Grid Synchronization of Wind Turbines during Severe Symmetrical Faults with Phase Jumps

Abstract

This paper investigates the performance of a converter synchronization unit during severe symmetrical faults with phase jumps. The loss of synchronization of power converters during low-voltage situations is described and restrictive current limits for stable operation are derived. In order to achieve zero-voltage ride-through capability, the phase-locked loop can be frozen during a fault to ensure stability while complying with grid codes. Since the frozen PLL approach is only applicable in the case of constant frequency and phase angle of the grid voltage, this paper investigates the performance of the frozen PLL during phase jumps and reveals whether a proposed phase compensation technique can be utilized to improve the power transfer of the converter during a severe symmetrical fault. This is done through a comprehensive simulation study where the frozen PLL is analyzed with and without phase compensation for different types of line impedance configurations. It is revealed, that even though the proposed phase compensation method can improve the injected power during a fault situation with phase jumps, a non-compensated frozen PLL can inherently ensure stability and having less complex implementation and acceptable injection of currents when compared to state-of-the-art solutions for loss of synchronization. The ride-through capability of the frozen PLL with and without the proposed compensation method is experimentally verified.