The stability range of power transfer in high impedance, weak, grids with Voltage Source Converters (VSC) can be extended a simple modification of the conventional Synchronous Reference Frame Phase Locked Loop (PLL) used for grid synchronization. By introducing an impedance-conditioning term in the voltage used by the PLL, the VSC control system is virtually synchronized to the voltage at a stronger point in the grid. By synchronization to the voltage behind the corresponding virtual impedance, the proposed approach can be utilized to influence the reactive power flow of the VSC in a way that will inherently enhance the power transfer capability of the high-impedance grid. To verify the impact of the proposed approach, an analytical model of a VSC HVDC terminal is used to calculate the achievable steady-state power transfer capability, as well as the range where small-signal stability is ensured, in a grid with low SCR when applying different levels of virtual impedance in the grid synchronization. The validity of the stability assessment based on the analytical model, and the increased stability range achieved with the proposed impedance-conditioned PLL, are also verified by time-domain simulations in the Matlab/Simulink environment.
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