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Grid-forming (GFM) converters are recognized for their stabilizing effects in renewable energy systems. Integrating GFM converters into high voltage direct current (HVDC) systems requires DC voltage control. However, there can be a conflict between GFM converter and DC voltage control when they are used in combination. This paper presents a rigorous control design for a GFM converter that connects the DC-link voltage to the power angle of the converter, thereby integrating DC voltage control with GFM capability. The proposed control is validated through small signal and transient-stability analyses on a modular multilevel converter (MMC)-based HVDC system with a point-to-point (P2P) GFM-GFM configuration. The results demonstrate that employing a GFM-GFM configuration with the proposed control enhances the stability of the AC system to which it is connected. The system exhibits low sensitivity to grid strength and can sustain islanding conditions. The high stability limit of the system with varying grid strength using the proposed control is validated using a system with four voltage source converters.

The future multi-terminal direct-current (MTDC) grid will require the interconnection of point-to-point high-voltage (HV) DC links with different specifications such as DC voltage level, system grounding configuration and HVDC technology. To adapt these differences, it is obligatory for DC/DC converters to interconnect HVDC links. Additionally, they are capable of providing supplementary functionalities as they are highly controllable devices. In this article, a primary virtual resistance DC voltage controller associated with DC/DC converter is proposed for managing DC grid voltages of the interconnected HVDC grids, increasing the reliability of the system. The commonly known topology, Front-to-Front Modular Multilevel Converter (F2F-MMC) is adopted for DC/DC converter. Time-domain simulations are performed using EMTP software for validating the controller behaviour under power disturbances and large events of loss of one converter in a MMC-based MTDC system. The converters are modelled using reduced order modelling (ROM) methodology. Apart from this, dynamic studies have been carried out using a linear state space model for small-signal stability analysis of a HVDC system integrating DC/DC converter with a virtual resistance DC voltage controller. The results are examined through parametric sensitivity analysis.

This paper presents an MTDC test case integrating a DC/DC converter where the converter is working with a DC voltage controller and participating in the DC voltage management system. The influence of voltage-controlled DC/DC converter is studied by introducing power disturbances in the MTDC system.

The development of multi-terminal DC (MTDC) networks has various challenges as interconnecting grids of different voltages and grounding schemes, DC grid protection and power flow. DC/DC converter has emerged as the solution for interconnecting HVDC links with different specifications. In this paper, the Front-to-Front Modular Multilevel Converter (F2F-MMC) topology is adopted for DC/DC converter, which can act as a firewall between the healthy and faulty grid during DC faults. Along with this, DC/DC converters when operated in DC voltage control mode can provide supplementary functionalities such as participating in DC grid voltage management and increasing the reliability of the system. In this study, the F2F-MMC converter is operated with a virtual resistance DC voltage controller integrated into an MTDC system. A pole-to-pole DC fault is applied on the MTDC grid and the influence of the virtual resistance controller associated with the DC/DC converter is studied for re-establishing the power flow after DC faults.

The M2DC is a non-isolated DC/DC converter candidate suitable for multi-terminal HVDC interconnection. The main challenge of this converter topology is the presence of large circulating AC currents that lead to the use of higher current rating IGBT and higher losses. To reduce those, the impacts from the design and control strategy have been extensively studied. The present work demonstrates that adapting the arm inductance values despite the asymmetrical nature of the M2DC may lead to a higher efficiency and lower capacitor size, especially when the voltage ratio of the converter is close to one-to-one.

The Modular Multi-Level DC-DC Converter (M2DC) is an attractive non-isolated DC-DC converter topology for HVDC grid. In order to carry out MTDC grid stability studies, the development of reduce order models of converters is necessary. This article first presents the M2DC converter. Then, the reduce order model will be developed in the second part. The development of the control of this model will be carried out in the third part. Atlast, the comparison of the reduce order model and its control with the average arm model will be performed in the later section of the paper.

Afin de réaliser des études de stabilité des réseaux MTDC, le développement de modèles d’ordre réduit des convertisseurs s’avère nécessaire. Cet article présente un modèle réduit du convertisseur DC/DC modulaire multiniveaux (M2DC) : une topologie de convertisseur DC-DC non isolée attrayante pour le réseau HVDC. Cet article présente d’abord le convertisseur M2DC. Dans un second temps, le modèle réduit sera développé. Le développement de la commande de ce modèle sera effectué dans la troisième partie. Enfin, la comparaison du modèle réduit et de son contrôle avec le modèle moyen des bras sera effectuée dans la dernière section de l’article.