MMC-UPFC And Its Treatment Of Unbalanced Grid Voltage Conditions

Unified power flow controller(UPFC)plays an important role in the rapid development of strong smart grid and energy internet.It is the 3rd generation device of flexible AC transmission technology devices(FACTS).It helps to promote the development of modern power grid to be more energy-saving and environment-friendly,and to tap the potential of the existing power grid.On the other hand,some problems in modern power grids have become increasingly prominent.For example,there are still unbalanced power generation and load distribution,unbalanced power flow distribution across transmission lines,unbalanced three-phase voltage in heavy load regions,etc.They all urgently need to be solved.A passive-sliding mode variable structure control strategy for modular multilevel converter(MMC)based UPFC is proposed in this paper.The control strategy is based on the theory of passive control and sliding mode control.The dynamic model of UPFC series-side and parallel-side converters and the corresponding passive sliding mode variable structure controllers are established as well.On this basis,the mathematical model of MMC circulation and the corresponding circulation suppression controller are established.The MMC-UPFC controller is designed afterwards.The 27-level MMC-UPFC simulation model is built in MATLAB/Simulink.The simulation results verify the effectiveness of the proposed control strategy in MMC-UPFC four different operating modes.Three-phase unbalanced voltage treatment is proposed in this paper based on the above control strategy of the MMC-UPFC.On one hand,the negative sequence current component is injected by the parallel-side converter.On the other hand,negative sequence voltage component is compensated by the series-side converter to the line to treat unbalanced voltage.The mathematical model of MMC-UPFC in the positive-and negative-sequence system is established.The three-phase unbalanced controller of series and parallel converter is designed.Two unbalanced grid scenarios of asymmetrical lines parameters and unbalanced receiving side voltage are set for simulation verification.The simulation results show that the proposed three-phase unbalanced treatment scheme can achieve three-phase balance of the power grid.