Research On Control Strategy Of MMC-UPFC Under Unbalanced Transmission Line Conditions

With the construction of global energy interconnection and the expansion of power consumption scale,problems such as the relatively weak structure and unreasonable layout of the existing power system become increasingly evident.As a series-parallel integrated compensation device with excellent performance,unified power flow controller(UPFC)can flexibly adjust line power flow and has broad prospects for development.Nowadays,with the increase of asymmetric load and the influence of line coupling,the three-phase unbalance of power grid becomes more and more common.Therefore it is vital to research the control strategy of UPFC under unbalanced conditions.On the basis of carefully studying the domestic and foreign references,the working modes of UPFC series and parallel side are analyzed respectively in this thesis.It is explained that the power flow regulation of UPFC is realized by controlling the magnitude and phase of output voltage in series side.The common topologies of the UPFC converter are compared,and the advancement of the MMC structure is described.The power injection model of UPFC,the transient model and the cycle by cycle switch average model of MMC are established.Based on the mathematical model,a double-loop decoupling device level controller for UPFC series and parallel sides is designed.Then two kinds of MMC modulation algorithms are introduced,namely carrier phase-shifted SPWM and nearest level modulation(NLM).The instantaneous power components of MMC upper and lower arm are analyzed.Based on NLM,an improved method considering sub-module voltage estimation is proposed,which can effectively suppress the harmonic circulating current of MMC.In order to further suppress the internal circulating current,a suppressor is also designed.In view of the three-phase unbalanced conditions of power grid,the device level controller of UPFC is improved.On the basis of the original control structure,the parallel side is controlled by the positive sequence component.The positive and negative sequence controllers and simplified controllers are designed in the series side so that the UPFC can operate stably when the system is unbalanced,and it has the function of restraining the negative sequence current of the grid.The extraction method of positive and negative sequence of voltage and current is studied.Second-order generalized integrator(SOGI)is introduced to separate positive and negative sequence components.Compared with the commonly used T/4 delay method,the SOGI method has higher separation accuracy and can suppress harmonics.Then the integrated control structure diagram of UPFC series side under three-phase unbalanced conditions is given.The operating range of UPFC series side under steady and unbalanced conditions is analyzed,and the relevant constraint conditions are given.The simulation model of the main circuit and control system is built by using MATLAB/Simulink.The simulation experiments are carried out under three-phase balanced and unbalanced conditions respectively.The effectiveness of the proposed control strategy is verified.