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• Proposes novel dc grid device based on capacitive energy storage. • Reduces fault current and voltage drop of voltage source converters under dc faults. • Reduces overvoltages under pole-to-ground faults in symmetrical monopole grids. • Improves power flow security by preventing converter blocking. • Improves transient dc grid stability. A major and very important challenge in dc grid development is maintaining continuous converter operation under dc faults. This paper proposes a novel capacitive energy storage device which improves security of dc grids by avoiding terminal blocking. The device provides current from the capacitor bank during dc faults, reducing fault current contribution and voltage drop of dc grid converters. Moreover, the device also helps in balancing pole voltages which is of particular significance during pole-to-ground faults in symmetrical monopole systems. Other benefits like improved transient grid stability are also demonstrated. Device's design and performance is assessed using theoretical analysis and verified on a three-terminal offshore dc grid model in PSCAD. The cost of the device's electronics is minimized and the total cost and weight estimation are also shown. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents an integrated design of an offshore wind farm and an interconnection circuit based on a series multiterminal HVDC link with current source inverters (CSI). The transmission converters are used to achieve variable speed operation for a group of generators, and this enables use of very simple generators. The series converter connection eliminates offshore transformers. The paper discusses the control systems for both, generator side and grid side converters. A 200MW wind farm is simulated on PSCAD/EMTDC platform and the responses confirm satisfactory operation for a range of wind speed changes. It is shown that each generator group can operate with a different and optimal frequency and that wind variations on individual units cannot jeopardize system stability. The main challenges for the proposed topology are system insulation and management of transmission line losses, and the paper studies some possible solutions. [Copyright &y& Elsevier]

Abstract: This paper investigates a suitable control system for a DC transmission system based on voltage source converters (VSC). Each of the two VSC converters has two control inputs and the four control channels on a VSC transmission system offer potential for a versatile control. However, the main challenge is the dynamic interaction among the control loops. It is proposed in this study that the overall system stability and good robustness should be achieved with two high-gain feedback loops, one at each converter. Eigenvalue and robustness analysis with MATLAB software shows that the best fast feedback signals are inverter DC voltage and rectifier AC voltage q component. The slow controller consists of three PI controllers that regulate: rectifier AC voltage, inverter AC voltage and the DC power. The VSC transmission control under fault conditions is achieved with a separate controller that takes over system control for fault-level currents. The fault controller regulates the local DC currents at each converter. The proposed controller is tested using PSCAD/EMTDC for a wide range of small-signal step inputs and the design performance is confirmed. The design is also tested for typical fault scenarios on AC and DC side to verify the fault controller operation. [Copyright &y& Elsevier]