This paper proposes a non-linear DC power modulation strategy for expandable point-to-point (PtP) high-voltage DC (HVDC) systems. The goal is to enhance the active power management during post-fault conditions of the interconnected AC networks. The proposed strategy is developed by defining exponentially decaying functions, which, depending on the HVDC network configuration of the expandable HVDC system, alter the active current reference in a voltage source converter (VSC) affected by an AC network's disturbance, without utilizing proportional-integral (PI) controllers. Furthermore, it is investigated whether the produced alteration can fulfill the post-fault active power recovery (PFAPR) requirements of VSC-HVDC systems, even in situations when no communication protocols between the VSC units are used. Lastly, it is demonstrated, through simulation experiments, that the expandable HVDC system (working in a point-to-point (PtP) or a multi-terminal (MT) network configuration), shows a better performance (in terms of the PFAPR profile and the DC voltage response) when the proposed strategy is utilized instead of conventional main-supplementary or droop control strategies.

The increasing need of cost-effective and large-scale energy storage systems is motivating a strong focus on the deployment of hydrogen based conversion and storage. Hence, the potential of achieving fast dynamic response and high efficiency of electrolyzers makes them attractive to support the mitigation of reliability and stability threats due to the inherent variable power supply from renewables. Suitable dynamic models of new combined solutions, i.e. renewable generation with electrolyzers, are urgently needed to properly characterize and mitigate such threats. Therefore, this paper presents an EMT real-time simulation model of an electrolyzer connected to an offshore 20 MW DC wind power generation system. The envisioned power electronic layout along with the necessary controlling actions are explained in detail. Two modes of electrolyzer operation are proposed the master and slave mode. Based on whether hydrogen production is the priority or grid power injection is priority, the controlling action of the proposed model can be switched. The proposed model is developed as a building block for the study and design of multi-gigawatt scale offshore wind power plants, with stability support from electrolyzers.