Comparison of Communication-based and Coordinated-based Frequency Control Schemes for HVDC-Connected Offshore Wind Farms

As the penetration of wind generation increases, the frequency support from wind turbines is necessary. There are many advantages for the connection between offshore wind farms and electricity grid via a high-voltage direct current (HVDC) system. HVDC transmission systems help wind farms provide frequency regulation and voltage control, which have more flexibility compared to HVAC transmission systems. Several control methods for frequency support have been developed. However, few works compared various control methods for a HVDC-connected offshore wind farm.This work proposes a detailed comparison on the main frequency-support control methods for the HVDC-connected offshore wind farm. These controls are mainly based on the communication-based schemes and the coordinated-based methods. The communication-based scheme uses communication devices to transfer the information of grid frequency to an offshore wind farm directly. The coordinated-based methods transfer the dynamic information of the DC voltage on HVDC via generator emulation control (GEC), and then enable the voltage source converter (VSC) at the wind-farm side to supply frequency regulation. In this work, the synthetic inertial control of wind turbines is considered, which include inertia and droop controllers; furthermore, the capacitors of the HVDC system can release the stored energy to help wind farms for the frequency-regulation function.