Voltage Stability and Control of Offshore Wind Farms With AC Collection and HVDC Transmission

This paper investigates the stability and control of offshore wind farms employing medium-voltage ac collection and high-voltage dc (HVDC) transmission to the onshore power grids. Type-IV (full power conversion) turbines and HVDC rectifier based on voltage-source converters are assumed. Output impedance models of the wind turbines and input impedance models of the HVDC rectifier in the positive-and negative-sequence are developed using the harmonic linearization method. An impedance-based stability criterion is then applied to determine the stability of the offshore ac collection bus. Possible instability of the ac bus voltage and resonance between the wind farm and the HVDC rectifier are examined through analysis of the system impedance model. The analytical impedance models are used to identify the root causes of such instability and resonance problems, and to develop possible solutions. Detailed circuit simulation is used to validate the analysis. Individual converter impedance models are also validated by experimental measurements of scaled-down prototypes.