Abstract
This paper addresses the problem of providing frequency control services, including inertia emulation and primary frequency control, from offshore wind farms connected through a multiterminal HVDC network. The proposed strategy consists of a cascading control mechanism based on dc voltage regulation at the onshore converters and frequency regulation at the offshore converters. The control scheme involves only local measurements and actions avoiding security and reliability issues of control structures based on remote information. The effectiveness of the proposed strategy is illustrated in a test system that consists of two nonsynchronous areas linked by a multiterminal HVDC grid where two offshore wind farms are also connected.

Abstract
The massive interconnection of offshore Wind Farms (WF) brings challenges for the operation of electric grids. The predicted amount of offshore wind power will lead to a smaller ratio of conventional units operating in the system. Thus, the power system will have less capability to provide fast dynamic regulation. Despite of offshore WF being able to inject power on the AC grid through High Voltage Direct Current (HVDC) convertors, they cannot participate on frequency support by the intrinsic decoupling that DC adoption brings. This paper proposes a control methodology, based on local controllers, to enable the participation of offshore WF in primary frequency control. Additionally, enhancements were made on the Wind Energy Converters (WEC) controller to make them capable of emulating inertial behaviour. Tests were performed in a multi-terminal DC network with two off shore wind farms to assess the feasibility and effectiveness of the concept in a communication-free framework.