unbalance and distortion, the two detection methods of instruction voltage based on Park transformation and resonant control without phase-locked loop are researched in this paper. According to the inherent time delay of digital system, the detection method of instruction current based on adaptive prediction is studied. Through the improvement of matrix C, a detection method of instruction current for the graded compensation is proposed, it is suitable for three-phase unbalanced load that contain any harmonic component. The direct and indirect prediction methods of instruction current for full compensation are compared from the two aspects of steady-state accuracy and dynamic response speed respectively. For the power fluctuations of micro-grid caused by intermittent power, the influence mechanism of PCC voltage quality by the power fluctuation is studied. The simulation results show that the detection method of instruction signal is effective in the Matlab/Simulink environment.Finally, the multi-objective control strategy to improve the power quality of micro-grid based on UPQC is researched. In order to improve the compensation effect of UPQC series unit when distribution network has short voltage surge or deep voltage drop, the voltage tracking control strategy based on adaptive triangular carrier is studied, the capacitors on the DC bus is equalizing controlled by UPQC series unit at the same time. In the master-slave control structure of micro-grid system, in order to reduce the transient impact of voltage and current when the operation mode of micro-grid is changed, the smooth transition control strategy and grid synchronization control strategy of UPQC parallel unit which has the dual function of main power supply and power quality control are researched. The compensation strategy of instruction current value is added in the same current loop which uses incremental PI regulator, so the continuity of instruction current value is ensured when the operation mode of micro-grid is changed. The grid synchronization control strategy is proposed which adjusts the reference phase for the target and the smooth switching of micro-grid is realized. To stabilize the micro-grid power fluctuations and improve the quality of the PCC voltage, the double loop control strategy of two bidirectional DC/DC converters in the hybrid energy storage unit is studied. According to the different characteristics of super capacitor and battery, an energy management method of hybrid energy storage unit is designed.It can not only exert respective advantage of the energy storage components but also control the operation of energy storage components in the safe range. So the service life of the storage elements are extended and the stabilize effect is improved.In order to verify the effectiveness of the proposed control strategy in this paper, the simulation model is builded in Matlab/Simulink environment. By setting different conditions, the simulation research has been carried on from four aspects of improving voltage quality, current quality, stabilizing power fluctuations and achieving smooth transition of micro-grid respectively. The simulation results show that the proposed multi-objective control strategy in this paper to improve power quality of micro-grid based on UPQC is effective and feasible.