HVDC systems are playing an increasingly significant role in energy transmission due to their technical and economic superiority over HVAC systems for long distance transmission. HVDC is preferable beyond 300-800 km for overhead point-to-point transmission projects and for the cable based interconnection or the grid integration of remote offshore wind farms beyond 50-100 km. Several HVDC review papers exist in literature but often focus on specific geographic locations or system components. In contrast, this paper presents a detailed, up-to-date, analysis and assessment of HVDC transmission systems on a global scale, targeting expert and general audience alike. The paper covers the following aspects: technical and economic comparison of HVAC and HVDC systems; investigation of international HVDC market size, conditions, geographic sparsity of the technology adoption, as well as the main suppliers landscape; and high-level comparisons and analysis of HVDC system components such as Voltage Source Converters (VSCs) and Line Commutated Converters (LCCs), etc. The presented analysis are supported by practical case studies from existing projects in an effort to reveal the complex technical and economic considerations, factors and rationale involved in the evaluation and selection of transmission system technology for a given project. The contemporary operational challenges such as the ownership of Multi-Terminal DC (MTDC) networks are also discussed. Subsequently, the required development factors, both technically and regulatory, for proper MTDC networks operation are highlighted, including a future outlook of different HVDC system components. Collectively, the role of HVDC transmission in achieving national renewable energy targets in light of the Paris agreement commitments is highlighted with relevant examples of potential HVDC corridors.

This paper details a methodology for investigating the reliability of different offshore grid design options for the connection of offshore wind power to shore. The methodology uses a sequential Monte Carlo based technique that allows investigation of realistic offshore phenomena such as the weather dependency of component repair times. A number of case studies are examined for the connection to shore of a cluster of far offshore wind farms and a cost benefit analysis is performed which compares the capital costs, electrical losses and reliability of each. There is shown to be clear value in options that have inherent redundancy and alternative protection strategies which avoid the use of expensive DC circuit breakers are shown to be potentially viable. It is also found that low probability, high impact faults such as transmission branch failures are a key driver behind overall grid reliability.

This paper outlines possibilities for connecting 2.4 GW of power from two separate wind farms at Dogger Bank in the North Sea to the GB transmission system in Great Britain. Three options based on HVDC with Voltage Source Converters (VSC HVDC) are investigated: two separate point-to-point connections, a four-terminal multi-terminal network and a four-terminal network with the addition of an AC auxiliary cable between the two wind farms. Each option is investigated in terms of investment cost, controllability and reliability against expected fault scenarios. The paper concludes that a VSC-HVDC point-to-point connection is the cheapest option in terms of capital cost and has the additional advantage that it uses technology that is commercially available. However, while multi-terminal connections are more expensive to build it is found that they can offer significant advantages over point to point systems in terms of security of supply and so could offer better value for money overall. A multi-terminal option with an auxiliary AC connection between wind farms is found to be lower cost than a full multi-terminal DC grid option although the latter network would offer ability to operate at greater connection distances between substations

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The changing power landscape introduces concerns about frequency management in a power system with significant amounts of non-synchronous sources of power. In islanded power systems like Great Britain and Ireland, electricity system operators are sometimes forced to undertake very expensive redispatch actions, including curtailing large amounts of renewable generation to meet statutory frequency stability constraints. Consequently, there is an imminent need to understand and quantify the limits that these constraints pose on the power system and develop metrics that can be easily integrated into current system planning and operational paradigm. This paper proposes three such metrics for quantifying the containment limits of a power system at a given operating point. The paper further argues that while the penetration of non-synchronous dispatch can indeed be used as the basis of a metric to define the containment limits of a power system, it does not account for variations in the contributions of other containment factors such as inertia. To address the aforementioned issue two alternatives are proposed: the first defines the containment limits of a power system without direct reference to penetration of non-synchronous power, instead it determines a relationship in terms of critical inertia. The second alternative improves upon the first and it considers the components of frequency stability constraints, offering an increased degree of flexibility in quantifying containment limits, and understanding the influence that certain key factors have on frequency containment.