In the AC surrounded Multi-terminal HVDC (AC-MTDC) grids, the substantial growth in Renewables-based Power Generation (RPG) out-turn high inertia into low or Zero-Inertia systems and are the near future of the transiting power systems. In this context, it necessitates the development of a Zero-Inertia AC-MTDC grid test system and appropriate control strategies for the satisfactory operation of the system. Thus, this paper develops the Zero-Inertia AC-MTDC grid while replacing the synchronous generators with the Inverter-Based Resources (IBRs). Besides, it utilizes the Grid Forming (GFM), Grid Supporting (GST), and Grid Following (GFL) control strategies for the Zero-Inertia AC-MTDC grids. The GFM and GST control modes are utilized to start up the system and improve the system’s frequency and dc voltage profiles in a decentralized manner during any imbalances in the grid. Moreover, the GST mode is achieved in this paper through a conventional fixed droop and proposed Modified-Operating Point-based droop (MOPD) control strategies for Voltage Source Converters (VSCs) connected to grid and wind farms in the AC-MTDC grids. The significance and validation of these control modes are perceived through the various case studies with a developed Zero-Inertia AC-MTDC test system and inertia AC-MTDC grid in the PSCAD/EMTDC simulation.

The holistic nature of primary and secondary control leads to the satisfactory operation of the AC surrounded Multi-Terminal HVDC (AC-MTDC) grids. In this paper, a Lower Order Response Model  (LORM) is developed and utilized in the hierarchical control structure. The LORM based secondary control is proposed while operating primary control in droop for a requisite operation of the AC-MTDC grids. The objective of this secondary control is to ensure reduced frequency and DC-voltage deviation with enhanced stability of the AC-MTDC grids. Further, the stability margins of the AC-MTDC grid are identified by employing an eigenvalue analysis of the LORM. These margins decide the droop value limits, which are utilized in secondary control for the stable operation of the AC-MTDC grid. The proposed framework is validated in MATLAB and PSCAD/EMTDC co-simulation platforms while considering two case studies and an AC-MTDC test system consisting of five terminals integrated into a two-area power system.

The PVF or PV2F double droop control is commended for its ability to regulate both the dc voltage and frequency in a decentralized approach. However, a convincing response is not achieved due to an interaction between the droop characteristics of dc voltage and frequency. This interaction affects the dc voltage and frequency support of the AC system surrounded Multi-Terminal HVDC (AC-MTDC) grid. To overcome this effect, a Duo control strategy is proposed in this paper, which takes advantage of a Bi-polar Voltage Source Converter (B-VSC) topology in the MTDC grid. The virtue of proposed control technique is emphasized by comparing it with the existing $ PV2F double droop control along with three case studies and two test systems. The validation of interaction less Duo control strategy is carried out on five terminal CIGRE DC grid benchmark model integrated into two area power system (AC-MTDC grid-1) and New England IEEE 39 bus system (AC-MTDC grid-2). These test systems are simulated in PSCAD/EMTDC software.