A Frequency-Based Synchronization Approach for the VSC-HVDC Station Connected to a Weak AC Grid

This paper proposes a frequency-based synchronization control (FSC) approach for the voltage-sourced converter in rectification and inversion modes of an HVDC station that is connected to a weak ac grid. Similar to the conventional vector current control (VCC) approach, FSC includes a conventional phase-locked loop (PLL), and its outer loop controller generates the current reference for the inner loop. Different from VCC, the inner loop of FSC utilizes a supplementary frequency-based control loop to provide damping for the oscillations due to the PLL and generates the angle/magnitude for the converter ac-side voltage. The inner loop of FSC switches to VCC to limit over-current during fault scenario. This paper develops a linearized dynamic model of VSC-HVDC station, analytically evaluates behavior of the FSC dynamics, and validates the results based on time-domain simulation studies in the PSCAD platform. The eigenanalysis and simulation results show that FSC can effectively mitigate the instability issues of the PLL even under very weak ac grid condition.