Inertial Support from Offshore Wind Farms Interfaced through MTDC Grids

Extracting inertial frequency support from an offshore wind farm (OWF), which is connected to asynchronous AC areas via a Multiterminal Direct Current (MTDC) grid is studied. The OWF controls are modified to emulate the entire AC-MTDC system with OWF as N-asynchronous-area MTDC system. When an AC-side disturbance occurs, a distress signal is transmitted from the corresponding converter. The distress signal carries basic information and is transmitted through the DC lines or existing communication channels, which are used for common DC voltage-droop control. After the distress signal is received, inertial and frequency-droop controllers are activated in converters of the participating asynchronous AC areas with values that are determined ahead of time by means of a proposed design process. This ensures that apart from the inertial support provided by the OWF operating in maximum power point tracking mode, all participating asynchronous AC areas also exchange both inertial and primary frequency support in a prescribed manner, i.e, obeying a ratio-based criterion. The proposed strategies are validated through rigorous nonlinear time-domain simulation studies using a detailed model of the study system.