Dear colleagues,

Multi-Terminal HVDC (MT-HVDC) grids are expected to play a key role in future electricity delivery systems. The main drivers for the development of MT-HVDC grids are the large-scale integration of renewable energy resources, particularly off-shore wind farms, and the promotion of international energy markets through the concept of super-grids. The voltage source converter (VSC) technology, practically implemented as modular multilevel converters (MMCs) based on half-bridge or full bridge submodules, enables the realization of MT-HVDC grids by offering flexibility to change the power flow direction and the possibility of connecting to weak AC systems. This Special Issue covers both MT-HVDC grids comprising more than two terminals and meshed DC paths and MT-HVDC systems comprising more than two terminals but no meshed DC paths. There are a few MMC based MT-HVDC systems in operation while the world’s first large-scale MT-HVDC grid, the Zhangbei four-terminal HVDC grid in China, is expected to be operational in 2022.

The protection of MT-HVDC grids is of utmost important to ensure the safety of equipment, preserve the stability of the AC grids they are connected to, and attain the full reliability benefits of MT-HVDC grids. The protection should prevent faults in an AC system that result in the loss of generation or load in a MT-HVDC grid, impacting the other AC systems via the MT-HVDC grid. Similarly, a fault inside a converter station should not result in complete shutdown of a MT-HVDC grid. The faults on the DC side lines, cables, or busbars are the most challenging, as all converters contribute to the fault current, causing a sudden reduction in the voltage of DC buses, severely restricting the power flow, and risking the shutdown of the entire HVDC grid. DC side fault currents can rise very rapidly, peaking well above the steady state fault current due to discharge of converter capacitance and stored energy in inductive and capacitive elements in the fault current paths. A DC side fault is reflected on the AC side as a three-phase short circuit, and a MMC based on half-bridge submodules is unable to block the steady state fault current contribution from the AC side. On the other hand, limitations of the DC circuit breaker technology, both technical and economic, make the problem even more challenging. Therefore, the protection of MT-HVDC grids has recently become a subject of intense research.

This Special Issue of Energies aims to discuss the challenges and solutions to the problem of the protection of future MT-HVDC grids. The topics of interest include, but are not limited to

• Fault detection and discrimination techniques for MT-HVDC grids;
• Identification of faults inside converter stations;
• Developments in DC circuit breaker technology and practical testing;
• Fast and sensitive protection techniques for submarine cables and mixed transmission system with both cables and overhead lines;
• Impacts of different grid configurations and grounding schemes on fault discrimination and protection;
• Fault current limiters and their impacts on the protection of MT-HVDC grids;
• Impact of fault tolerant converters on the protection of MT-HVDC grids;
• Co-ordinating strategies for fault handling;
• Temporary fault clearing strategies;
• Methodologies for specifying MT-HVDC grid protection requirements and testing;
• Analytical models for fault current characterization and fault location.

Prof. Dr. Athula D. Rajapakse
Guest Editor

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• DC Fault Clearing in HVDC Grids
• Multi-terminal HVDC Grids
• Protection of HVDC Grids
• DC Fault Detection and Discrimination
• HVDC Circuit Breakers
• HVDC Fault Current Limiters
• HVDC Grid Fault Analysis
• Fault Tolerant Converters
• HVDC Grid Grounding
• DC Cable Protection