Stability Analysis of Grid-Strengthening Geared Direct Grid-Connected Dual-Speed Slip-Synchronous Wind Turbines

Abstract

With the recent proliferation in penetration levels of converter-based grid-connected wind turbine systems, a decrease in grid strength has been observed. In pursuit of improving grid strength and power quality, the newly proposed slip-synchronous wind turbine system (SS-WTS) is employed, whereby a synchronous generator (SG) is directly connected to the grid and damping is provided through the use of a slip permanent magnet coupling (S-PMC). However, the proper operation of the turbine drivetrain has yet to be fully investigated under transient loading conditions. This article examines various drivetrain topologies to ensure the optimal placement of the S-PMC for stability and load mitigation. From the stability analysis, various S-PMC slip ranges, SG synchronous reactance values and inertia ratios between the turbine hub and SG rotor were evaluated. This led to more thorough design criteria being established to ensure the proper stability of the SS-WTS under transient conditions, with specific regard to low-voltage ride-throughs (LVRTs). This article found that, although connecting the SS-WTS to the grid is a feasible solution to improve grid strength, instability is observed under certain transient conditions, such as LVRTs. It was noted that direct-drive SS-WTSs are less susceptible to transient instability, as they possess a far larger inertia ratio between the wind turbine and the SG rotor. This article found the optimal slip and synchronous reactance for all SS-WTS variants, where stability under all transient conditions is dependent on the turbine’s rated torque, the turbine’s inertia and the SG inertia. This made it possible to formulate adequate design criteria based on the application and capability needs of the SS-WTS.