Impedance Modeling and Stability Analysis of All-DC Delivered Offshore Wind Farm

The all-DC delivered offshore wind farm has great potential in power engineering application due to its advantages in large capacity and long distance delivery. Considering the cost and reliability, the all-DC wind farm can be connected to the onshore grid via the Line-Commuted Converter High Voltage Direct Current (LCC-HVDC) but more interaction may also occur in the form of sub-/super-synchronous oscillations. In order to analyze the stability issue of such offshore wind farm system, the DC-port impedance are firstly established for the aggregated wind farm based on permanent magnet synchronous generator (PMSG) and LCC-inverter by using the harmonic linearization method, where the power outer loop cascaded by the current inner loop of PMSG wind farm and the phase-lock loop (PLL) of the LCC inverter are taken into consideration. Furthermore, the interaction between these two subsystems is analyzed based on the stability criteria through their impedance characteristics. Finally, simulation results are conducted by reproducing the time-domain oscillation in order to evaluate the effectiveness of impedance model for the all-DC delivered offshore wind farm system.