Research On Fault Ride Through Strategy Of Unified Power Flow Controller And Its Coordination With Protection

Unified power flow controller(UPFC)can realize flexible and fast adjustment of transmission line parameters and precise and real-time control of system power flow.,which has important research significance.This article takes UPFC’s fault ride through(FRT)strategy and its cooperation with line protection as the core.It focuses on the principles and functions of UPFC,the UPFC FRT strategy,the impact and countermeasures when UPFC equipped in the transmission line on the fault direction discriminating element,and the impact and countermeasures when UPFC equipped in the transmission line on distance protection.First of all,this article introduces the overall structure and main functions of UPFC,gives the switching states of the modular multilevel converter(MMC)submodules,introduces the control methods of UPFC series and shunt side converters,and lays the theoretical foundation for UPFC FRT strategy and its cooperation with line protection.Secondly,based on the 500kV UPFC demonstration project in south of Jiangsu,a UPFC simulation model was built in PSCAD/EMTDC to analyze the fault characteristics of the short circuit when UPFC equipped in the line.Aiming at the fault mechanis of UPFC shunt side blocking after a fault,two FRT strategies are proposed.One is to insert a current limiting reactor(CLR)on the DC bus,and the other is to express the physical characteristics of the CLR by mathematical formulas,and improve the outer voltage loop control strategy of the shunt side MMC.Simulation results built in PSCAD/EMTDC verify the feasibility of the two methods.Then,the influence of UPFC on the direction discriminating element of the positive sequence fault component(PSFC)is discussed.Aiming at the phenomenon that the direction discriminating element may malfunction when the fault is in the reverse direction,a fault direction discriminating element for UPFC lines based on fuzzy logic is proposed.Obtain the PSFC impedance angle of the protection installation in the event of a fault.Combine the division of the action area of the fault direction discriminating element with fuzzy logic,construct a comprehensive membership function of the angle of the PSFC impedance and judge its relationship with the threshold.So that the fault direction can be judged correctly.Finally,analyze the impact on distance protection when different types of faults occur on the line equipped with UPFC,the expression of the measured impedance at the protection installation is derived.And the impact when UPFC equipped in the line is analyzed.Two countermeasures are proposed,one is to close the high voltage breaker(HVB)after the fault,the series side of UPFC is out of operation,which can convert UPFC to Static Synchronous Compensator(STATCOM).The other is to change the position of the voltage transformer and connect the voltage transformer to the right outlet of the UPFC series transformer.Simulation results built in PSCAD/EMTDC verify the feasibility of the two methods.