Ratio-based selective inertial and primary frequency support through MTDC grids with offshore wind farms

Provision of inertial and primary frequency support in a controlled manner through multiterminal direct current (MTDC) grid connecting asynchronous AC areas and offshore wind farms (OWFs) is explored. Under nominal condition, only voltage droop is considered to allow the AC systems operate asynchronously as they are meant to. Following an AC-side disturbance, the corresponding converter transmits basic information via a distress signal through the DC lines or existing communication channels used for common DC voltage-droop control. Upon receiving the distress signal, a new decentralized inertial-droop-based controller and the traditional decentralized frequency-droop controller are activated in preselected participating converters with values that are predetermined based on our proposed design procedure. The procedure ensures a new ratio-based performance while providing frequency support. The proposed scheme is based on an Nth-order model of N-asynchronous-area MTDC system. Furthermore, to extract frequency support from an OWF connected to an AC-MTDC system, its power reference is modified such that the whole of the AC-MTDC system with OWF is emulated as an N-asynchronous-area MTDC system. The proposed strategies are first implemented in the Nth-order model for theoretical validation and later, in full-order models of study systems (with and without OWF) for a more rigorous verification.