Research On Improving Power Quality Control Strategy Of Microgrid Using Photovoltaic-Hybrid Energy Storage-Based UPQC

In order to optimize the existing energy source structure and realize the sustainable utilization of energy,microgrid has advanced rapidly in recent years.A series of crucial technologies in connection to microgrid have been extensively investigated.Among them,power quality control strategy is one of the crucial technologies.Since the power quality of the microgrid is not only correlated with the operating characteristics of distributed power sources,energy storage devices,and various loads,but also influenced by grid faults,a novel control and energy management strategy is proposed in this paper for unified power quality conditioner(UPQC)based on photovoltaic(PV)and hybrid energy storage system(HESS).The proposed scheme can not only comprehensively deal with the power quality issues,but also balance power quickly in microgrid.The details are as follows:(1)The tracking control strategy of UPQC with the dual functions of solving power quality problems and photovoltaic power generation is studied.Firstly,the sliding mode SVPWM control strategy is proposed to improve the compensation performance of the UPQC series converter for microgrid voltage under the grid fault conditions,especially under the asymmetric fault conditions.Secondly,the multiobjective control strategy for controlling the UPQC parallel converter in the gridconnected mode is studied,which solves the internal power quality issues of the microgrid and implements the photovoltaic generation.In addition,the U/f control strategy for controlling the parallel converter in island mode is investigated,which supplied voltage and frequency supports for the loads on the AC bus.Finally,the islanding phase-locking technology and pre-synchronization control strategy are studied to ensure the smooth switching of the microgrid operation mode.(2)Sliding mode droop control strategy is proposed for HESS based on battery energy storage system(BESS)and superconducting magnetic energy storage(SMES).According to the operating characteristics of BESS and SMES,SMES is utilized as a primary energy storage device,using sliding mode control method to maintain a constant DC bus voltage for prioritizing the suppression of power fluctuations in the microgrid.While BESS is used as backup energy storage for SMES,the input/output power of BESS is determined according to the state of charge(SOC)of SMES using droop control,and then the sliding mode method is utilized to control the charge/discharge operation of the BESS,so that the SOC of SMES is always within the limit.Meanwhile,the charge/discharge times and its own discharge rate of BESS are greatly decreased,extending its service life.(3)An energy management scheme is proposed for PV-HESS-based system.In the grid-connected and island operation modes of microgrid,the energy management scheme based on the SOCs of BESS and SMES is able to flexibly and effectively manage the power flows in the various parts of the microgrid,ultimately to implement the power balance quickly in microgrid.(4)In Matlab/Simulink environment,the proposed control and energy management strategies for PV-HESS-based UPQC are simulated and analyzed.Simulation results validate the feasibility and effectiveness of the control and energy management strategy proposed in this paper.