Research On Controll And Application In Microgrid Imbalance Suppression Of Improved Unified Power Flow Controller

In the face of increasingly prominent global environmental problems,China has put forward the goal of "carbon neutral" and "carbon peak",and a large number of distributed new energy is connected to the public grid through the micro grid.However,the controllability of new energy generation power is poor,which requires specific equipment to control and compensate its power exchange with grid.The different length of three-phase feeders and unbalanced load also lead to the imbalance of voltage and current in the microgrid.The Unified Power Flow Controller(UPFC)can meet the requirement of the microgrid power flow controllability,but the UPFC with large capacity covers a wide area and costs a lot.In view of the problems above,this article takes UPFC as the research object and combines it with Sen transformer to obtain an improved UPFC topology structure which can reduce the capacity of the converter,designs multi-function control methods such as power flow control and voltage control of the improved UPFC,and realizes three-phase imbalance suppression of voltage and current in microgrid.Firstly,the basic working principle of the traditional UPFC is analyzed,and the power flow compensation and control mechanism of the shunt and series side are studied respectively.The positive and negative sequence models of the series and shunt sides in dq coordinate system are obtained by mathematical modeling,which provides a basis for the design of control methods and strategies.The power flow model of dual terminal system considering UPFC access is derived,and the influence of UPFC on power flow distribution is estimated.In order to reduce the cost of converters in traditional UPFC with high capacity,the two-way improved Sen transformer(TIST)is further promoted on the basis of improved Sen transformer and combined with UPFC to form the improved unified power flow controller(IUPFC)topology.The control methods of the TIST taps,series side and shunt side are designed,and the multi-function coordinated control of IUPFC series side is realized.In order to avoid the mutation of voltage and current caused by switching of the TIST taps and multi-functional switching of the series side,the soft switching method is designed.A semi physical experiment platform based on RT-Box is built,and DSP control codes is programed to conduct the semi-physical experiment.The simulation and experimental results show that the IUPFC structure can effectively reduce the series side converter capacity,and the designed control and soft switching methods can achieve multiple control functions with smooth switching.Aiming at the three-phase imbalance of voltage and current in microgrid,a fast and accurate extraction method of positive and negative sequence components is studied.The synthesis method of negative sequence voltage by TIST is proposed.The shunt and series side of IUPFC are used to compensate the negative sequence component of the grid current and voltage respectively,so as to achieve imbalance suppression.The simulation and experimental results show that the designed IUPFC three-phase imbalance suppression strategy can effectively reduce the ratio of threephase imbalance of voltage and current and improve the power quality of the connected grid.