Study On The Impact Of D-PMSG Access On Distribution Network Protection Under Asymmetric Faults

The development of clean energy has received more and more attention with the introduction of the "double carbon" target,and the development of clean energy generation methods represented by wind power is developing very rapidly.As the installed capacity of wind farms continues to increase,more and more Direct-driven Wind Generation System(DPMSG)wind turbines are being connected to traditional single-supply distribution grids,leading to a shift towards active distribution grids.However,due to the differences in the fault output characteristics and grid connection methods of D-PMSGs compared to AC synchronous motors,the traditional fault location analysis theory and methods based on AC synchronous motor power supply are not able to meet the existing distribution network protection requirements.Therefore,in distribution networks with high penetration of D-PMSGs,the impact of the output characteristics of D-PMSGs on the distribution network protection under asymmetric faults needs to be fully considered,which is of great significance for the safe and stable operation of the distribution network.In this paper,the modelling and grid-connection control strategy of direct-drive permanent magnet wind turbines,the control strategy of the grid-side converter under asymmetric fault conditions and the fault location after connection to the distribution network are studied in depth,while simulation models are built using the Matlab/Simulink simulation software platform for verification.The main work is as follows:(1)This paper presents a mathematical modelling of the wind turbine,the D-PMSG,the machine side and the grid-side converter of the D-PMSG,and investigates the control strategy of the machine side and grid-side converter of the D-PMSG.(2)This paper focuses on the control strategy of the network-side converter of a D-PMSG under asymmetric faults in the distribution network,using a double current loop control strategy for positive and negative sequence in the dq coordinate system.The aim is to suppress the negative sequence component of the output current of the D-PMSG under asymmetric faults so that only the positive sequence current component is present in the output current under voltage asymmetry.A low voltage ride through(LVRT)control strategy for direct-drive permanent magnet wind turbines in the absence of faults is investigated to meet the requirements for grid connection in the event of a voltage dip.The simulation software is used to build a model of the D-PMSG with the above control strategy and the simulation results show the feasibility of the control strategy proposed in this paper.(3)The fault output characteristics of D-PMSGs are significantly different from those of conventional AC power supplies,while the fault types in the distribution network are diverse and the fault characteristics are complex and variable,so existing fault location methods have limitations.Based on the analysis of the impact of D-PMSG access to the distribution network on conventional relay protection devices and fault location,a fault location method is proposed for D-PMSG access to the distribution network under asymmetric faults,based on the output characteristics of D-PMSG as a positive sequence voltage controlled current source.A simulation model of a typical 10.5 k V distribution network with D-PMSG access is constructed by means of simulation software,and the simulation analysis verifies the effectiveness of the location method for different short-circuit faults and different fault locations.