London This paper addresses the control of multiterminal voltage-source converters at high-voltage direct current in the context of offshore wind farms. Droop control is commonly used to regulate the dc voltage in this kind of grid, and droop parameters are selected on the basis of steady-state analyses. Here, a control design methodology is proposed based on the frequency-response analysis. This methodology provides a criterion to select the droop gains, taking into account the performance specifications [i.e., the desired voltage errors and the maximum control inputs (currents)]. The application of the methodology is illustrated with a four-terminal grid.

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This article is focused on the droop-based DC voltage control design for multi-terminal VSC-HVDC grid systems, considering the AC and the DC system dynamics. The droop control design relies on detailed linearized models of the complete multi-terminal grid, including the different system dynamics, such as the DC grid, the AC grid, the AC connection filters and the converter inner controllers. Based on the derived linear models, classical and modern control techniques are applied to design the different controllers, including a multi-variable frequency analysis to design the grid voltage droop control. In combination with the droop control, a DC oscillation damping scheme is proposed, in order to improve the system performance. The control design is validated through simulations of a threeterminal system.

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An analysis of control structures for Modular Multilevel Converters (MMC) used in High-Voltage Direct Current (HVDC) applications is addressed. In particular, the study focuses on the case of a point-to-point link with masterslave control, considering an energy-based scheme (also known as closed-loop or energy-controlled) for the MMC, meaning that the internal energy of the converter is explicitly controlled. With such an approach, the MMC internal energy can be controlled independently from the energy of the HVDC link, and whereas the internal capacitance of the MMC depends on the converter's rating, the capacitance at the DC terminal depends on the cable length. Therefore, several possibilities regarding the outer control structure (internal energy and DC voltage) arise, affecting the overall dynamics differently. Whereas for a long link the classic control structures should perform well, for shorter links the transient performance might not be acceptable and other alternatives shall be used instead. Different control structures are presented and evaluated in this paper through small-signal and frequency-domain analysis, and validated through time-domain simulation with Matlab R Simscape Power Systems TM .

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Abstract-This article is focused on the modelling and control of an interline Current Flow Controller (CFC) for meshed HVDC grids. The operation states of the CFC are presented and an average model is derived. The average model is used to perform steady-state analysis on a 3-terminal meshed grid, showing the current change capabilities and the benefits on the operation area. The converter control is designed using a linearised model. The system performance of the CFC is tested by means of simulation in a 3-terminal grid and in a 5-terminal grid.

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