Integrated Design and Verification of Multi-Vendor VSC-HVDC and Multi-Terminal DC Grids

<jats:p>The transition from point-to-point high-voltage direct-current links to multi-terminal, multi-vendor voltage-source-converter (VSC) grids changes the engineering problem from equipment procurement to system integration. Converter controls, DC protection, offshore AC networks, cable resonances, communications and operating procedures become mutually dependent, while key controller details remain proprietary and system configurations evolve over decades. This paper presents a structured critical review of VSC-HVDC and multi-terminal DC-grid design across technology selection, modelling, weak-grid and multi-infeed stability, grid-forming control, DC-voltage coordination, fault clearing, interoperability, model validation, testing, reliability and standards. The evidence base combines peer-reviewed research, IEC and IEEE standards, CIGRE guidance, network codes and recent multi-vendor initiatives, with emphasis on literature from 2018–2026 and seminal earlier work. The review finds that three recurrent discontinuities dominate risk: requirements that are not expressed as measurable behaviours; models that are not governed as configuration-controlled evidence; and acceptance tests that are not traceable to system-level failure consequences. To address these discontinuities, an integrated lifecycle assurance framework is proposed: requirements → architecture → models → stability → control → protection → validation → acceptance → operation. The framework adds explicit interface contracts, model-fidelity escalation rules, executable acceptance cases, failure-propagation analysis and controlled regression after change. It also clarifies why component compliance, short-circuit ratio screening or a single factory test cannot establish system acceptability. Research priorities are identified for open-but-protective model interfaces, energy-aware grid-forming specifications, interoperable protection envelopes, automated scenario coverage and evidence-preserving upgrades. The resulting synthesis is intended to support owners, system operators, manufacturers, consultants and researchers developing extensible offshore and interregional DC grids.</jats:p>